Traffic lights, also known as traffic signals, control the flow of vehicles and pedestrians at intersections by switching through red, yellow and green phases.

At the simplest level, a controller follows a timing plan that decides how long each movement gets a green signal. Those phases repeat in sequence to form a full signal cycle.

Modern traffic lights often use sensors to detect vehicles or pedestrian demand. These can include inductive loops in the pavement, radar, cameras or other detectors connected to the controller.

The controller receives that data and adjusts the phase timing to improve traffic flow, reduce unnecessary waiting and maintain safety at the intersection.

Traffic engineers can further tune the cycle by time of day. For example, green time may be increased during peak traffic periods, while quieter roads may rely more heavily on vehicle detection.

In larger traffic systems, intersections can also be coordinated so that multiple traffic lights work together along a route, improving progression and reducing congestion.