How Do Traffic Lights Work?

There are a number of ways in which a traffic light works depending on the system put in place. Basically there are three main types of traffic lights: the old fashioned timer based traffic lights, then the coil wire based sensory traffic lights which work on the principle of electromagnetism and the new age light emitting diode traffic lights (LED).

Many other new technologies too are in the offing which make use of lasers and cameras but most of them are still in the experimental stage. The timer based traffic lights are ubiquitous in the cities mainly at busy intersections as there is hardly any need of any sensory devices or detectors due to the continuous flow of traffic.

But in the suburban areas or on roads where the traffic flow is intermittent the coil wire based traffic lights are mostly used in which a wire embedded in the road detects the presence of a vehicle above it and conveys the signal to the traffic lights; often for accurate detection a number of wires are laid down at different lengths to maintain a proper flow of vehicles according to the volume of traffic. LED traffic lights use a lot less voltage and also have a longer life as they work on a different principle from the conventional filament bulbs.

Emergency vehicles can change traffic lights from red to green by way of a transmitter in the vehicle which emits a preset high or low priority signal which is picked up by a receiver mounted on top of the traffic lights.

Most traffic lights have some sort of controller connected to some or all of
  - a time-of-day clock,
  - emergency vehicle sensors,
  - traffic sensors,
  - walk signal buttons,
  - pedestrian signals, and
  - one or more traffic signals for each lane in each direction, including left and right turn lanes.
The controller is designed with safety in mind. For example, it never allows traffic to cross an active pedestrian lane, or to cross an active traffic lane in another direction. Most will have a period in the cycle when all signals are red. Most will go to blinking red in all directions if they lose their mind.

The controller may have one or more microprocessors in it, or may be hard-wired or pre-programmed logic. It will probably have special-purpose interface circuitry connected to the items listed above.

The general sort of algorithm used seems to be: If there is traffic waiting and there is no emergency vehicle requesting priority, give the waiting traffic a green if the other direction has no traffic or has been green for some period of time. The detection of traffic waiting seems to take place before the light turns yellow for cross traffic. (If you miss that yellow, you will wait on a left turn signal through an entire cycle, most places.) Some controllers make provisions for synchronizing lights across more or less wide areas of a city, or for changing programs depending on day of the week or time of day. Some have sensors at a distance from the intersection, so they can change to green before traffic arrives. (It doesn't pay to speed.)

A usual controller does 2-way control: N/S traffic goes, then E/W traffic. Some do 3-way control: N/S goes, then E, then W. Some do 4- or 5- way control, depending on the intersection and the sorts of turns most traffic makes.

Some are on timers, but most if not nearly all, work off of magnetic sensors placed under the road at intersections that sense traffic and stopped cars which in turn triggers the light.