How Do Traffic Lights Detect Cars?

The most common detector is the inductive loop, a coil of wire cut into the road surface near the stop line. An electric current makes the loop act like a metal detector; when a large metal object such as a car stops over it, the loop's inductance changes and the controller registers a waiting vehicle. The controller then adjusts the light sequence, extending green time for busy approaches. Newer junctions use video cameras, microwave radar or infrared sensors mounted on poles. Many cities also coordinate signals along main roads to create 'green waves'.