How do Red-light Cameras work?

Red-light systems rely on some sophisticated technology, but conceptually they are very simple. The system includes only three essential elements:

One or more cameras.

One or more triggers.

A computer.

In a typical system, cameras are positioned at the corners of an intersection, on poles a few yards high. The cameras point inward, so they can photograph cars driving through the intersection. Generally, a red-light system has cameras at all four corners of an intersection, to photograph cars going in different directions and get pictures from different angles. Some systems use film cameras, but most newer systems use digital cameras.

There are a number of trigger technologies, but they all serve the same purpose: They detect when a car has moved past a particular point in the road. Red-light systems typically have two induction-loop triggers positioned under the road near the stop line.

The computer is the brains behind the operation. It is wired to the cameras, the triggers and the traffic-light circuit itself. The computer constantly monitors the traffic signal and the triggers. If a car sets off a trigger when the light is red, the computer takes two pictures to document the violation. The first picture shows the car just on the edge of the intersection and the second picture shows the car in the middle of the intersection.

The main trigger technology used in red-light systems is the induction loop. An induction-loop trigger is a length of electrical wire buried just under the asphalt. Usually, the wire is laid out in a couple of rectangular loops resting on top of each other (see diagram below).

This wire is hooked up to an electrical power source and a meter. If you've read How Electromagnets Work, you know that when you send electrical current through a wire, it generates a magnetic field. Positioning the wire in concentric loops, as in any electromagnet, amplifies this field.

The meter in the system constantly monitors the total inductance level of the circuit. When the inductance changes significantly, the computer recognizes this shift and knows that a car has passed over the loop.

This is the most common trigger mechanism, but it's not the only one in use. Some areas have had success with radar, laser or air-tube sensors.

One emerging trigger mechanism is the video loop. In this system, a computer analyzes a video feed from the intersection. As the computer receives each new video frame, it checks for substantial changes at specific points in the image. The computer is programmed to recognize the particular changes that indicate a car moving through the intersection. If the light is red and the computer recognizes this sort of change, it activates the still cameras. The main advantage of this system is you don't have to dig up the road to install it, and you can adjust the trigger areas at any time. Essentially, it is a virtual inductive-loop trigger.