A traffic command measure used to assign the right of way to traffic flow in time at intersections where traffic separation is not possible. The function of traffic signals is to scientifically allocate the right of way for vehicles and pedestrians on the road so that they can pass through orderly and smoothly. The installation of traffic signals, traffic signs and traffic lines shall conform to the national standards.
Traffic signal is divided into: traffic signal light, traffic sign, traffic line and traffic police's command. China implements unified road traffic signals. 
1. Origin of traffic lights
A traffic light (traffic signal) is a light and colour signal alternating at specified times and placed at intersections or other special points to assign the right of way to the driver and the pedestrian to control their stopping and turning. Use red, yellow and green lights or sound system to instruct vehicles and pedestrians to stop, pay attention and proceed.
The origins of traffic lights can be traced back to Britain in the early 19th century. At that time, in york city, England, a woman in red was "married", while a woman in green was "unmarried". At that time, carriage accidents often happened in front of the houses of parliament in London. Inspired by the red and green suit, the British mechanic? Hart designed the red and green gas traffic lights in 1868, which were changed by a policeman with a long pole pulling a belt.
By 1914, traffic lights had evolved from gas to electricity, not far behind modern traffic lights.
The earliest traffic lights in China appeared in the British concession in Shanghai. Some materials show that as early as 1923, Shanghai public concession began to use mechanical devices at some intersections to instruct vehicles to stop and move forward. On April 13, that year, nanjing road was the first to install traffic light traffic signal devices at two important intersections.
In 1968, the United Nations agreement on road traffic and road signs and signals defined the meaning of various signals: a green light is a traffic signal, and vehicles facing a green light can go straight, turn left or turn right accordingly, unless another sign prohibits a certain kind of turning. Turning vehicles must be allowed to legally driving in the intersection, pedestrians crossing the crosswalk to have priority; A red light is a stop signal. Vehicles facing a red light must stop behind the stop line at the intersection. Yellow light is a warning signal, facing the yellow light of the vehicle can not cross the parking line, but the vehicle is very close to the parking line and not safe parking can enter the intersection. Since then, the rule has been adopted around the world.
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