How do traffic lights work?

          The growing urbanization is a direct outcome of rapid industrialization owing to which a large population gets concentrated in the cities. This consequently results in a heavy rush of vehicular traffic, which necessitates effective control and efficient management to avoid chaos and accidents.

          Regulation of traffic is done either by uniformed men who use different hand gestures to send different signals to the drivers or by the use of traffic lights which indicate the desired signals to the drivers.

          Traffic lights are found at most of the important crossings and junctions of a city. These lights are arranged one above the other on a pole and covered by glasses of three different colours, viz. green, orange and red. Red light indicates to stop, and green gives the ‘go ahead’ signal. Orange signals are used in between to prepare or get ready for the next signal—either slowing down if the next signal is red or getting ready to move if the oncoming signal is green. These lights either change at a pre-set time or in some cities sets of timed lights are connected together so that traffic can proceed along the main road-route at a certain controlled speed without stopping. But the modern day traffic lights are computerized and time intervals are not fixed but vary in tune with vehicular presence. But what is the working mechanism of all these types of traffic lights?


          The lights with fixed timings run according to a preset timed cycle. If a vehicle pulls up just as the signal changes from green to red then it has to wait for the cycle to be completed till the light changes to green again even if no other vehicle is present in other directions of the crossing.

          But in the modern style timings of signals are not pre-set but respond to a vehicle’s presence. The technique used in this case is the setting up of a weak magnetic field through an electric wire buried under the road which is sensitive to the metal parts of the vehicle on the road. A computer operated control box senses this through the wire and if there are no other vehicles waiting to move in other directions then it flashes the green signal to the waiting vehicle. The computers also change the lights and in the case of a centrally computerized traffic system it is possible to operate sets of lights simultaneously to allow an easy and smooth flow of traffic.