VTInotat

Nummer: 08 Datum: 1987-02-12

Titel: The Effect of Overtaking Sight Distance on Journey Speed in Two

' Lane Rural Road

Forfattare: Tang Shumei

Avdelning: Trafikavdelningen

Projektnummer: 7700501-5

Projektnamn: Kinesisk gastforskare

Uppdragsgivare: VTI

Distribution: fri

Statens vag- och trafikinstitut

w6Eff-och Trafih- Pa: 58101 LinkGping. Tel. 013-1152 00. Telex 50125 VTISGI! SIIIStit tet Besok: Olaus Magnus vag 37, Linkoping

ACKNOWLEDGEMENTS

The author would like to thank Mr Arne Carlsson for his valuable

comments and Miss Gunilla Berg for her very substantial assistance in

some figures.

TABLE OF CONTENTS

ABSTRACT

INTRODUCTION

METHODOLOGY

SIMULATION RESULTS AND ANALYSIS

The relationship between sight distance andovertaking concentrationThe relationship between overtaking concentra-tion and journey speedThe effect of sight distance on journey speedThe effect of the percentage of sight distanceabove 300 m and 600 m on journey speedThe effect of traffic volume on overtaking rate

DISCUSSION

CONCLUSIONS

REFERENCES

Page

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The Effect of Overtaking Sight Distance on Journey Speed in Two LaneRural Road

by Tang ShumeiSwedish Road and Traffic Research Institute

5-581 01 LINKOPING Sweden

ABSTRACT

This paper presents some findings of a study of the effect of overtaking

sight distance on journey speed by means of Swedish VTI simulation

model.

The findings reveal that there are strong relationship between sight

distance and overtaking concentration, overtaking concentration and

journey speed.

The implication of the findings is the provision of the level of service

evaluation and the determination of the criteria of controling geometric

design for two lane rural roads.

This paper concludes with a recommendation that the criteria of over

taking sight distance be an essential element in two lane rural road

planning and design. Overtaking lanes, climbing lanes and paved shoulders

are all viable alternatives to new roads.

1 INTRODUCTION

In designing two lane rural roads, overtaking sight distance as well as

stoping sight distance is needed. Overtaking sight distance is normally

stipulated according to the design speeds. These values are based on some

hypotheses of design speed and acceleration. It has been also proposed

that road sections with better sight distance should constitute certain

proportion in the case of heavier traffic and higher speed. But there is no

criteria yet. Since the consideration for different sight distance in

geometric design has a direct impact on the cost of roads, normal design

criteria is the major concern for road designers. However, despite the

accordance with standards, different sight distance condition will result

in different journey speeds and capacities. In the sections with

heterogeneous traffic, if no segregation, the poor sight distance will lead

to platoons and delay thereby a lower level of service.

How sight distance influences journey speeds. Very few literatures dealing

with this are available. It is needed to study the quantitative relationship

(1). This paper studies the effect of sight distance on travel speeds by

using a traffic simulation technique.

2 METHODOLOGY

Two types of method can be applied to study the effect of sight distance

on journey speeds: emperical method and traffic simulation technique.

Since the interactions between vehicles are very complicated, it has been

adviced that traffic simulation technique be a useful aid.

This study was carried out by means of Swedish VTI traffic simulation

model (2) which has shown its potential in long experience. This model is a

kind of microscopic simulation model which describes a single vehicle in

the flow along defined stretches of road forgiven traffic volumes and

traffic compisition. The vehicle are assigned the following characteri-

stics:

a) Vehicle type

b) Basis desired speed in m/s

c) p-value (power/mass ratio) in W/Kg

The stretch of a road considered consists of a sequence of consecutive

road block objects and sight distance function in each direction of travel.

Each object is homogeneous with regard to the follows:

a) Road width

b) Auxiliary lane/Wide shoulder

c) Slope

d) Horizontal curvature

e) Speed limit and

f) Overtaking restrictions

The input data for this study involves road sight distance profile, road

width and traffic data. They are discribed as follows:

a) Sight distance data

The minimum sight distance 200 m

The maximum sight distance 1 000, 800, 600, 400 (m)

The frequency of maximum sight distance 2, l, 0.5/Km respectively

b) Road data

Road width

Road length

c) Traffic data

Volume

Composition

1. cars

2. lorries

3. semi-trailers

4. truck-trailers

9m

8km

1200, 750, 300 Veh/h (both directions)*

88%

4%

4%

4%

Figure l is one of the examples showing the maximum and the minimum

sight distance as well as the frequency of maximum sight distance in each

kilometer.

1000 q

800

600

400

200

Figure 1.

ll SIGHT DISTANCE (m)

A

LENGTH (mlT

1600 2000

Sight distance profile

The main outputs of the simulation studied in this paper are journey

speeds of cars, overtaking concentration (overtakings/km. h) and overe

taking rate (overtakings/ car. hm). .

*Depending of a random distribution for generating traffic, the actual

flow was 1 240, 725 and 300 veh/h at the simulation.

3 SIMULATION RESULTS AND ANALYSIS

3.1 The relationship between sight distance and overtaking con-

centration

Overtaking is defined as such a manoeuvre that a driver moves from a

position behind to one in front of another vehicle travelling in the same

direction. To complete an overtake manoeuvre, the requirement needed

are as follows:

a) The available sight distance should be large enough for overtaking

safely

b) The acceleration ability should be available

If requirement b) is fulfilled, according to the simulation, the better the

sight distance condition, the higher the overtaking concentration. This

implies better sight distance can disperse the traffic queues. Fig. 2 shows

the effect of different sight distance on overtaking concentration under

the condition of different traffic volumes. As can be seen, overtaking

concentration is directly proportional to sight distance. The range of

incremental rate (of overtaking concentration with different sight distan-

ce) is from 0.02 o 0.19 with different traffic and maximum sight distance

frequency, shown in the table of Fig. 2. Again, with large maximum sight

distance frequency, incremental rate of overtaking increases more largely

than small maximum sight distance frequency.

ll OVERTAKINGI kmhr

Traffic volume Q 1 240 725 300

Z 0.18 0.14 0.03

Frequency F l 0.11 0.08 0.03

0.5 0.06 0.05 0.02

150 -

100

50-

o zoo 200 360 1.50 560 600 760 860 960 1600 5.07;)

Figure 2 The relationship between sight distance and overtaking

concentration.

3.2 The relationship between overtaking concentration and

journey speed

It is always the case that faster vehicles and slower vehicles are

travelling on the same lane in rural two lane road. When a car travelling

on the road is catching up a slower one, it is constrained and has to tail

the slower vehicle at a lower speed, which subsequently constrains the

following cars behind it and results in extending the platoon and affects

the capacity.

If the traffic and road conditions are suitable for overtaking, the

constrained vehicles would be released. After overtaking, the faster

vehicles can travel at its normal speed. As can be seen, overtaking is

playing a positive role in increasing the average journey speed and

therefore save time.

The relationship between overtaking concentration and journey speed

from the simulation is shown in figure 3. It is concluded that

a) journey speed is linear with overtaking concentration;

b) The heavier the traffic, the smaller the influence of overtaking

concentration on journey speed, namely, for the traffic volume of 300

veh/h, the increase in journey speed with increased overtaking con-

centraion is faster than that for a traffic volume of l 240 veh/h.

c) With the same traffic flow, large sight distance results in larger

overtaking concentration.

d) Lines of journey speed varying with overtaking concentration form

three lines which can be expressed by three regression equations with

traffic flow Q being 300, 725, 1240 (veh/h) respectively are as

follows:

For Q = 300 veh/h V : 0.233D + 78.5

Q = 725 veh/h V : 0.048D + 74.0

Q = l leO veh/h V : 0.025D + 70.9

Where, V is the journey speed (Km/h);

D is the overtaking concentration (overtaking/Km/hr)

_ 90 _ JOURNEY SPEED {mm

531000

9: 300

S: N

5:1000

80" 0"" s- 1.00

0:750

S = 1000

S 600 Q: 1200

5:600

$ 0

o 10 £035 46 56 so i036 923160110120130150OVERTAKING/ kmhr

Figure 3. The relationship between overtaking concentration andjourney speed of cars.

3.3 The effect of sight distance on journey speed

From the findings of previous sections, it is very clear that journey speed

is directly proportional to sight distance. Figures in Table l are calculated

'from FiO. 2 and the regression equations.

Journey speed and overtaking concentration of cars under

Table 1.different traffic flow and sight condition

_- .. _ , _._-, - __ - a ..- WWW if..- s _3n ago i mo mm m _, WWW)

Q L"r" 0.3" " ' l n 5 ' l _~ 0.5 m ,_ i _________ j J - _L , 6 . l0 [3 m- _, l '6 H r i » T71 '50 4 iv [K ) I m 1

300 236 ' 0 38')

v 79.89 80.18 31:? 7M-. i3 :R °1 ??3_ 3~l§ a. 3'-39 3} 5 R 75 - . °2 3] 9 »7 37'71C 1 }, ""275 "3'2"" " , _G K, (31 3a )1 0r) + a"; 7n 1 m

750 100*; 77: l07a

L V 7 4-72 7"; 36 ,_ 25.7? 7, ' 77-2., 73.73 7.5 3 75:63. 7.6. E L w. h _-1.1_._77 36.-. .7'3.(:¢;_.-.(; , r .. r ' 4 : U ,l 4, ll] (,7 ll;, + (:3 r ) l ,

1200 :33 I ms 9%

v 71.23 71.n5 71.73 72.n2 77 no *v.a3 7|.W? vs 71 #3 *7 >* a :3 7a.n:L........ ., _~ ...-,._._, .. l -,_M.-.. _. .M, .a .i__..- .. -- _ ._ -- - _..l,. ...--

Note:

1) SD - Sight distance (m)

F - Sight distance frequency/Km

C - Overtaking concentration

V - Journey speed of cars

2) Figures with " * " are the time (in sec.) saved for all cars in the traffic

flow comparing sight distance frequency 2 to sight distance frequency

0.5.

Table 1 gives alternatives of sight distance condition with same journey

speed under different terrian condition. For instance, when traffic flow is

300 veh/h, journey speed for two 800 m sight distance in one kilometer

and for one 1 000 m sight distance in one kilometer is the same.

In addition, with the same sight distance condition, the time saved for car

traffic is more when traffic volume is 725 veh/h than 300 veh/h and l 240

veh/ h.

10

3.4 The effect of the percentage of sight distance above 300 m

and 600 m on journey speed

The percentage of sight distance above a certain limit can be calculated

as follows if L is the total length. See Fig. ll.

P(2A):._Z_ £sT _{_3_A_)_x100

SIGHT DlSTANCE (M)ll

T 1.

A

L +LENGTH (M)

_LF L 1

Figure 4. Determination of percentage of sight distance above acertain limit.

The dependance of journey speed to sight distance above 300 m and 600 m

is shown in Fig. 5 a and Fig. 5 b repectively.

Figure 5 a

Figure 5 b

Figure 5

ll

WYSFEEDvsWOFSlG-TTDSTANE

SIGHT MAXIMUMA 5:11:00 :1 5:600

88.mwm x 3:500 o 5:400

86- AA x

3 0:300 veh/hA x D

82-0 ( x DO

80 o O A

x78 A Q=725veh/ ..I

A X D

76 D Ax DO0 ° o A

74 xA

724 5k 6* x D:1° °° <2:va

70 r r T l lO 20 4O 60 80 100

Percentage of sight distance > 300m (2:)

The dependance of journey speed to sight distance above300 m.

JOlREYSFEEDvsFERSEN'MOl- SG-ITDSTANCE

SGHTMAXIMUM

as~ ' A3 (2:3me

84

882 XA

80 A

X78 A 0:725de

A<75 xA

A74"

K Q"'12.40v°h/h72" XA E

70 1 I I T 1

O 20 40 50 80 100

Pereenfageofelgf dietance>600m(%)

The dependance of journey speedito sight distance above600 m.

The effect of the percentage of sight distance on journeyspeed.

12

For SD _>_ 300 m

Q = 300 veh/h V = 5.68Ps + 79.9

Q = 725 veh/h V = 4.50Ps + 74.3

Q =1 240 veh/h V = 2.63Ps + 71.1

For SD _>_ 600 m

Q = 300 veh/h v = 5.37Ps + 81.5

Q = 725 veh/h v -_- 4.37135 + 75.7

Q = 1 240 veh/h v = 2.75Ps + 71.8

where SD, Q and V are defined as before. P5 is the percentage of sight

distance above certain value.

Table 2. Journey speed of cars under different traffic flow and sightcondition.

Sight Distancelm) SD 3 300 m SD _>_ 600 m

percentage (%) 20 40 60 80 100 20 40 60 80 100

300 (veh/h) 81.0 82.2 83.3 84.4 85.6 82.6 83.6 84.7 85.8 86.9

725 " 75.2 76.1 76.7 77.9 78.8 76.6 77.4 78.3 79.2 80.1

1240 " 71.6 72.2 72.7 73.2 73.7 72.4 72.9 73.5 74.0 74.6

Figures in table 2 are calculated according to the regression equations

above. Comparison between Table 2 and Table 1 show that the same speed

may be received from different sight distance condition. For instance, the

journey speeds of the road with one maximum sight distance 1 000 meters

per kilometer and the road with the percentage P5 (3 300 m) being 80%

are the same when traffic flow is l 240 veh/h. This implies that if the

terrain condition is limited, the alignment is controlled by certain sight

distance limits, the determination of the fixed overtaking sight distance

value is not necessary, because any suitable sight distance percentage of

above certain sight distance can be used for this purpose. Therefore, the

most economical sight distance alternative can be chosen.

13

3.5 The effect of traffic volume on overtaking rate

Overtaking rate (overtakings/car/km) is notincreased with the increase-

ment of traffic volume all the time. Fig. 6 gives an example and indicates

that the largest overtaking rate happens when traffic volume is between

#00 veh/h and 700 veh/h under different sight condition. Combining the

analysis made above, in which the larger overtaking concentration results

in higher journey speed, it may be concluded that in the design of a two

lane rural road with traffic volume being 400 veh/h - 700 veh/h the

consideration of the improvement of sight distance will get high potential

benefit. This outcome is in accordance with the one made above: the

travel time saved for the car traffic is more when the traffic volume is

725 veh/h than 300 veh/h and l 2% veh/h.

OVERTAKlNGI [All in

0.20

018- \

(:16 - \

\ \

01m \\ \

\\ \

\ \ m\\

-4 \011 \

\\.

\\\n

0105:

\ _,

\\0.06 / \

\\

, \/ \// \// /-\ \.

00!:

0.02

FLDV (VEHW,

o 550 vsooq

Figure 6. The effect of traffic flow on overtaking rate.

14

4 DISCUSSION

Platoons are caused by slow-moving vehicles constraining faster-moving

vehicles, the latter ones having difficulties in overtaking because of

alignment or high oncoming traffic flow. In effect the very existence of

platoons and the opportunity or lack of opportunity to overtake slow-

moving vehicles may be considered as a measure of the level of service

provided by a highway (3). The study of the effect of sight distance on

journey speed reveals that to improve the road capacity and the level of

service, it is necessary to improve overtaking condition. This can be

achieved through follows:

1) Build roads with better alignment so that vehicles have better sight

distance to carry out overtakings. This is achieved by avoiding sharp

curves both horizontally and vertically.

2) Build auxiliary lanes (climbing lanes or overtaking lanes) or paved

shoulders with enough width, so that slower vehicles can use them for

faster vehicles to pass. According to the estimation made in Sweden,

100% of vehicle drivers use climbing lanes to overtake, 70% of car

drivers and 90% of other vehicle drivers use paved shoulders to

overtake.

In planning a new road or determining an improvement measure, one has

to take terrain, traffic volume, funds etc. into account and take more

suitable way to better sight condition thereby the capacity.

Arne Carlsson studied three road width alternatives of a road of length 20

km to be built by VTI simulation model (4). The three alternatives are:

l) 9 m

2) 9 m + 4 climbing lanes (both directions)

3) 13 m

Simulation shows that the slope of journey speed - traffic flow curve of 9

m + 4 climbing lanes road and 13 m road has slight difference. The cost of

9 m + 3 climbing lanes road is only about 80% of 13 m road.

15

As can be seen, climbing lane is one of the effective and economical way

to improve road capacity.

Another potential way to improve capacity is the change of earth

shoulders to paved shoulders. A study made by India (5) shows that, by

changing earth shoulders to brick shoulders, the journey speed of different

vehicles or road users can increase as follows: car by 9%, bus by 8%, lorry

by 15%, two wheel vehicle by 6%.

In China, two lane rural roads contribute 83% of national road net work, it

will remain the main part in the future. Owing to the heterogeneous

traffic in two lane rural roads, road capacity and safety is more

conspicuous with traffic increasing. How to improve road capacity is

becoming more urgent. From this study, climbing lanes or overtaking

lanes and hard shoulders offer a potential for substantial improvement in

the level of service on rural roads at relative cost. Due to the frequently

current shortage of road funds and uncertainities about travel trend on

some roads, auxiliary lanes and paved shoulders merit particular atten-

tion.

16

CONCLUSIONS

This study of the effect of sight distance on journey speed for different

traffic volume can be concluded as follows:

1)

2)

5)

3)

4)

6)

Overtaking concentration increases with the increment of traffic flow

and sight distance. With better sight distance condition, incremental

rate of overtaking concetration increases more largely than worse

sight distance condition with the increment of traffic.

With the same traffic volume, better sight distance results in higher

journey speed. With the same sight distance condition, the time saved

for car traffic is more when traffic volume is 725 veh/h than 300

veh/h and l 240 veh/h. This is because overtaking rate is larger when

traffic flow is between 400-800 veh/h. This means that with traffic

volume being 400 veh/h - 800 veh/h, the consideration of improvement

of sight distance will obtain greater benefit.

In the case of the limitation of terrain condition, the determining of

fixed overtaking sight distance value is not necessary. Any suitbal

sight percentage of above certain sight distance can be used for this

purpose. Therefore, the most economical alternative can be chosen.

Sight distance condition or the percentage of sight distance above

certain value can be used for measuring the level of service. Since

delay reduction results from better sight distance.

In planning a two lane rural road, the sight percentage can be chosen

based on design speed and level of service for controlling geometric

design.

Auxiliary lanes (climbing lanes and overtaking lanes) and paved shoul-

ders are effective and economical way to improve road capacity. It is

suggested that to some bottle neck sections in existing roads, to build

auxiliary lanes and hard shoulders or paved shoulders show great

benefit.

l7

7) It is necessary to further study the effect of overtaking sight distance

on journey speed under Chinese condition to establish the criteria of

the percentage of above certain sight distance for different traffic

volume, design speed and the level of service.

18

REFERENCES

S LylyHelsinki University of Technology, Finland.

"The Importance of passing sight distance in Highway Design"Proceeding of the symposium on method for determing Geometric Road Design Standard (1976).

Anders Brodin and Arne Carlsson

"The VTI Traffic Simulation model and programme systemSwedish Road and Traffic Institute".Meddelande nr 321 A 1986

John F MorralDepartment of Civil Engineering. The University of Calgary.

A1 WernerAlberta Transportation

"Measurement of level of service for two-lane rural high-ways.

Arne Carlsson

Redovisning av trafiksimulering pa Rv 31.bggestorp-NassjoSwedish Road and Traffic InstituteMemoranda (in Swedish) 1984-02-29

Central Road Research Institute

"Road cost study in India"Final ReportIndia 1982