How do traffic lights know a car is waiting?

Traffic lights change in one of two ways. Some run on a fixed timer that cycles through green, yellow and red regardless of who is waiting. Others are actuated: sensors report that a vehicle, bicycle or pedestrian is present, and a controller uses that information to decide when to switch. The sensors are most often wire loops in the pavement, video cameras, or radar units. [1] [2]

Pre-timed signals change at set intervals and work well where traffic patterns are predictable and speeds are slow. South Carolina's transportation department says fewer than 15 percent of its nearly 4,000 signals are pre-timed. Minneapolis, by contrast, reports that most of its roughly 815 signals are pre-timed, with a typical cycle of about 100 seconds that is lengthened or shortened for rush hours and major events. [1] [3]

Actuated signals suit intersections where traffic varies and speeds are higher. They typically keep the main street green until a vehicle or pedestrian shows up on a side street, then process that request. Pedestrians usually make the request with a push button on a corner pole. [1]

The inductive loop is a wire laid in a groove cut into the pavement near the stop bar. A vehicle over it changes the loop's magnetic field, and a computer registers the change. Pennsylvania's transportation department calls it very reliable and cheaper to maintain than other detectors, though utility work nearby can disrupt it. It responds to metal, not to weight, as a Minneapolis engineer pointed out when dispelling a common belief. [2] [1] [3]

Video detection uses a camera mounted near the signal. It learns what the empty road looks like and registers a vehicle when the pixels in a defined zone change. Pennsylvania notes that it picks up motorcycles more easily than loops but is less effective in bad weather. Bismarck, North Dakota says its cameras only detect presence in zones, do not record or store images, and cannot read plates or faces. [2] [4] [3]

Radar units send microwaves that bounce off waiting vehicles. They detect small vehicles and work in poor weather, but Pennsylvania says they are usually the most expensive to install and maintain. One engineering firm reports that loops remain the most common method, while new installations typically use video or radar, which need no pavement cutting. [2] [5]

Detectors send a call to the controller, the computer in a roadside cabinet that sets the timing. Florida's transportation department says traffic-responsive controllers can adjust green time to the amount of traffic, letting as many vehicles through as possible before serving another approach. Even with detectors, a timing mechanism still limits how long each lane gets green. Other inputs can also matter, including pedestrians, light rail, bus priority systems, and coordination with neighboring intersections. Pennsylvania decides detector type and green times case by case with local municipalities. [6] [2] [3] [4]