Why a traffic light in the US may not respond to a car — The Hill

In the United States, some traffic lights may not change if the system has not detected a vehicle in the sensor zone. Some intersections operate on a set timer, while others use underground inductive loops, magnetic sensors, cameras or infrared sensors, The Hill reports.

How traffic lights detect vehicles

An inductive loop is a wire coil embedded beneath the road surface, usually in the shape of a rectangle. When a large metal object, including a car, stops above it, the magnetic field changes, sending a signal to the traffic light control system.

An area with such a sensor can often be noticed before the stop line: it may differ in asphalt color or be outlined by tar lines. For the system to register a vehicle, a driver should pull up to the solid white stop line and stop within the detection zone. Motorcycle, moped and bicycle owners in the city of Neenah, Wisconsin, are advised to position their wheels on the lines marking the loop's outline.

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Cameras, infrared and magnetic sensors

Some intersections use magnetic detectors and magnetometers, which also respond to changes in the magnetic field. Infrared sensors may activate when a vehicle crosses a light beam or detect heat from an engine. Cameras can be part of a network that analyzes traffic and adjusts signal operations.

In 2021, Los Angeles introduced a system of magnetic sensors, cameras and a central computer network to control all 4,400 traffic lights in the city in order to combat congestion. Some infrared laser sensors are placed in metal boxes above the road.

If a traffic light does not respond, flashing high beams may help in some cases, while honking does not affect such systems. The publication stresses that drivers should not run a red light: if necessary, they should safely change their route, including by turning right where permitted.