Traffic Signal Timings
August 11, 2017
Traffic Signals. No one really likes them but we all rely on them daily as we drive around town. But who controls when they turn red? Or green? Why is the yellow longer at some intersections and shorter at others? And for the love, why am I getting stopped at every single red light on my way to work?
Those questions all come back to what we traffic engineers call traffic signal timings. Traffic signal timings are an intricate, complex balancing act between many factors. The volume on the main street versus the volume on the intersecting street. The number of left turns. The geometry of the intersection. The distance between signals. The time of day. The split in traffic on the mainline. The number of pedestrians trying to cross.
Many traffic signals operate in isolation and don’t “talk” to those around them. Others are “coordinated” and interact with one another as traffic patterns change throughout the day.
The goal of traffic signal timings is to move traffic as efficiently as possible through a corridor with as few stops as possible. The idea is to move a group of cars (called a platoon) together through the corridor.
To establish or “re-time” the signals in a corridor, we start by collecting traffic counts at each of the intersections. We also drive the corridor many times to see how long it takes to get from one end to the other at different points in the day. This is called the travel time.
This is what we DON'T want.
From there, we calculate the amount of time it takes to clear the intersection (red time) and the time it takes to see and react to a change in the signal (yellow time). These are calculated carefully to minimize the chance of crashes within the intersection.
We load all this information into several computer models and run simulations to determine the best timings. Typically, since the traffic patterns change throughout the day, separate plans are developed for the morning rush hour, lunchtime, and the evening rush hour. A fourth plan, called the off-peak, runs during all other times of the day.
One key aspect to signal timings is the offset between intersections. The offset is a set time that controls when a series of signals turns green. In other words, if you driving a corridor and are stopped at a light, the light you are at will turn green first and the next light will turn green a few seconds later. This offset time allows the platoon to travel to and through the next light without stopping. If a corridor is timed well, the offsets will turn green in succession and the platoon will not have to stop (or stop minimally).
Once we have established the new timing plans, we load them into the traffic signals. We then spend 2-3 days observing the traffic and making any necessary tweaks to the timings.
Finally, we drive the corridor again (many times) and compare the time to the earlier travel time. If everything goes well, it should take less time to travel through the corridor with fewer stops at red lights.
Our work here is done!