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Do you know when the traffic light appeared? It turns out that they began to regulate the traffic using a mechanical device even 140 years ago, in London. The first traffic light was installed in the centre of the city on a 6-meter pole. It [be controlled|controlled|was controlled|were controlled] by a specially assigned person. Using a system of belts, he put up and down the arrow of the device. Then the arrow was replaced by a lamp that worked on gas. There were green and red signals in the lamp. The yellow lamps [hadn't been invented|haven't been invented|hasn't been invented|didn't invented] yet.

The first electric traffic light appeared in the USA, in the city of Cleveland, in 1914. It also had red and green signals. It was still mechanical. The policeman's warning whistle[used| was using|was used|be used] instead of the yellow signal at that time. An electric traffic light with three colours and automatic control appeared in New York in 4 years.

It is interesting that at first the green signal was at the top of the traffic light, but then they [was decided|deciding|decided|have decided] that it was better to put a red signal on top. And now, in all countries of the world, traffic signals [is arranged|are arranged|arranges| were arranged] according to a single rule: red at the top, yellow in the middle, and green at the bottom.