Intergreen Time

The following excerpt was taken from section 12-306.2 of the 1995 Idaho Transportation Department Traffic Manual.

Vehicle Signal Change Interval

A vehicle signal change interval is that period of time in a traffic signal cycle between conflicting green intervals. It is the time required to terminate one green indication before initiating a conflicting green indication characterized by either a yellow interval or a yellow and all-red interval. At the present time, there is considerable discussion of proper timing for change interval with no recommended national practice adopted at this time.

The Idaho Motor Vehicle Code permits vehicles to enter the intersection on a yellow indication - termed as a permissive yellow rule. These vehicles have lawfully entered the intersection and accordingly are permitted to clear the intersection on the remaining yellow interval, an all-red interval, or subsequent green indication. It should also be noted that Idaho Code permits vehicles to enter the intersection on a green indication only after yielding the right-of-way to vehicles lawfully within the intersection.

However, drivers are not always that observant of vehicles entering the intersection, particularly at the far side of an intersection, which can lead to a conflict between the two vehicles.

The recommended formula for determining an appropriate change interval is:

Y + R = t + V/(2a ± 2 Gg) + (W + L)/V

Where:
Y = length of the yellow interval
R = length of the all-red interval
t = driver perception/reaction time, recommended at 1.0 seconds.
V = velocity of approaching vehicle in feet/second, recommended that the 85 percentile signal approach speed or the posted speed limit, converted to feet/second, be used.
a = vehicle deceleration rate, recommended as 10 feet per second2.
g = acceleration due to gravity at 32 feet per second2.
G = grade of the signal approach in percent divided by 100 or 2 percent is 0.02. A downhill grade results in a negative term, i.e., -2 Gg.
W = width of intersection measured in feet from the near side stop line to the far edge of the conflicting traffic lane along the vehicle path.
L = length of vehicle on clearance, recommended as 20 feet for passenger cars.

The above formula will determine the total change interval composed of a yellow interval and all-red interval. The recommended minimum yellow intervals for traffic signals on the state highway system in Idaho are as follows:

 Approach Speed Standard Yellow Interval All-Red Clearance Interval 25 mph 3.2 sec Optional 30 mph 3.2 sec Optional 35 mph 3.2 sec Optional 40 mph 4.0 sec Required 45 mph 4.0 sec Required 50 mph 4.0 sec Required 55 mph 4.0 sec Required > 55 mph 5.0 sec Required

The all-red clearance interval is determined by computing the change interval, "Y+R," noted above and subtracting the standard yellow interval. The yellow interval has been standardized to present the drivers the same yellow interval at comparable intersections. Additional clearance time is then provided by adding an all-red interval for a longer change interval.

It should be recognized that longer change intervals detract from the available intersection green time and are only needed if there are potential vehicle or vehicle-pedestrian conflicts between signal phases. Note that the term (W+L)/V provides additional clearance time for a vehicle to clear the intersection conflict zone. However, it is desirable to set a minimum yellow interval based on engineering judgment and then adjust the change interval using an all-red interval if needed.

An all-red interval may be desirable at an intersection to provide additional time for a vehicle to clear the intersection before there are conflicts with pedestrians or other vehicles. The need for an all-red interval must consider a number of factors as follows:

• Sight distance between vehicles or vehicle/pedestrian conflicts.
• Phasing of signal indications resulting in location of clearing vehicle versus conflicting vehicle or pedestrian movements.
• Width of intersection or length of turning path of vehicle.
• Start up delay of a conflicting pedestrian or vehicle movement plus the time to reach a point of conflict with the clearing vehicle.
• Speed of the approaching vehicle.
• Required intersection clearance for a protected left-turn movement relative to position in intersection versus conflicting pedestrians or vehicles.
• Field observation of intersection operations relative to vehicle conflicts with only a yellow interval and intersection accidents attributable to vehicle change interval.

The all-red intervals should not be less than 0.5 seconds and would normally be limited to 2.0 seconds. The determination of the all-red interval should be based on the factors noted above, calculated values, intersection observations, vehicle clearance practices at comparable intersections, and engineer judgment.