How does a traffic light signal work?

Answer 1

Traffic lights have the light sets, which emit colored light to "control" traffic. Additionally, there is support equipment that includes programmable logic controllers, or PLCs and sensors in the street. Let's take an intersection with two lanes of traffic in each direction on both roads. We'll include a left turn lane in all directions, too. Ready to look at how things work? Then let's do that by starting with the sensors.

The sensors are basically coils of wire (inductors) below the pavement. A vehicle that passes over one will cause a change in the inductance of the coil. The controlling circuits will sense this and know a vehicle has passed over it. All the lanes will generally have these coils buried in the street near the intersection. Let's move on to the lights.

The lights, the red, yellow and green ones, simply respond to programmed instructions. These include a combination of timing and sensor data. The timing is set by traffic engineers who use traffic data to "dial in" the controls. When enough time has passed, as set, the light will change to stop traffic along one street and allow traffic on the other street to proceed. If a vehicle passes over the sensor in the turn lane, that traffic is given a chance to cross at the beginning (or end) of flow in that direction on that street. If no traffic has crossed the sensor, that traffic lane will be ignored.

In cases where a "minor street" meets a "major street" and the hour is late, the timing circuit might be "paused" to allow a constant green on the major street unless a vehicle is sensed on the minor street. Different combinations of traffic flow can be accommodated by considered programming of the controllers. One instance of this is to set up a "cascade" of green lights for traffic moving along a long street with a lot of lights. If a driver maintains a "regular" speed and traffic isn't exceptionally heavy, the driver will see the signals turning green as he approaches them. Other variations are possible, as any driver experienced in an urban setting can explain.