How do traffic lights work?

In 1858, in London, England, the main street installed to coal gas as a light source of red, blue mechanical wrench type signals, used to direct the passage of horse-drawn carriages. This is the world's earliest traffic signals. 1868, British mechanical engineers Nayt in the Westminster district of London, the square in front of the Houses of Parliament, the installation of the world's first gas traffic lights. It consists of red and green two to rotating square glass lamps, red means "stop", green means "attention". 1869 January 2, the gas lamp exploded, so that the police were injured, and was canceled.

In 1914, the electrical start traffic lights appeared in the United States. This traffic light consists of a red, green and yellow circular projector, installed on a tower on 5th Street in New York City. A red light meant "stop" and a green light meant "go".

In 1918, the traffic light with control and infrared traffic lights. With the control of traffic lights, a pressure detector is installed in the ground, the vehicle is close to the red light will change to green; another is to use a loudspeaker to start the traffic lights, the Division opportunity to press the Loudspeaker when the red light, so that the red light to green. Infrared traffic light when pedestrians step on the pressure-sensitive pavement, it can detect that someone wants to cross the street. The infrared beam extends the red light of the signal for a longer period of time, delaying the release of cars to avoid traffic accidents.

The emergence of signal lights, so that traffic can be effectively controlled, for the diversion of traffic flow, improve road capacity, reduce traffic accidents have a significant effect. 1968, the United Nations "road traffic and road signals agreement" on the meaning of a variety of signals provided. Green light is a passing signal, facing the green light vehicles can go straight, left turn and right turn, unless another sign prohibits a certain kind of turn. Both left and right turning vehicles must give priority to vehicles legally moving through the intersection and to pedestrians crossing the crosswalk. A red light is a no-traffic signal, and vehicles facing a red light must stop behind the stop line at the intersection. A yellow light is a warning signal. Vehicles facing a yellow light must not cross the stop line, but may enter the intersection if they are too close to the stop line to stop safely. Since then, this rule has become common throughout the world.