Do some red traffic signals change only when a car is at the stop line?

Yes, some red traffic signals are equipped with vehicle detection systems that only change when a car is at the stop line. These systems use sensors, such as inductive loops or cameras, to detect the presence of a vehicle. This design helps reduce unnecessary waiting times for vehicles at intersections when no traffic is present, improving overall traffic flow. However, not all traffic signals operate this way; some are timed or controlled by other factors regardless of vehicle presence.