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
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:
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:
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.