Jeff Stephenson, P.E., is the Team Leader of [...]
As civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.
While traffic signals are probably part of your everyday life, they may not always get much thought beyond annoyance when the light turns red. But there is actually a lot of interesting information about them, and believe it or not, we get quite a few questions regarding traffic signals.
We answer the most frequently asked traffic signal questions in this two part edition of our “Did You Know” series.
How do traffic signals detect vehicles?
We most frequently work with three types of vehicle detection. One is commonly referred to as a “loop”, where electrical wire is placed in the pavement, typically in a rectangular or square shape. The loop detects a vehicle via electromagnetic disturbance when the vehicle is over the loop. A second type involves a video camera attached to the signal pole or arm that detects a vehicle in a defined “zone”. The third type of vehicle detection we work with includes a microwave device attached to the signal pole or arm to detect a vehicle in a manner similar to sonar. There are a few other less common types of vehicle detection, and each option has its pros and cons.
How do you program a traffic signal?
The operation of the traffic signal is determined by several devices working together which reside in the large silver cabinet you may have noticed on the corner of an intersection. All of these devices are either providing input or receiving output from a device inside of the cabinet known as the controller. The controller is essentially a computer which tells all of the traffic signal heads whether they should be red, yellow, or green, and for how long. All of the timings required for red, yellow, green, and a number of other settings are programmed into the controller. Controllers can be programmed manually or using a computer program, and they can be programmed in the “shop” before they are placed in the field or after they are in operation.
What are those cameras on traffic signal poles, and what do they do?
Most of the time, when you see a camera on a traffic signal pole, it is there for the purpose of detecting vehicles at the intersection, as mentioned in the previous question. The cameras are connected to video processors, which evaluate the image and tell the traffic signal controller when vehicles are present. Cameras can also be used to record traffic counts. In some instances, cameras are connected to a system where an operator can view a live feed from the camera for the purposes of monitoring traffic.
How do you determine signal timing?
Green, yellow and red timing are all determined by different factors. The volume of traffic for each movement (left turn, straight or right turn) at the intersection is used to develop the green time for each interval of the traffic signal. The yellow time and red time are both critical to intersection safety. Yellow time should be set to provide drivers adequate time to stop before entering the intersection or proceed through the intersection before the traffic signal changes to red. Yellow time is determined based on the vehicle speed for each intersection approach, and it can be adjusted for uphill and downhill road conditions. Red time is another “clearance interval” where every approach to the intersection briefly sees a red display, or “all-red,” in order to clear the intersection before green is given to a conflicting movement.
Why are some signals on timers, loops or video?
Each interval of a traffic signal is always on a timer. If the traffic signal can detect vehicles, the traffic signal can be programmed to cut the green time short, or “gap out” when there are no more vehicles to serve from that direction. Traffic signals equipped to detect vehicles are commonly programmed to not give a green display if there is no vehicle there waiting. This is called a traffic-actuated traffic signal. If the traffic signal cannot detect vehicles, each interval will always run its assigned amount of green time. This is called a pre-timed or “fixed” traffic signal. Pre-timed traffic signals are common in downtown grids and one-way pairs.