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How do traffic lights work?

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Quick answer

Traffic lights are run by a controller at each junction that cycles red, amber and green so that crossing streams never get green together. Fixed-time controllers follow a set schedule, while smarter ones use buried wire loops, cameras or push buttons to give more green time to whichever approach has traffic waiting.

Traffic lights work by switching red, amber and green lamps in a timed sequence set by a controller box at the junction, so conflicting streams of vehicles take turns. Many controllers also use road sensors, cameras or push buttons to lengthen or skip phases based on the traffic actually waiting.

Step by step

  1. Engineers count traffic at the junction and programme the controller with phases, green durations and safety intervals.
  2. The controller turns one group of signals green while every conflicting approach is held on red.
  3. Detectors such as road loops or cameras report waiting vehicles, and the controller extends or shortens the green.
  4. When the phase ends, amber shows, followed by a brief all-red pause to clear the junction.
  5. The next phase gets green, and the cycle repeats, often synchronised with nearby junctions by a central computer.

Key things to understand

  1. Each junction has a controller cabinet that holds the programme deciding which approaches get green and for how long.
  2. The programme splits a full cycle into phases, and no two conflicting directions are ever given green together.
  3. Amber warns drivers to stop, and a short all-red interval clears the junction before the next green begins.
  4. Wire loops buried in the road, cameras or radar detect waiting vehicles so the controller can adapt timings.
  5. Neighbouring signals along a main road can be coordinated so a platoon of cars meets successive greens.

Labelled diagram

  1. Detectors

    Road loops, cameras and push buttons report vehicles or pedestrians waiting at the junction.

  2. Controller

    A roadside computer runs the phase plan and applies safety rules on every change.

  3. Signal heads

    Red, amber and green lamps show each approach whether to stop or go.

  4. All-red interval

    A short pause with every signal red lets the junction clear safely.

  5. Central link

    A traffic centre can coordinate many junctions to create green waves.

How a junction decides who goes next

Real-world examples

  • Many Indian cities fit countdown timers beside signals so drivers can see how many seconds remain on red or green.
  • A quiet side road gets green only when a car rolls over the buried loop, keeping the main road flowing.
  • On long city avenues, coordinated signals create a green wave for traffic moving at the posted speed.
  • Buses in some cities send a radio request that lets the signal hold green a few seconds longer.

Common mistake

✗ Myth: Flashing your headlights or honking makes a traffic light change faster.

✓ Fact: Ordinary signals respond only to their detectors and timing plan. Lights and horns do nothing; only authorised emergency or transit vehicles with special transmitters can request priority.

People also ask

How do traffic lights detect cars?
Most detect vehicles with inductive loops, which are coils of wire set into slots in the road surface. When a car stops over the loop, its metal body alters the loop's inductance, and the controller registers a waiting vehicle. Overhead video cameras, radar and microwave sensors are increasingly used instead of loops.
Are traffic lights on a timer?
Some are. Fixed-time signals run the same cycle all day or switch between preset plans for rush hour and night. Actuated and adaptive signals change timing based on detected traffic, so a green may end early if no cars are waiting or stay longer when a queue is still moving.
Why are traffic lights red amber and green?
Red was taken from railway signalling, where it already meant danger and stop, and it is easy to see from a distance. Green became the go colour, and amber or yellow sits between them as a warning. The fixed order also helps colour-blind drivers, who can rely on the lamp's position.
How does a traffic light know a car is waiting?
A common method is an inductive loop, a coil of wire cut into the road surface. A car's metal body changes the loop's magnetic field, and the controller reads that change as a waiting vehicle. Cameras and radar units do the same job above the road.
Why is there a moment when all lights are red?
The all-red interval gives vehicles that entered on amber time to clear the junction before crossing traffic gets green. It lasts only a second or two but prevents collisions between late runners and early starters.
What does a pedestrian button actually do?
At most crossings the button registers a demand, so the controller adds a pedestrian phase in the next suitable slot of the cycle. Pressing it repeatedly does not speed things up, and at some busy junctions the walk phase runs automatically anyway.
Why do traffic lights use LEDs now?
LED signal heads use far less electricity than old incandescent bulbs and last many years longer, cutting maintenance visits. Many tiny LEDs make up each lamp, so the signal stays visible even if a few fail.

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