What makes a Traffic Light Work?


Urbanization has brought rapid development in industries due to which the majority of the population is settling down in the cities. This results in a heavy rush in vehicular traffic requiring efficient control to avoid accidents. Traffic lights can be seen arranged one above the other at crucial crossings and junctions in a city. The colors used for traffic lights are green, yellow, and red, out of which red denotes stop, green is used to indicate ‘carry on,’ and the yellow color is used in between to be ready for either red or green.

Lights changes are either preset with a timer or based on current conditions and connected with one another in such a way that traffic can move on the main road controlling their speed. However, these days the interval between traffic lights is computerized and not set. They change, but vary in sync with vehicular traffic.

History of traffic lights

The question of traffic safety existed even before automobiles were used. This led to the invention of the world’s first traffic light in London in 1868. A revolving lantern comprising of red and green signals was set up at a junction in London. These lights were lit with the help of gas and operated by men. A lever at the base of the light was used to turn the colors of the light and allow the traffic to flow. Regrettably, in January 1869, this piece of equipment blew out and hurt the police officer who was controlling it at the time.

The contemporary red and green electric traffic light was invented in 1912 by a Utah police officer named Lester Wire. In 1914, the first traffic signal was installed in Cleveland, Ohio. This device had a buzzer apart from the normal red and green colors. The buzzer was used to provide a warning of the color changes. This device was designed by James Hoge and was used by both the police and the fire stations to manage the signals in an emergency. Police officer William Potts invented the first four-way, three-color traffic light in 1920.

The first coordinated traffic signal arrangement system was set up in 1917 and had six linked points of intersections operated concurrently by a manual switch. In 1922 in Houston, Texas control of the interconnected lights was done automatically. Inter-linked traffic lights with automatic control were introduced in Wolverhampton, England only in 1927.

Traffic lights

Traffic Signal Lights

Color Code for traffic signal lights

All of us know that red, green and yellow are used as traffic light signals. Why were these colors selected?

The rules which governed the right of way in maritime to recognize port was red and starboard was green, which signaled that the vessel on the left had to stop to allow the one on the right to cross.

Actually, the colors for traffic lights were adopted from the color code used by railway engineers as a traffic signal invented to control the trains on the rail lines. They used red to represent a signal to stop since it represents danger or warning and this color caught the attention of passersby as well. The other two colors used by railway engineers were green symbolizing ‘caution’ and white meaning ‘go.’ Filters were used in traffic signals and because of this the white color had some problems associated with it. Streetlights, stars, and the glare of sunlight or other lights could be misunderstood as a ‘go’ signal since they also shone as white from a distance. In order to solve this problem engineers used yellow to denote caution.

Working Mechanism of Traffic Lights

In olden times, the change in color of traffic lights was preset with fixed timings. This means that when a vehicle draws up exactly as the signal alters from green to red, it has to wait for the next change to green, even if there are no other vehicles in any other directions.

Yet the contemporary style consists of signals that are not fixed, but act in response to the presence of vehicles. This system uses a sensor loop embedded in the pavement that detects weak magnetic fields such as the metallic parts of cars. A controller box installed nearby contains a computer that senses the vehicle’s presence. If no other cars are found waiting, the controller blinks the green signal.

The waiting vehicle then moves on. If a centrally computerized traffic arrangement is installed, then a set of lights can be controlled concurrently allowing smooth passage of traffic.

New Research (Adaptive Traffic Lights)

Traffic lights acting locally will better the control of traffic globally, and this is the new research going on at present. With regard to adaptive traffic lights, a mechanical engineer, Gábor Orosz of the University of Michigan says, “It’s very interesting- the approach is adaptive and the system can react. That’s how it should be- that’s how we can get the most out of our current system.”


Gartner N., 1983. “A demand responsive strategy for traffic signal control”. Transportation Research Record, no.906.

A dynamic and automatic traffic light control expert system for solving the road congestion problem by W. Wen

Rachel Ehrenberg – wiredscience