TRAFFIC MANAGEMENT

AND SPEED

WORKSHOP-CUM-TRAINING PROGRAMME ON ROAD SAFETY

17th ndash 21st September 2018

Dr K Ramachandra RaoProfessor

Department of Civil Engineering and Transportation Research and Injury Prevention Programmme (TRIPP)

Indian Institute of Technology Delhi

Outline

bull Introduction

bull Evidence of speed impacts on injury

bull Reaction time

bull Braking distance

bull Speed ndash injury severity relationship

bull Speed limits

bull Road structure - speed

bull ITS and Safety ndash Michelin Report

bull Way forward

Speed - safety 2

Speed - safety 3

Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle

use speed WAS NOT CONSIDERED to be a MAJOR

RISK

bull Many studies in 1960rsquos attempted to understand various

factors associated with incidence of road traffic crashes

including variables

bull Traffic volume age sex of drivers width and curvature of roads

weather driver error but did not include SPEED

bull A study in 1964 reports that ldquoOne of the important

findings of this study is that the greater the differential in

speed of a driver and his vehicle from the average speed

of all traffic the greater the chance of that driver being

involved in an accidentrdquo

Speed - safety 4

Introductionbull Further ldquoThe accident-involvement injury and property-damage rates

were highest at very low speeds lowest at about the average speed

of all traffic and increased at the very high speeds particularly at

nightrdquo

bull Thus the greater the variation in speed of any vehicle from the

average speed of all traffic the greater its chance of being involved in

an accidentrdquo

bull ldquoThe fatality rate was highest at very high speeds and lowest at about

the average speedrdquo (Solomon 1964)

bull ldquoAs is well known the primary factor affecting the likelihood of driver

injury is speedrdquo (White and Clayton 1972)

bull Another study concluded that there was a reduction in speeds

brought about by the 55 mph law and the interstate rural system had

a 30 reduction in fatality rates which prevented 4700 fatalities and

81000 injuries in 1974

Speed - safety 5

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Outline

bull Introduction

bull Evidence of speed impacts on injury

bull Reaction time

bull Braking distance

bull Speed ndash injury severity relationship

bull Speed limits

bull Road structure - speed

bull ITS and Safety ndash Michelin Report

bull Way forward

Speed - safety 2

Speed - safety 3

Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle

use speed WAS NOT CONSIDERED to be a MAJOR

RISK

bull Many studies in 1960rsquos attempted to understand various

factors associated with incidence of road traffic crashes

including variables

bull Traffic volume age sex of drivers width and curvature of roads

weather driver error but did not include SPEED

bull A study in 1964 reports that ldquoOne of the important

findings of this study is that the greater the differential in

speed of a driver and his vehicle from the average speed

of all traffic the greater the chance of that driver being

involved in an accidentrdquo

Speed - safety 4

Introductionbull Further ldquoThe accident-involvement injury and property-damage rates

were highest at very low speeds lowest at about the average speed

of all traffic and increased at the very high speeds particularly at

nightrdquo

bull Thus the greater the variation in speed of any vehicle from the

average speed of all traffic the greater its chance of being involved in

an accidentrdquo

bull ldquoThe fatality rate was highest at very high speeds and lowest at about

the average speedrdquo (Solomon 1964)

bull ldquoAs is well known the primary factor affecting the likelihood of driver

injury is speedrdquo (White and Clayton 1972)

bull Another study concluded that there was a reduction in speeds

brought about by the 55 mph law and the interstate rural system had

a 30 reduction in fatality rates which prevented 4700 fatalities and

81000 injuries in 1974

Speed - safety 5

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed - safety 3

Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle

use speed WAS NOT CONSIDERED to be a MAJOR

RISK

bull Many studies in 1960rsquos attempted to understand various

factors associated with incidence of road traffic crashes

including variables

bull Traffic volume age sex of drivers width and curvature of roads

weather driver error but did not include SPEED

bull A study in 1964 reports that ldquoOne of the important

findings of this study is that the greater the differential in

speed of a driver and his vehicle from the average speed

of all traffic the greater the chance of that driver being

involved in an accidentrdquo

Speed - safety 4

Introductionbull Further ldquoThe accident-involvement injury and property-damage rates

were highest at very low speeds lowest at about the average speed

of all traffic and increased at the very high speeds particularly at

nightrdquo

bull Thus the greater the variation in speed of any vehicle from the

average speed of all traffic the greater its chance of being involved in

an accidentrdquo

bull ldquoThe fatality rate was highest at very high speeds and lowest at about

the average speedrdquo (Solomon 1964)

bull ldquoAs is well known the primary factor affecting the likelihood of driver

injury is speedrdquo (White and Clayton 1972)

bull Another study concluded that there was a reduction in speeds

brought about by the 55 mph law and the interstate rural system had

a 30 reduction in fatality rates which prevented 4700 fatalities and

81000 injuries in 1974

Speed - safety 5

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle

use speed WAS NOT CONSIDERED to be a MAJOR

RISK

bull Many studies in 1960rsquos attempted to understand various

factors associated with incidence of road traffic crashes

including variables

bull Traffic volume age sex of drivers width and curvature of roads

weather driver error but did not include SPEED

bull A study in 1964 reports that ldquoOne of the important

findings of this study is that the greater the differential in

speed of a driver and his vehicle from the average speed

of all traffic the greater the chance of that driver being

involved in an accidentrdquo

Speed - safety 4

Introductionbull Further ldquoThe accident-involvement injury and property-damage rates

were highest at very low speeds lowest at about the average speed

of all traffic and increased at the very high speeds particularly at

nightrdquo

bull Thus the greater the variation in speed of any vehicle from the

average speed of all traffic the greater its chance of being involved in

an accidentrdquo

bull ldquoThe fatality rate was highest at very high speeds and lowest at about

the average speedrdquo (Solomon 1964)

bull ldquoAs is well known the primary factor affecting the likelihood of driver

injury is speedrdquo (White and Clayton 1972)

bull Another study concluded that there was a reduction in speeds

brought about by the 55 mph law and the interstate rural system had

a 30 reduction in fatality rates which prevented 4700 fatalities and

81000 injuries in 1974

Speed - safety 5

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Introductionbull Further ldquoThe accident-involvement injury and property-damage rates

were highest at very low speeds lowest at about the average speed

of all traffic and increased at the very high speeds particularly at

nightrdquo

bull Thus the greater the variation in speed of any vehicle from the

average speed of all traffic the greater its chance of being involved in

an accidentrdquo

bull ldquoThe fatality rate was highest at very high speeds and lowest at about

the average speedrdquo (Solomon 1964)

bull ldquoAs is well known the primary factor affecting the likelihood of driver

injury is speedrdquo (White and Clayton 1972)

bull Another study concluded that there was a reduction in speeds

brought about by the 55 mph law and the interstate rural system had

a 30 reduction in fatality rates which prevented 4700 fatalities and

81000 injuries in 1974

Speed - safety 5

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidencebull Over whelming evidence ndash

bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)

bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500

bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby

bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce

Speed - safety 6

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden

United Kingdom and the Netherlands and concluded that

bull Fatalities tend to change proportionally with change of average speed in a double quadratic way

bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)

bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)

bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities

Speed - safety 7

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speeding - highways

Speed - safety 8

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speeding

Speed - safety 9

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Person struck by car gt 5 death rate

Field of Vision at 30 kmh

Livable StreetsMore than just traffic volume ndash lower speeds

IIT Delhi 17 September 2018

Speed - safety 10

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Field of Vision at 50 kmh

Person struck by car gt 45 death rate

IIT Delhi 17 September 2018

Speed - safety 11

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Field of Vision at 65 kmh

IIT Delhi 17 September 2018

Speed - safety 12

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Probability of pedestrian fatality

Speed - safety 13

Probability of

pedestrian fatality

increases 4-fold for

a 10 kmh (40 to 50

kmh) increase in

impacting speed

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better

control of the vehicle and the vehicle can stop much

earlier and reduce the probability of a crash

bull Say a vehicle travelling at 80 kmh (22 ms) than a

50 kmh (139 ms) to avoid crash

bull First uses steering 17 s (14 m extra)

bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m

extra) before brakes are applied

Speed - safety 14

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends

on the square of the original velocity

bull When this is combined with the extra reaction time we

see that distances covered at higher speeds are much

higher than those at lower speeds

bull Total stopping distance = reaction distance + braking

distance

d = dr + db

dr = vit

db = (v12-v2

2)2g(f001G)

bull Vehicles traveling at higher speeds continue to travel at

high speeds and the effect of braking manifests itself later

and later the higher the speed

Speed - safety 15

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Relationship of speed ndash braking distance

Speed - safety 16

Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh

can have the effect of an increase in impact speeds of 32

53 and 70 kmh respectively

bull This shows how increases in speed affect the outcome in

proportions that are disproportionately higher

Speed - safety 17

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at

lower impact velocities

bull This is because the severity of injuries depends to large

extent on the energy transferred to the human body

during an impact

bull The relationship between speed and energy is given

below

bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object

V ndash velocity of the object

bull Thus small increases in velocity effect much larger

increases in energy

Speed - safety 18

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed ndash impacts on car occupants and

pedestrians

Impact

velocity

(kmh)

Estimated injury

(Belted car)

Risk of fatality

(Belted car driver)

Road user fatality

(pedestrian Car impact)

30 004 001 008

50 010 003 087

80 042 021 100

100 080 061 100

120 100 100 100

Speed - safety 19

Mohan (2017)

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Probability of fatality ndash pedestrian and car

occupant

Speed - safety 20

Mohan (2016)

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed limits

bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)

bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design

bull Speed limits ndash setting and compliance

bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits

Speed - safety 21

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed limits

bull A key motivating factor in driversrsquo tendency to exceed the

speed limit is that they believe that the excess speed

does not threaten safety

bull The findings show that driversrsquo perception of the speed

above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash suggesting that enforcement

plays an important role in safety perceptions

Speed - safety 22

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed limits - perceptions

bull Manneringrsquos study show that driversrsquo perception of the

speed above the speed limit at which they will receive a

speeding ticket is a critical determinant of what they

believe is a safe speed ndash

bull Enforcement plays an important role in safety perceptions

bull Other significant factors in determining the speed above

the speed limit at which safety is first threatened include

bull age gender being previously stopped for speeding and driversrsquo

ethnicity

Speed - safety 23

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management

bull Speed limits

bull Good information about the local speed limit

bull Infrastructural measures

bull Police surveillance and enforcement

bull Education and information

bull Intelligent Speed Assistance (ISA)

Speed - safety 24

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Speed management consists of a combination of

measures in a logical order (Wegman amp Aarts 2006 Van

Schagen amp Feypell 2011)

bull Step 1 Determine the safe speed limit

bull When motorized traffic mixes with pedestrians and cyclists the

speed must be low

bull the possibility of certain conflicts such as a lateral or a head-on

conflict influences the safe speed

Speed - safety 25

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Step 2 Make sure the limit is credible

bull Credible means that the limit is consistent with the

expectations

bull drivers are more inclined to keep to the limit (Van

Schagen Wegman amp Roszbach 2004)

Speed - safety 26

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Step 3 Give good information about the local limit

bull In practice it is not always clear which limit applies at a certain

location

bull Information on the local limit is usually given by road signs

Speed - safety 27

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Step 4 Support the limit with speed inhibitors

bull Where necessary (eg near schools pedestrian and bicycle

crossings single-level intersections) physical speed inhibitors help

drivers to choose a lower speed

bull speed humps road narrowings plateaus or roundabouts

Speed - safety 28

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Step 5 Police enforcement

bull With the above measures we may assume that many of

the speeding offences are prevented

bull But because drivers can ultimately choose their own

speed offences will always be committed

bull Therefore focused police enforcement remains necessary

for the time being aimed at both general deterrence and

prevention

Speed - safety 29

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed management - measures

bull Step 5 Education and information

bull Education and information offer support for each of the

above measures

They can be used to explain speed measures such as

police enforcement and speed humps and make people

aware of the risks of (too) fast driving

bull It appears very difficult to influence speed behaviour with

education and information (Van Schagen et al 2016)

Speed - safety 30

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Road features ndash enhancereduce speeds

Speed - safety 31

SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012

Reproduction is only permitted with due acknowledgement

lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible

If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option

adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the

road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits

All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit

a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed

Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators

Accelerators Decelerators

1 Tangents Long tangents Short tangents (many bends or intersections)

2 Physical speed limiters Physical speed limiters not present Physical speed limiters present

3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment

4 Road width Wide road Narrow road

5 Road surface Smooth road surface Rough road surface

Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left

and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Road design standards - safety

The truth is that roads designed to standards are not safe

not unsafe nor are they appropriately safe roads designed

to standards have an unpremeditated level of safety

- Ezra Hauer

Speed - safety 32

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Swiss cheese model

Speed - safety 33

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed - safety 34

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed - safety 35

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Emergency vehicle pre-emption

Speed - safety 36

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Way forward

bull Speed management is mandatory in high activity

precincts

bull Credible speed limits

bull Identification of road features ndash reducing speed

bull ITS implementations for safety ndash needs further exploration

Speed - safety 37

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport

Planning and Traffic Safety Making cities roads and vehicles safer Taylor

and Francis

bull Mohan D (2017) Class handouts ndash Traffic Safety and environment

(CVL747) IIT Delhi

bull SWOV (2017) What measures for speed management are there

bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-

management-are-there)

bull Mannering FL (2009) An empirical analysis of driver perceptions of the

relationship between speed limits and safety Transportation Research Part F

12 99ndash106

bull Elvik R (2017) Can evolutionary theory explain the slow development of

knowledge about the level of safety built into roads Accident Analysis and

Prevention 106 166ndash172

bull Michelin Report (2014) Road safety and connected mobility International

Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum

Speed - safety 38

Speed - safety 39

Thank you