I Didn’t Know That...

Q: How does a traffic light know that a car is on a side street waiting for it to change?

A: A variety of techniques have been used, but by far the most common is a simple induction loop buried in a groove in the pavement. An induction loop is a coil of wire containing four or five loops. Electrical current passing through it creates a small magnetic field. When a car enters the field, it alters the amount of current passing through the circuit. The alteration can be easily detected and used to signal the light to change. The filled-in grooves are often quite visible at intersections.

The same technology is also used in cameras that identify drivers running red lights. The induction loop, usually buried in the crosswalk, can be cycled on and off rapidly to tell if a car is moving or just sitting in the crosswalk. More information can be found at: https://www.photocop.com/redlight.htm.