How traffic lights work and how they help a city’s traffic

How traffic lights work and how they help a city’s traffic

By carolc

For all of us, it is extremely common when you hear about traffic lights, it is simple and perhaps ordinary to know what device we are talking about. But this was not always the case, in remote times long before the automotive revolution began a system had to be devised to be able to control the flow of passers-by or devices on wheels as means of transport. By this time, the systems they used to operate it were already known as traffic lights, using flags or lights, transmitting information between two points faster than sending a courier on horseback.

It was not until the mid-19th century, specifically on 9 December 1868 that the first traffic light was installed in London more or less as we know them today. Although that design of the John Peake Knight railway consisted of lifting arms and two gas lamps, one red and one green.

A failed attempt

But this invention was a total failure, was placed on the outskirts of the House of Parliament in 1868 and had an appearance to control a railway signal, at night. A month after being installed, the traffic light exploded due to a gas leak and severely burned the policeman who operated on it. This brought much controversy regarding these devices and they decided to continue using their pennants that posed no apparent danger.

But by 1920, the automotive revolution began where carriages were already beginning to be traded for motor cars that flooded the streets of America’s big cities and towns. It was there that it all began, they were a symbol of progress where every city, town wanted to have one. The growth was excessive and being only a few devices, became hundreds, at first the lights were programmed according to the time they would remain on, thirty seconds in red, thirty seconds in green, but that’s where theories began to appear that sought to speed up traffic by coordinating traffic lights between them.

What is a Traffic Light?

Traffic lights are signaling devices regulating the movement of vehicles, bicycles and pedestrians on tracks, assigning the right of passage or priority of vehicles and pedestrians sequentially, by the indications of red, yellow and green lights, operated by an electronic control unit.

How to bring order to chaos

The dense traffic seen daily in large cities is difficult to keep it tidy, so the authorities impose their guidelines and do not trust the rationality of the individuals behind the wheel. When certain levels of traffic are reached, traditional signaling is not enough; rather other parameters such as passing time at intersections are involved, where chaos is prominent without a good system that controls it.

Not long ago these parameters were set fixed, but the main traffic controllers were noticing the behavior of the vehicle flow, its patterns, its variations and the way drivers behaved, you can vary the distribution of the times of each light to improve traffic and reduce polluting emissions.

How a traffic light works and the technologies it uses

The traffic lights seem simple, showing a change of lights at different times, but behind this electronic transit signal, there is a technology that allows us to understand how it works.

The lights at a traffic light don’t change in a way they want. Older ones use a system of times that mark the duration of a light color, while the more modern ones feed on remote programming and control technologies.

Through signals and sensors that detect traffic flows in the streets of the city, traffic lights receive remote orders to control the duration of their times, thus avoiding traffic jams and, generating a system of coordination between several traffic lights, trying to minimize the arrests of the vehicles or, the time they will spend in a red light.

Today, the lights of the traffic lights are usually LED, thus delivering better visual and energy efficiency, in addition to being clearly visible in the dark or, when the sun faces.

Another technology present in some traffic lights is a camera that detects when a vehicle goes through with a red light taking an image that it sends to a control center. So that it curses the corresponding violation in a registered way.

Corrective maintenance to the traffic light network

It is necessary to consider the eventualities that may occur, for example, the replacement of burnt lamps and the replacement of poles and heads in the event of demolitions or damage caused by external factors, communication failures, and problems with some technological component.

The resolution of the problems that are detected must occur within the period indicated in the respective administrator’s manual and considering the seriousness of the inconveniences that they may generate. There are failures that require immediate solutions and others whose solution you can expect, of course, as long as the equipment in question is not in a high-traffic zone.

Types of coordination at traffic lights

Simultaneous system

In a simultaneous system all traffic lights show the same indication along the track at approximately the same time

Alternate system

In the alternate system adjacent traffic lights or groups of adjacent traffic lights show alternate indications along a given route. In the simple alternate system, adjacent traffic lights show opposite or opposite directions.

Limited progressive system

The limited progressive system sets duration common to cycles and green light indications are given independent, in accordance with the requirements of each intersection and in accordance with a time programmer to allow continuous or near-continuous movement of groups of vehicles operating at the project speed.

Flexible progressive system

The flexible progressive system encompasses all the features of the limited progressive system and has a number of additional features that depend on the type of intersection controller, master control and other accessories. A common system-round cycle is used.

Pros and Cons of traffic lights


  • Orders the circulation of traffic and by appropriate allocation of the right to use the intersection.
  • Reduces the frequency of certain types of accidents.
  • With favorable spacing can be synchronized to maintain continuous circulation.
  • Allow to periodically interrupt the intensive traffic volumes of an artery, to grant the passage of vehicles.


  • Unjustified costs are incurred for solutions that could have been resolved only with signals or otherwise economically.
  • Cause unjustified delays for a certain number of users, especially in the case of small transit volumes.
  • Produce unfavorable reaction in the public.
  • Increase the number of accidents of the range type.
  • Cause unnecessary wastes of time in the hours of the day.