How Things Work

Once merely timers attached to a pole, traffic signals today are way more sophisticated than that. They have the greatest impact on and virtually control a city’s traffic.

There are three main kinds of traffic signals.

To begin with, there are the pre-timed signals. Those are simply timers that change the lights from red to green at fixed intervals.
Although pre-timed signals change at a constant rate irrespective of the traffic, the time span between two intervals may vary in different areas depending on the estimated traffic flow of the area. The schedule of intervals may also change throughout the day according to traffic flow.

Next, there are semi-actuated traffic signals that ‘sense’ the traffic at the sides of the streets and change lights accordingly. They are able to do this with the help of sensors called electronic detector loops that are fitted beneath pavements. The electronic detector loops (similar in function to metal detectors) detect vehicles that stop at the traffic lights. This enables vehicles to pass without waiting long when roads are empty.

These type of signals are normally installed where a main road intersects a low-paved one. The efficiency of semi-actuated traffic lights falls apart when smaller vehicles, like motorcycles and bicycles, approach the traffic signal. A more “detectable” vehicle needs to roll by in order for the semi-actuated signal to sense the presence of a vehicle on the road.

Finally, there are fully-actuated traffic signals that sense the amount of traffic (the number of cars) and change the light to green automatically. These are also known as synchronized signals.

Unlike semi-actuated signals, fully-actuated signals have sensors on major and minor roads. The entire system works together to conciliate the traffic.

A variation of fully-actuated signals are coordinated signals. Those are controlled by a chief controller and are programmed so that the green lights glow for a lengthy stretch of time. Thus, a great amount of vehicles can pass, leading to a substantial reduction in speeding.

Generic semi- and fully-actuated traffic signals can sense the number of cars and their frequency by a coil fitted below the road. These wires are connected to controller equipment. There is a small amount of electric field present on the wire. When a car passes over the coil, the magnetic material present in the car changes the inductance on the wire.

The controller equipment recognizes the change in inductance as vehicles pass by and changes the frequency of green light. The “white” bar painted on the road is usually where the last coil in that particular lane is located.

In places where it is difficult to fit the coils, different methods of traffic detection are used. For example, the City of Oak Ridges uses microwaves to sense traffic. The microwaves are detected using a closed circuit monitor fixed on a pole. This device works similarly to the radar guns used by the police and it can barely detect extremely slow-moving vehicles.

Areas that are likely to experience emergency traffic on a regular basis, such as roads with hospitals, are installed with signals that are somewhat interruptible. In other words, emergency vehicles like police cars and ambulances can break the signal without causing any harm to other cars.

A preemption sequence makes this possible, as it blocks all other intersections by changing those signals to red.

Traffic signals were initially established in 1868, near the British Houses of Parliament in London, England. However, the modern version of the traffic signal was developed in America in 1912.