The pavement becomes part of a sensor
Under many intersections, a wire is laid in a loop—often visible as a rectangle of narrow cuts in the road, later sealed over. Electronics send alternating current through it, producing a magnetic field that changes direction rapidly. The loop and its circuit behave like a tuned system, with a characteristic electrical frequency.
When a vehicle stops over the loop, its steel body interacts with that field. The interaction creates tiny circulating currents in the metal, which in turn alter the loop’s electrical properties. A detector monitors the change, usually as a shift in the circuit’s frequency, and reports that something is occupying the sensor. It is not reading a license plate or recognizing a particular kind of car; it is detecting a change in the field.
A request, not an instant green
The loop’s location helps the signal controller decide what the detection means. A loop near the stop line can register a vehicle waiting for a turn, while sensors farther back may help track traffic moving through an approach. The controller uses those inputs alongside the intersection’s programmed timing and other traffic demands to decide when to serve a phase.
That is why pulling onto the marked sensor area matters: a vehicle stopped beyond the loop may not be detected. And because the system senses metal’s effect rather than wheels or motion alone, bicycles and motorcycles can sometimes be harder for a particular setup to detect. The traffic light is not responding to a car’s arrival as a command; the buried loop is supplying one piece of information to the controller.
