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

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

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)

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

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.”

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

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 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)

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

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

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

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

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

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

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”

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

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.

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

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”

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 =

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 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)

120’ 122’

Yellow Intervals v Y = t + 2a ± Gg Peak Northbound Y = 1+ 46.9/(2*10) = 3.345 sec Southbound Y = 1+ 42.5/(2*10) = 3.125 sec Eastbound Y = 1+ 51.3/(2*10) = 3.565 sec Westbound Y = 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

Yellow Intervals Off Peak Northbound Southbound Eastbound Westbound Y = 1+ 55.7/(2*10) = 3.785 sec Southbound Y = 1+ 52.8/(2*10) = 3.640 sec Eastbound Y = 1+ 61.6/(2*10) = 4.08 sec Westbound Y = 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 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

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

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

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 x L W Clearing line S

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

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

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)

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

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

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 (W+L) vo min = (xc+ W+L)/vo OR min = 2 + ½ vo/2a2*+ (W+L) vo

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

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  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: 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 (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 satisfied Drivers 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 satisfied Drivers 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 Modules Freeways Weaving Ramps Multi-lane Highways Two-lane Highways Signalized Intersections Unsignalized Intersections Arterials Transit 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

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

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

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 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