How does the traffic light detect a car when a car is at ared light how does it trigger the light to change.?

Coils of wire are buried under the road's surface to detect the metal in the car. Next time you are riding in a car look and you can usually see the lines in the asphalt where cuts were made to bury the wires. The signal from the detector is sent to the computer that runs the lights so it knows there are cars waiting. Good question!