Wavetronix
For motorists, the safest possible condition with current intersection control strategies exists when a red light is already displayed as they approach an intersection. But what happens if the light changes as they approach the intersection?
The stop on red problem occurs if a motorist is about 2.5 to 5.5 seconds away from entering an intersection when the light turns to yellow. This creates a “decision dilemma zone” in which the driver must decide to speed through the intersection, and risk running the red light or right-angle collision; or stop suddenly and risk the more common rear-end collision. As illustrated in the following table, red light running, acceleration through the intersection and abruptly stopping are all major components of the stop on red problem.
| Component of Stop on Red Problem | Statistical Significance |
|---|---|
| Driver enters intersection after red | 15% of signalized crashes and 33% of deaths |
| Driver accelerates through intersection on yellow/red | 28% of signalized crashes are right-angle collisions |
| Driver abruptly stops on yellow/red | 42% of signalized crashes are rear-end collisions |
As stated before, the decision dilemma zone describes the area upstream of the intersection where the stop on red problem originates. It should not be confused with the classical definition of the physical dilemma zone, which exists when insufficient yellow time is programmed into the traffic controller, creating actual locations on the road where motorists, driving at or below the speed limit, physically cannot stop at, nor proceed safely and legally through, the intersection.
Classical analysis of the physical dilemma zone is sufficient to select a reasonable yellow time, but it ignores several fundamentals of the stop on red problem: motorists' expectation of the duration of the yellow light; motorists' estimated arrival time at the intersection; the presence and separation of multiple vehicles on the roadway; motorists' differing reaction capabilities; and the varying speeds at which different motorists travel.
The driver decision dilemma zone takes into account all of these factors, and it is important to note that the decision dilemma zone exists no matter what yellow time is programmed into the traffic controller. The following figure illustrates the different decision dilemma zones that exist for motorists driving at different speeds. In this example, the decision dilemma zone starts at 5.5 seconds and ends at 2.5 seconds from the stop bar. Notice how the dilemma zone is wider and further back for higher speed vehicles.
Programming correct yellow times can help, but this approach is really only effective if all motorists drive at the same speed with the same capabilities. Since most motorists drive at a range of speeds and react to things in different ways, there is not one “correct” or “safe” duration for the yellow light.
Generally-accepted yellow clearance intervals range from three to six seconds, but most drivers are unaware of the actual duration of the yellow clearance interval when they estimate their arrival time to the intersection in deciding whether to stop or go. Often motorists are caught off-guard by the duration of yellow and end up running a red light, accelerating through the intersection, or stopping abruptly. When other vehicles are involved, these conflicts frequently result in collisions.
The following table shows yellow clearance intervals calculated for intersection approaches with different design speeds using the “ITE equation.” The customary assumption is a deceleration rate of 11.2 feet per second squared (a=11.2 ft/s2) and 1.0 second reaction time.
| Yellow Clearance Interval (sec) | ||||
|---|---|---|---|---|
| a=10.0 ft/s2 | a=11.2 ft/s2 | a=12.5 ft/s2 | ||
| Speed (mph) | 75 | 6.5 | 5.9 | 5.4 |
| 70 | 6.1 | 5.6 | 5.1 | |
| 65 | 5.8 | 5.3 | 4.8 | |
| 60 | 5.4 | 4.9 | 4.5 | |
| 55 | 5.0 | 4.6 | 4.2 | |
| 50 | 4.7 | 4.3 | 3.9 | |
| 45 | 4.3 | 3.9 | 3.6 | |
| 40 | 3.9 | 3.6 | 3.3 | |
| 35 | 3.6 | 3.3 | 3.1 | |
| 30 | 3.2 | 3.0 | 2.8 | |