How do traffic lights work?

A friend of mine wanted me to prove to him how traffic signals (lights) knew when a car was waiting to cross an intersection or make a controlled turn. I had consistently told him that most systems (especially on rural or country roads) use an "inductor" loop-style circuit that senses the magnetic field of the stopped car to tell the system a vehicle is waiting.

The car's mass, most of it some type of metal (frequently steel), acts as the inductor material in the loop of wires placed in the ground. You can frequently see the loop of wires at the stopping points of an intersection with a traffic control light (the wires are typically covered by a black rubber compound). The car's mass has a magnetic field and that field is sensed by the circuit in the ground which is read by an induction meter. When the meter value changes the system knows there is some considerable mass (car) waiting for the light to give it the indication to proceed.

This is precisely why it can be difficult to get a traffic signal to change for a motorcycle. It is also why it is nearly impossible for it to change for a bicycle or human being. The system was set up to measure the field of an object with considerable mass (in all cases, a motor vehicle) and the above mentioned objects have little mass with even smaller magnetic fields because of it.

There are many other types of traffic control systems but the most common is the induction-style circuit. Timed systems that change on a schedule with no consideration of actual traffic flow are common in urban areas with many intersections and consistent traffic patterns. There are also dual-systems that utilize both technologies in many cities. It just depends on the right system for the specific application.

For more information on traffic signal (light) systems you can read the How Stuff Works article regarding this subject. It also explains induction circuits in more detail.

- Mitch G