Setting Change Intervals at Signalized Intersections

T he change interval (yellow plus all red) is essentially put into the traffic light cycle to allow drivers to make “stop” and “go” decisions and to allow time for clearing the intersection. A long change interval is obviously much safer than a short one, because it allows more room for mistakes.” However, an unduly long interval would introduce inefficiency simply because the change interval is really an unproductive period in the traffic cycle. An unduly long interval would not only cut down the through volume of a roadway, but it would invite drivers’ disrespect for the light, resulting in higher risks for accidents. For this reason, the Traffic Control Devices Handbook suggests a maximum of about 5 seconds under normal circumstances.3 Technically speaking, the length of the change interval should allow enough time for the driver who cannot stop in time to 1. Drive the minimum distance needed to stop, including the “reaction” time, or 2. Pass completely through the intersection. It goes without saying that the assumption is drivers who see the yellow light soon enough will use the braking distance to stop their vehicles. If the prevailing speed on the approach roadway is V, the width of the cross street is w, and the typical vehicle length is I (commonly assumed to be 20 feet), then the time needed to clear the intersection is (w + /)/V. This includes time for the rear end of the vehicle to clear the intersection. Assuming for the time being that the driver brakes his vehicle at a constant rate, the braking force on the vehicle on a level road is f7, where T is the weight of the vehicle and f is the coefficient of friction (also assumed to be constant at this time). For a level road, the minimal distance traveled in stopping can be found by Newtonian mechanics to be