How do traffic light signals works???

Traffic Light Signals

When We all have stuck in traffic jam,we wonder how these light changes ? Does some person change the light watching the traffic or they automatically change? 

Traffic lights is a set of automatic,coloured lights for controlling traffic.
Traffic lights are signaling devices positioned at road intersection, pedestrian crossing , and other locations to control the flows of traffic.

Traffic lights was initially designed for railway system.
In 1830's the signal used for railway is Red-Stop , Green - Caution and White - Go. This system was not safe as white colour is not visible in night with stars at horizon or with white background  . In 1914 a red lens was fall from a signal and indicate White-Go result in a huge train tragedy. After that white colour is replaced by Green and Green by Yellow for caution.

The world's first road traffic light was a manually operated gas lit signal installed outside House of Parliament London. It exploded less than a month due to fuel leakage, injuring its policeman operator.

The inventor of the early Traffic lights are Garrett Morgan, traffic lights of today are based on Morgan's original design. 

• Red - Stop,when to stop
• Yellow (Amber) - Caution , go slow 
•  Green -go, move

 Types of traffic lights  :

1) Timer based                       2) Traffic based

• Timer based : Timer based traffic signal display green signal after a fixed time cycle regardless of the traffic condition . For Example : If a timer of 90 sec is set the light will go green after 90 sec for each cycle. It is very wasteful if in some cycles there are no vehicles waiting as the time could be better allocated to a busier approach.
• Traffic based : Traffic based signal changes or display green signal according to the flow of traffic.It will give more time to the crossing which has more vehicles. This can be done by using : i) Grounded detector ( induction loop ) method : In this method an electromagnet is grounded below the road near the crossing and connected to a microprocessor through cables. We can identify such signals by stretches on road as shown in figure

Induction Loop 

  When electricity is flowing through the wire it produces magnetic field when the car  (metal object) move through the magnetic field, the field affects the electrons in the car and produce a current opposite to the current in the loop. This opposite current produce a magnetic field in car which opposes the field of the loop due to that a signal is sent to the microprocessor or controller which turn the signal green.

 ii) Above the ground detection (microwave detection) method : The microwave detectors are placed on the top of signal bar and vehicles can be detected anywhere with in the field of vision as long as vehicles is travelling faster than 4 km/h . In cases where a slow moving queue occurs the approach should also have a loop at the stop line to ensure that traffic is detected. 
 Camera are also used in this method and these are mounted on the top of the traffic light . Camera senses your vehicles motion and turn light green.