Inside South Carolina Department of Transportation

How Traffic Signals Work

How are signals controlled?

The SCDOT uses two methods to control traffic signals. The first employs a timing mechanism to change the signal at specific intervals. Pre-timed traffic signals work well where traffic patterns are predictable and speeds are slow. However, less than 15 percent of South Carolina's nearly 4,000 traffic signals are pre-timed.

This means over 3,300 of the state's traffic signals rely on a second method to manage traffic.

At intersections where traffic patterns vary and speeds are higher, a traffic signal that detects a vehicle's presence is needed. It must not only recognize automobiles, trucks, and buses but must also respond to motorcycles, bicycles, and pedestrians. Such traffic signals are known as actuated traffic signals.

Actuated traffic signals maintain a green signal on the busiest street until a pedestrian or a vehicle on the less traveled side street approaches the intersection. The traffic signal detects and responds to this new demand.

To make traffic signals responsive to pedestrians, traffic engineers install push buttons located on poles at intersection corners. The pedestrian pushes the button to change the signal.

To detect the presence of motorists and bicyclists on the side street, traffic engineers use a metal sensing device known as a loop detector.

Vehicle users can often spot the telltale signs of loop detectors by the grooves cut into the roadway behind the stop bars at intersections. Wire loops that sense metal objects above them are placed in these grooves. Once a vehicle is detected the traffic signal processes the request, allowing traffic from the side street to enter the intersection at the appropriate time.