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:
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.