Traffic signal lights consist of red lights, green lights and yellow lights. Red light indicates no traffic, green light indicates permission, and yellow light indicates warning.
In 1858, red and blue mechanical spanner signal lights were installed on the main streets of London, England, to direct the carriage to pass. This is the earliest traffic signal light in the world. In 1868, British mechanical engineer Naite installed the world's first gas traffic signal lights in the square in front of the Parliament Building in Westminster, London. It consists of two rotary square glass lanterns, red and green. Red indicates stop and green indicates attention. On the 23rd day of its operation, the gas lamp suddenly exploded, causing a policeman on duty to die instantly. Then the gaslight was cancelled.
In 1914, electric traffic signal lights appeared in America. This kind of traffic signal light is composed of red, green and yellow circular projectors, which are installed on a tower in No.5 Street, New York. Red light indicates stop and green light indicates passage.
In 1918, crossing traffic lights with control and infrared lights appeared. One of the traffic signal lights is with the pressure detector underground. Once the vehicle approaches the red light, the red light will turn into a green light. Monitoring pole type is another kind of traffic light started by loudspeakers. When drivers meet red lights, they can turn them into green lights by pressing their horn.
The appearance of monitoring pole signal lights made the traffic be effectively controlled. It has obvious effect on guiding traffic flow, improving road capacity and reducing traffic accidents. In 1968, the United Nations Agreement on Road Traffic and Road Signs and Signals stipulated the meanings of various signal lights. Green light is a permission signal. Vehicles facing green lights can go straight, turn left and turn right, unless there is another sign prohibiting a certain turn. Vehicles turning left and right must give priority to those legally driving at intersections and pedestrians crossing crosswalks. Red light is a no traffic signal. Vehicles facing red light must park behind the parking lines at intersections. Yellow light is a warning signal. Vehicles facing yellow lights cannot cross the parking line. However, vehicles can enter the intersection when they are too close to the parking line to park safely.