How Loop Detectors Work

Types of Loop Detectors.

Traditional loop detectors (shown right) are roughly rectangular in shape. They easily detect larger vehicles. However, motorcyclists, bicyclists, and even motorists in smaller cars may find that some of these traditional loop detectors do not easily detect their presence.

Empty Loop Detector
Figure Eight Loop Detector

To improve detection, South Carolina uses a figure "8" style loop detector known as a quadrupole loop (shown left). This pattern improves the sensitivity in the middle of the loop. This middle area is where the problem occurs for smaller vehicles at intersections employing traditional loop detectors.

South Carolina is installing these quadrupole loops statewide as traffic signal upgrades are made. However, it will take several years for quadrupole loops to be installed at all actuated traffic signals. Meanwhile, the SCDOT maintains all existing loop detectors to ensure that they are properly tuned to detect small and large vehicles alike.