Clearance Intervals

NCHRP 172: Signal Timing Improvement Practices

Clearance Interval

Clearance Interval = Yellow + All Red

Yellow = Y = t + v2a ± Gg

All Red = AR =w + L v or

Pv or

P + L v

According to the ITE recommended practice:

Where:Y = yellow interval (seconds)t = driver perception-reaction time for stopping, taken as 1 secv = approach speed (ft/sec) taken as the 85th percentile speed or the speed limita = deceleration rate for stopping taken as 10 ft/sec2 G = percent of grade divided by 100 (positive for upgrade, negative for downgrade)

L = length of the clearing vehicle, normally 20 feet

W = width of the intersection in feet, measured from the upstream stop bar to the

downstream extended edge of pavement P = width of the intersection (feet) measured

from the near-side stop line to the far side of the farthest conflicting pedestrian crosswalk along an actual vehicle path

Clearance Interval

w

P

Uses a comfortable and attainable deceleration rate of 10 ft/sec/sec As opposed to the ‘emergency’ rate of

15 ft/sec/sec used earlier Adds one second to the calculated

yellow time

Yellow Interval At least half the states use the

“permissive yellow rule” allows vehicles to enter the intersection

on a yellow signal and to be in the intersection when the signal turns red

Specifies the length of the yellow change interval as:

“The yellow vehicle change intervals should have a range of approximately 3 to 6 seconds. Generally, the longer intervals are appropriate to higher approach speeds.”

National MUTCD

Fraley vs. the City of Flint, MITort suit in Michigan (1974)

Court’s opinion:

“it is not enough that a yellow time merely be between 3 and 6 seconds”

The yellow interval must be designed for intersection-specific conditions

truck use intersection geometry other site specific characteristics

Yellow Interval

ITE formula gives a yellow interval long enough so that a clearing driver will not be forced to enter the intersection on the red, which is an unlawful act

Yellow Interval Based on equation for stopping:

S = vot + vo2/2a

vot: gives the distance traveled at initial speed vo during braking perception-reaction time tVo

2/2a: braking distance to a final speed v = 0, from the fundamental equation of linear kinematics

v2 = vo2 + 2as

where v = final speed (ft/sec) vo = initial speed (ft/sec)

a = deceleration rate (ft/sec/sec)s = distance traveled during braking (ft)

If the yellow begins when a vehicle is further away from the intersection than the minimum stopping distance required The driver will be able to stop

If the vehicle has < the calculated stopping distance Reasonable for the driver to decide to

clear

Minimum required yellow time will carry the clearing vehicle into just into the intersection Legally entered (permissive rule) Just before the red begins

Minimum yellow time

Y = t + v2a ± Gg

Figure 6. Possible scenario with no all-red clearance

Eastbound car is clearing after having barely entered the intersection by the time the red begins. There is no all-red interval, so northbound car receives the green immediately

No all-red interval is used

Yellow time calculated according to the ITE formula will carry the clearing vehicle just into the intersection by the time it ends As shown by vehicle A in the preceding slide

If there is no all-red interval, then oncoming traffic is released on a green signal (vehicle B) Vehicle A will not be protected

Driver of vehicle B has a duty to yield the right-of-way to vehicle A legally within the intersection Permissive rule

However, many drivers do not know this law

Naïve for traffic engineer to expect drivers to yield the ROW To ensure safety, use all-red intervals

Figure 6. Possible scenario with no all-red clearance

Northbound car fails to yield ROW to car A legally in the intersection, enters soon after receiving the

green and is struckNo all-red interval is used

All-Red Interval

“In order to time phase-change intervals for safety, traffic engineers sometimes need to go beyond the minimums implied by the rules of the road. An All-red clearance interval should be considered in some cases in addition to the yellow”

Signal Timing Improvement Practices

NCHRP 172

MUTCD “The yellow vehicle change interval

may be followed by a red clearance interval, of sufficient duration to permit traffic to clear the intersection before conflicting traffic movements are released”

TCDH “The policy of some jurisdictions is

to time the phase change interval to allow the outset of the green interval for conflicting movements without the intersection having been cleared”

TCDH “Some authorities believe that the timing of

a phase-change interval should enable a vehicle to clear the intersection before the onset of the green for conflicting movements. The following equation may be used to determine the phase change interval. It includes a reaction time, deceleration element and an intersection clearing time”

= t + v2a ± Gg

w + L v

+

Where CP is the non dilemma change period

CP

TCDH “the yellow change interval be equal to

the first two terms of the equation and the equation rounded up to the next ½ second, but no less than 3 seconds and no greater than 5 seconds. The remainder of the change period should consist of an all-red interval.

Eastbound car clears intersection by the time the northbound car receives green

Intersection where an all-red interval is used

Older Driver Highway Design Handbook Recommendations

and Guidelines

“To accommodate age differences in perception-reaction time, it is recommended that an all-red clearance interval be consistently implemented, with the length determined according to the Institute of Transportation Engineers (1992) expressions”

Where there is no pedestrian traffic, use:

Where there is the probability of pedestrian crossing, use the greater of:

Where there is significant pedestrian traffic or pedestrian signals protect the crosswalk, use:

All Red = r =w + L v

or Pv

P + L v

All Red = r =

P + L v

All Red = r =

According to traffic laws in Michigan, USA a vehicle must stop when confronted with a yellow light,

unless such an abrupt stop would endanger the safety of the driver as well as others

Law enforcement officials are reluctant to issue a citation for not stopping during the yellow interval Unless someone is observed to have accelerated

through the intersection•citation is rare•hard to prove when contested in a court of law

Entering the intersection when a signal turns red is what most officials consider a citable offense Red light violation Violations are affected by the duration of

the change interval of the traffic signal•yellow interval•all-red interval

When entering the intersection at the end of the clearance interval, motorist are exposed to the danger of being struck by the cross street traffic unless an all-red interval is present

Uniform Vehicle Code in the State of Michigan, USA

“If the signal exhibits a steady yellow indication, vehicular traffic facing the signal shall stop before entering the nearest crosswalk at the intersection or at a limit line when marked, but if the stop cannot be made safely, a vehicle must be driven cautiously through the intersection”.

A vehicle can enter an intersection legally, even a fraction before it turns red If it takes a vehicle two-seconds of time to

cross, then the vehicle is under eminent danger of being involved in a right angle crash in the absence of an all red interval

An intersection without an all red interval runs the risk of having right angle crashes, even if no one violated the red light

Example: Calculate Clearance Intervals for the Intersection of Middlebelt

Road and 5 Mile Road

Approach Speed Spot speed studies were taken at

each of the intersection approaches as follows:

APPROACH SPEED LIMIT

85th PERCENTILE SPEEDmph

Peak Off-peak

NB 40 32 (46.9 fps) 38 (55.7 fps)

SB 40 29 (42.5 fps) 36 (52.8 fps)

EB 45 35 (51.3 fps) 42 (61.6 fps)

WB 45 36 (52.8 fps) 46 (67.5 fps)

Figure 2. Condition Diagram of the Middlebelt Road and Five Mile Road Intersection

N

10’11’5’

39’ONLY ONLY

58’

11’

43’

MobileGasolineStation

Mid-FiveCenter

Drivewayand Alley

West FenkellBakery

QuickStop DryCleaning

M-5 Party Store

ONLY ONLY

6’11’ 7’

12’

12’

14’

14’

12’

10’

10’ 13’ 13’14’11’ 15’

14’10’12’14’ 13’ 11’

Precision Tune

83’

12’

14’

12’

11’

12’

12’

ONLY

ONLY

ONLY

ONLY

28’ 33’43’26’52’239’

Proposed Development Site

Thrifty Flowers and

Plants

Abandoned Office

Building

Co-Op ServicesCreditUnion

Street Name Signs

Center LaneTurn Left Only Sign

Center Two-Way Left-Turn Lane Sign

Center LaneTurn Left Only Sign

Center Two-Way Left-Turn Lane Sign

ONL

YO

NLY

ONL

YO

NLY

ONLYONLY

ONLYONLY

Middlebelt Road Sign

Speed Limit 45

Sign

Street Name Signs

20’ 11’5’ 19’ 70’ 33’

Left-Turn Signals

Right Turn Arrows

Right Turn Arrows

20’

12’

Alley

SunocoGasolineStation

6’

18’

Right Lane Must Turn Right Sign

44’

37’

52’

Parnin Janitorial Supplies

Faygo Discount

138’

13’

40’

BurgerKing

Restaurant

McDonald’sRestaurant

19’

52’

74’11’ 12’13’13’ 12’

Cambridge GroupOffice Building Speed

Limit 40 Sign

FIVE MILE ROAD

MID

DL

EB

EL

T R

OA

D

$2 Car Wash

Figure 2. Condition Diagram of the Middlebelt Road and Five Mile Road Intersection

N

10’11’5’

39’ONLY ONLY

58’

11’

43’

MobileGasolineStation

Mid-FiveCenter

Drivewayand Alley

West FenkellBakery

QuickStop DryCleaning

M-5 Party Store

ONLY ONLY

6’11’ 7’

12’

12’

14’

14’

12’

10’

10’ 13’ 13’14’11’ 15’

14’10’12’14’ 13’ 11’

Precision Tune

83’

12’

14’

12’

11’

12’

12’

ONLY

ONLY

ONLY

ONLY

28’ 33’43’26’52’239’

Proposed Development Site

Thrifty Flowers and

Plants

Abandoned Office

Building

Co-Op ServicesCreditUnion

Street Name Signs

Center LaneTurn Left Only Sign

Center Two-Way Left-Turn Lane Sign

Center LaneTurn Left Only Sign

Center Two-Way Left-Turn Lane Sign

ONL

YO

NLY

ONL

YO

NLY

ONL

YO

NLY

ONL

YO

NLY

ONLYONLY

ONLYONLY

Middlebelt Road Sign

Speed Limit 45

Sign

Street Name Signs

20’ 11’5’ 19’ 70’ 33’

Left-Turn Signals

Right Turn Arrows

Right Turn Arrows

20’

12’

Alley

SunocoGasolineStation

6’

18’

Right Lane Must Turn Right Sign

44’

37’

52’

Parnin Janitorial Supplies

Faygo Discount

138’

13’

40’

BurgerKing

Restaurant

McDonald’sRestaurant

19’

52’

74’11’ 12’13’13’ 12’

Cambridge GroupOffice Building Speed

Limit 40 Sign

FIVE MILE ROAD

MID

DL

EB

EL

T R

OA

D

$2 Car Wash

120’

122’

Yellow Intervals Peak

NorthboundY = 1+ 46.9/(2*10) = 3.345 sec

SouthboundY = 1+ 42.5/(2*10) = 3.125 sec

EastboundY = 1+ 51.3/(2*10) = 3.565 sec

WestboundY = 1+ 52.8/(2*10) = 3.64 sec

Peak N-S Yellow interval use 3.5 sec Peak E-W Yellow Interval use 4.0 sec

Y = t + v2a ± Gg

Yellow Intervals Off Peak

NorthboundY = 1+ 55.7/(2*10) = 3.785 sec

SouthboundY = 1+ 52.8/(2*10) = 3.640 sec

EastboundY = 1+ 61.6/(2*10) = 4.08 sec

WestboundY = 1+ 67.5/(2*10) = 4.375 sec

Off Peak N-S Yellow interval use 4.0 sec Off-Peak E-W Yellow Interval use 4.5 sec

All-Red Intervals Peak

Northbound(122+20)/46.9 = 3.0 sec

Southbound(122+20)/42.5 =3.3 sec

Eastbound(120+20)/51.3 =2.7 sec

Westbound(120+20)/52.8 =2.6 sec

Peak N-S All-Red interval use 3.3 sec Peak E-W All-Red Interval use 2.7 sec

All Red = r =w + L v

All-Red Intervals Off Peak

Northbound(122+20)/55.7 = 2.5 sec

Southbound(122+20)/52.8 =2.7 sec

Eastbound(120+20)/61.6 =2.3 sec

Westbound(120+20)/67.5 =2.1 sec

Off-Peak N-S All-Red interval use 2.7 sec Off-Peak E-W All-red Interval use 2.3 sec

All Red = r =w + L v

Clearance Intervals (CI) Peak Period

North-South East-WestY = 3.5 sec Y = 4.0 secAR = 3.3 sec AR = 2.7 sec CI = 6.8 sec CI = 6.7 sec

Off-Peak PeriodNorth-South East-West

Y = 4.0 sec Y = 4.5 sec AR = 2.7 sec AR = 2.3 secCI = 6.7 sec CI = 6.8 sec

Driver’s Decision Whether to stop or not stop at the

traffic signal may be related to:

vehicle approach speed color of the traffic signal when

noticed by the driver location of the vehicle with respect to

the intersection

Driver’s Decision natural driver behaviors: aggressive vs.

non-aggressive type of vehicle vehicle condition trip purpose

Gazis Research late 1950’s

W

xL

S

Clearing line

Gazis Study Car traveling at a constant speed = v0

Location of the car is at x feet from the stop bar, S

Driver has 2 options Must decelerate and stop before line S (stop bar) Must continue and go through the intersection Dilemma Zone

Dilemma Zone

xc

SCannot go xo

Dilemma zone

Cannot stop

Gazis Study

1, 2 : time at which acceleration or deceleration will begin after the starting of the yellow interval

a1: constant acceleration rate for crossing the intersection= 10 ft/sec2

a2: constant deceleration rate for stopping before the intersection = 10 ft/sec2

W: effective width of the intersection L: length of the car (usually 20’) : Length of the clearance interval

(Y+AR)

Gazis Study

WVehicle location when

light turns yellow

xL

S

Clearing line

Gazis Study

If the driver is to come to a complete stop before entering the intersection

(x - vo2) vo2/2a2

If the driver is to clear the intersection completely before the light turns red

(x + w + L) - vo 1 vo ( - 1) + ½ a1 ( - 1)2

Gazis Study

Assuming a maximum deceleration rate of a2

*, the critical distance is:

Xc = vo 1 + vo2/2a2

*

If x > xc the car can be stopped before the intersection

If x < xc it will be uncomfortable, unsafe or impossible to stop

Gazis Study

Maximum distance the car can be from the intersection of the yellow interval and still clear the intersection:Xo =vo - (W + L)

Gazis Study

Thus, if xo > xc the driver, once past the critical distance xc can clear the intersection before the signal turns red

If xo < xc , a driver at a distance x from the intersection, such that

xo <x< xc will find him/herself in an awkward position if the yellow interval begins at that moment

• cannot stop safely and has to attempt to go through the intersection

Gazis Study

Minimum length of the clearance interval min = (xc+ W+L)/vo OR

min = 2 + ½ vo/2a2*+

(W+L)vo

Gazis Study

Example:

W = 80’Vehicle location when

light turns yellow, traveling at 30 mph

(44 fps)

xL = 20’

S

Clearing line

For driver to stop:(X - vo2) vo

2/2a2

Assume 2 = 0.4 seconds and a2 = 10 ft/sec2

[X – (44 *0.4)] 442/ (2*10)X – 17.6 96.8X 114.4 ft

If the driver sees the yellow light 115 feet before the stop bar, the driver can stop in this distance

For driver to clear:(x + w + L) - vo 1 vo ( - 1 ) + ½ a1 ( - 1)2

Assume 1 = 0.2

For = 3.0 seconds(82+80+20)- 44(0.2) 44( 3 – 0.2 ) + ½ 10 (3-0.2)2

173.2 feet 162.4 feet – cannot clear

For = 5.0 seconds(82+80+20)- 44(0.2) 44( 5– 0.2) + ½ 10 (5-0.2)2

173.2 feet 326.4 feet

criteria satisfiedDrivers will be able to stop or clear.

For = 6.0 seconds(82+80+20)- 44(0.2) 44(6– 0.2) + ½ 10 (6-0.2)2

173.2 feet 423.4 feet – criteria satisfiedDrivers will be able to stop or clear.

Highway Capacity Software(HCS)

Highway Capacity Software

Based on the Highway Capacity Manual (HCM) Special Report 209 Transportation Research Board (TRB),

National Research Council (NRC)

Ten Modules1. Freeways2. Weaving 3. Ramps4. Multi-lane Highways5. Two-lane Highways6. Signalized Intersections7. Unsignalized Intersections8. Arterials9. Transit10. Pedestrians

Signalized Intersections Capacity

Defined for each lane group• Lane group: one or more lanes that accommodate

traffic and have a common stopline Lane group capacity: maximum rate of flow

for the subject lane group that may pass through the intersection under prevailing traffic, roadway and signalized conditions

Traffic Conditions Approach volumes (left, through, right) Vehicle type Location of bus stops Pedestrian crossing flows

Roadway Conditions Number and width of lanes Grades Lane use

• Including parking lanes

Signalized Conditions Signal phasing Signal timing Type of control Signal progression

Level of Service (LOS) for Signalized Intersections

Defined in terms of delay as a measure of driver discomfort Driver frustration Fuel consumption Lost travel time

Delay experienced by a motorist includes many factors: Signal control Geometrics Incidents

Total delay: Difference between actual travel time and

ideal travel time• In the absence of traffic control, geometric delay,

incidents and when there are no vehicles on the road

In HCS only control delay is quantified• initial deceleration delay• Queue move-up time• Stopped delay• Final acceleration delay

Previous versions of HCM/HCS (1994 version or earlier) Only included stopped time delay Latest version includes control delay

LOS LOS criteria are stated in terms of

average control delay per vehicle Delay is dependent on

• Quality of progression• Cycle length• Green ratio• V/c ratio for lane group• Phasing design

Designated by letters A - F

LEVEL OF SERVICE

CONTROL DELAY PER VEHICLE

(SEC/VEHICLES)

A

B

C

D

E

F

10

>10 and 20

>20 and 35

>35 and 55

>55 and 80

> 80

LOS Criteria for Signalized Intersections

LOS A Describes operations with very low

control delay, up to 10 sec/veh Occurs when progression is extremely

favorable When most cars arrive during the green Most vehicles do not stop at all Drivers can select speed and path

LOS B Describes operations with control

delay > 10 and up to 20 sec/veh Occurs with good progression, short

cycle lengths or both More vehicles stop than with LOS A

• Causing higher levels of average delay

LOS C Describes operations with control

delay greater than 20 and up to 35 sec/veh Fair progression, longer cycle lengths,

or both Individual cycle failures may begin to

appear at this level No. of vehicles stopping is significant

• Many still pass without stopping

LOS D Describes operations with control delay >

35 and up to 55 sec/veh Influence of congestion becomes more

noticeable Longer delays result

• Unfavorable progression• Long cycle lengths• High v/c ratios

Many vehicles stop• Proportion of vehicles not stopping declines

Individual cycle failures are noticeable

LOS E Describes operations with delay >

55 and up to 80 sec/veh The limit of acceptable delay Indicate poor progression, long cycle

lengths and high v/c ratios Individual cycle failures are frequent

occurrences

LOS F Describes operations with delay > 80

sec/veh Considered unacceptable to most drivers Occurs with oversaturation

• When arrival flow rates exceed the capacity of the intersection

Occurs at high v/c rations below 1.0 with many individual cycle failures

Poor progression and long cycle lengths may also contribute

Operational Analysis

Procedure

2.2. VOLUME ADJUSTMENTVOLUME ADJUSTMENT• Peak hour factor• Establish lane groups• Assign volumes to lane groups

1.1. INPUT INPUT • Geometric conditions• Traffic conditions• Signalization conditions

4.4. CAPACITY ANALYSIS MODULECAPACITY ANALYSIS MODULE• Compute lane group capacitiesCompute lane group capacities• Compute lane group v/c ratiosCompute lane group v/c ratios• Aggregate resultsAggregate results

5.5. LEVEL OF SERVICE MODULELEVEL OF SERVICE MODULE• Compute lane group delays• Aggregate delays• Determine levels of service

3. SATURATION FLOW RATE3. SATURATION FLOW RATE• Ideal saturation flow rate• Adjustments