Traffic Signal Timing

Updating traffic signal timing is a cost effective way to improve the flow of traffic on arterials and is one of the most basic strategies to help mitigate congestion. Of the estimated 330,000 traffic signals in the United States, and as many as 75 percent could be made to operate more efficiently by adjusting their timing plans, coordinating adjacent signals, or updating equipment. In fact, optimizing signal timing is considered a low-cost approach to reducing congestion. Optimizing traffic signal timing can produce benefits including:

• Shorter commute times
• Improved air quality
• Better fuel efficiency, and
• Decreased driver frustration

Signals that are in a coordinated mode are confined to a cycle length, which is governed by the cycle of a nearby major intersection. All signals along an arterial must have a common cycle length in order to achieve progression. Within that cycle length, a block of time is allocated to each movement. Each movement can appear only at a certain point in the cycle; once that has occurred, the movement cannot appear again until the next cycle. If a movement does not need all of its allocated time, the unused time becomes available to the next movement; this continues until all of the unused time, if any remains, ultimately is inherited by the main street movement where the cycle “zeros” itself out or begins again. Cycle lengths vary depending on the time of day. During the AM and PM rush hours, signals have their longest cycle lengths because the major roads must accommodate the greatest amount of traffic. In the midday the cycle lengths are slightly shorter and the shortest cycles typically occur at night and on weekends.