Red, Yellow, Green: How Traffic Signals Decide

How Does a Traffic Light Decide?

1At a busy intersection, the place where two roads cross, a traffic light changes from green to yellow to red again and again. It may seem as if the light simply blinks on its own, but most traffic signals are run by a small computer called a controller. The controller usually sits inside a metal cabinet near the corner, and it decides when each light should change.

2Some controllers work on a timer. They give each direction a set amount of green time, over and over, like a clock. Many timers follow different plans at different times of the day. In the morning and evening, when many people drive to and from work, a busy road may get a longer green light.

3Other controllers use sensors to find out where cars are waiting. A common sensor is a loop of wire buried just under the road, near the line where cars stop. Electricity flows through the wire. When a car stops on top of the loop, its metal body changes the way the electricity behaves, and the controller notices. You can often spot these loops by looking for thin lines cut into the road in the shape of a rectangle or a circle. Some signals use cameras on poles instead.

4Sensors help keep traffic moving. Without them, a driver on a quiet side street might wait at a red light even when no other cars are coming. With a sensor, the controller can leave the main road green until a car pulls up on the side street.

5Traffic signals also help people who walk. At many intersections, walkers press a button to ask for a turn to cross. The controller then adds a walk signal the next time the lights change. Many signals also have a short moment when every direction is red at once. This gives the last cars time to get through the intersection before anyone else starts to go.