What’s the purpose of the small cameras mounted on traffic signals? Curious Texas investigates

Affixed to many traffic light arms in the Dallas-Fort Worth area are what look like cameras aimed at oncoming vehicles.

Some people confuse them with red-light cameras, but the use of red-light cameras has been banned in Texas since 2019.

That’s why a reader asked Curious Texas, “What’s the purpose of the small cameras mounted on traffic signals?”

There are at least seven kinds of traffic sensors or detection systems throughout Texas, according to the Texas Department of Transportation. The systems manage traffic flow through intersections and long corridors.

At some intersections, cameras are mounted on top of a traffic light arm to allow for a wide view of approaching cars. These cameras, which are usually white and sometimes attached to a connecting pole, are part of a video imaging system that uses low-resolution cameras to detect the presence of cars.

A “zone” is drawn within the camera’s view, and the system monitors the number of waiting cars stopped at a light. When there is a specified change in the number of vehicles, the camera notifies the signal to change its lights.

Like a traffic camera, these devices are mounted to a traffic signal and are used to detect vehicles at an intersection to trigger the lights. The panels use radar technology.

Most traffic lights in Dallas are retrofitted with the detectors when funding becomes available, Jones said.

Some traffic signals have small, black devices that look similar to a camera. These are infrared sensors that are part of a “traffic preemption system” that allows emergency vehicles such as ambulances and fire trucks to safely pass through an intersection.

The sensor receives a signal from inside the emergency vehicle to cycle the traffic lights to green, allowing automatic right-of-way. Once the vehicle passes the intersection, the signal returns to regular operation. The city doesn’t use the devices, Jones said, but they can be found elsewhere in North Texas.

The Dallas transportation and police departments also have closed-circuit TV cameras that monitor traffic flow at intersections. At the Transportation Department, traffic signal staff monitors the city’s CCTV cameras as needed, Jones said.

Most of the devices attached to traffic signals are mounted on top, but underneath some stop light arms are “downstream lighting” indicators that help police catch red-light offenders.

The light, which can be white or blue, is wired to light up when a traffic signal is red. Once illuminated, an officer parked further down the stream of traffic can see when a driver has entered the intersection during a red light.

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