1. NCHRP 172: Signal Timing Improvement Practices
Clearance Intervals
NCHRP 172: Signal Timing Improvement Practices

2. Clearance Interval Clearance Interval = Yellow + All Red
According to the ITE recommended practice:
Clearance Interval = Yellow + All Red
Yellow = Y = t + v
2a ± Gg
w + L v P v
P + L v
All Red = AR = or or

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

4. 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

5. Clearance Interval w P

6. 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

7. 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

8. National MUTCD 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.”

9. Fraley vs. the City of Flint, MI
Tort 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

10. 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

11. 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 t
Vo2/2a: 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)

12. If the vehicle has < the calculated stopping distance
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

13. 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 + v
2a ± Gg

14. No all-red interval is used
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
Figure 6. Possible scenario with no all-red clearance

15. 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

16. However, many drivers do not know this law
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

17. No all-red interval is used
Northbound car fails to yield ROW to car A legally in the intersection, enters soon after receiving the green and is struck
No all-red interval is used
Figure 6. Possible scenario with no all-red clearance

18. Signal Timing Improvement Practices NCHRP 172
All-Red Interval
Signal Timing Improvement Practices NCHRP 172
“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”

19. 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”

20. 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”

21. 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 + v
2a ± Gg
w + L + CP
Where CP is the non dilemma change period

22. 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.

23. Intersection where an all-red interval is used
Eastbound car clears intersection by the time the northbound car receives green
Intersection where an all-red interval is used

24. 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”

25. w + L All Red = r = v P + L P All Red = r = or v v P + L All Red = r =
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:
w + L v
All Red = r =
P + L v P v
All Red = r = or
P + L v
All Red = r =

26. 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

27. 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

28. 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

29. 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”.

30. 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

31. Example: Calculate Clearance Intervals for the Intersection of Middlebelt Road and 5 Mile Road

32. Approach Speed
Spot speed studies were taken at each of the intersection approaches as follows:
APPROACH
SPEED LIMIT
85th PERCENTILE SPEED
mph
Peak
Off-peak NB 40
32 (46.9 fps)
38 (55.7 fps) SB
29 (42.5 fps)
36 (52.8 fps) EB 45
35 (51.3 fps)
42 (61.6 fps) WB
46 (67.5 fps)

33. 120’
122’

34. Yellow Intervals v Y = t + 2a ± Gg Peak
Northbound
Y = /(2*10) = sec
Southbound
Y = /(2*10) = sec
Eastbound
Y = /(2*10) = sec
Westbound
Y = /(2*10) = 3.64 sec
Peak N-S Yellow interval use 3.5 sec
Peak E-W Yellow Interval use 4.0 sec

35. Yellow Intervals Off Peak Northbound Southbound Eastbound Westbound
Y = /(2*10) = sec
Southbound
Y = /(2*10) = sec
Eastbound
Y = /(2*10) = 4.08 sec
Westbound
Y = /(2*10) = sec
Off Peak N-S Yellow interval use 4.0 sec
Off-Peak E-W Yellow Interval use 4.5 sec

36. All-Red Intervals w + L All Red = r = v Peak Northbound Southbound
(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

37. All-Red Intervals w + L All Red = r = v Off Peak Northbound Southbound
(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

38. Clearance Intervals (CI)
Peak Period
North-South East-West
Y = 3.5 sec Y = 4.0 sec
AR = 3.3 sec AR = 2.7 sec
CI = 6.8 sec CI = 6.7 sec
Off-Peak Period
Y = 4.0 sec Y = 4.5 sec
AR = 2.7 sec AR = 2.3 sec
CI = 6.7 sec CI = 6.8 sec

39. 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

40. Driver’s Decision
natural driver behaviors: aggressive vs. non-aggressive
type of vehicle
vehicle condition
trip purpose

41. Gazis Research late 1950’s x L W
Clearing line S

42. 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

43. Gazis Study
Dilemma Zone xc
Cannot stop
Dilemma zone
Cannot go xo S

44. 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= 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)

45. Vehicle location when light turns yellow
Gazis Study x W L
Vehicle location when light turns yellow
Clearing line S

46. Gazis Study
If the driver is to come to a complete stop before entering the intersection
(x - vo2)  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

47. 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

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

49. 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

50. Gazis Study Minimum length of the clearance interval (W+L) vo
min = (xc+ W+L)/vo OR
min = 2 + ½ vo/2a2*+
(W+L) vo

51. Vehicle location when light turns yellow, traveling at 30 mph (44 fps)
Example: x
L = 20’
W = 80’
Vehicle location when light turns yellow, traveling at 30 mph (44 fps)
Clearing line S

52. For driver to stop: (X - vo2)  vo2/2a2
Assume 2 = 0.4 seconds and a2 = 10 ft/sec2
[X – (44 *0.4)]  442/ (2*10)
X – 17.6  96.8
X  ft
If the driver sees the yellow light 115 feet before the stop bar, the driver can stop in this distance

53. For driver to clear: For  = 3.0 seconds For  = 5.0 seconds
(x + w + L) - vo 1  vo ( - 1 ) + ½ a1 ( - 1)2
Assume 1 = 0.2
For  = 3.0 seconds
( )- 44(0.2)  44( 3 – 0.2 ) + ½ 10 (3-0.2)2
173.2 feet  feet – cannot clear
For  = 5.0 seconds
( )- 44(0.2)  44( 5– 0.2) + ½ 10 (5-0.2)2
173.2 feet  feet
criteria satisfied
Drivers will be able to stop or clear.

54. For  = 6.0 seconds
( )- 44(0.2)  44(6– 0.2) + ½ 10 (6-0.2)2
173.2 feet  feet – criteria satisfied
Drivers will be able to stop or clear.

55. Highway Capacity Software (HCS)

56. Highway Capacity Software
Based on the Highway Capacity Manual (HCM)
Special Report 209
Transportation Research Board (TRB), National Research Council (NRC)

57. Ten Modules Freeways Weaving Ramps Multi-lane Highways
Two-lane Highways
Signalized Intersections
Unsignalized Intersections
Arterials
Transit
Pedestrians

58. 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

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

60. Signalized Conditions
Roadway Conditions
Number and width of lanes
Grades
Lane use
Including parking lanes
Signalized Conditions
Signal phasing
Signal timing
Type of control
Signal progression

61. 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

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

63. 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

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

65. 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

66. LOS Criteria for Signalized Intersections

67. 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

68. 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

69. 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

70. 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

71. 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

72. 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

73. Operational Analysis Procedure
INPUT
Geometric conditions
Traffic conditions
Signalization conditions
Operational Analysis Procedure
VOLUME ADJUSTMENT
Peak hour factor
Establish lane groups
Assign volumes to lane groups
3. SATURATION FLOW RATE
Ideal saturation flow rate
Adjustments
CAPACITY ANALYSIS MODULE
Compute lane group capacities
Compute lane group v/c ratios
Aggregate results
LEVEL OF SERVICE MODULE
Compute lane group delays
Aggregate delays
Determine levels of service