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.