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Why traffic lights are red, yellow and green

Generally considered the world's oldest traffic light appears in the United Kingdom. On December 10, 1868, United Kingdom engineer de·hate designed and manufactured a gas lamp traffic lights, lamp post 7 meters high, wearing a light red, Green Lantern, lamp feet, pulled by police armed with a long Rod like belt conversion lamp colors. Why use red and green? This was United Kingdom's culture, was wearing a "red" of women married, and "green" women are unmarried, so red "keep off", and green indicates "near". In fact, the penetration of the red and green lights are strong, not easy to spread, and as the opposite color, they are easy to identify, but also very suitable as a light.
However, the original signal light only red-green color, yellow is later in the United States only. After the first world war, United States rapid rise, more and more motor vehicles on the street, speed became fastest, red lights stopped rapidly more difficult, so people want to put a yellow light in the middle of the traffic lights, expressed "cautious". Many Chinese sources have referred to the inventor of the yellow lights were at that time in the United States Chinese Hu Ruding of General Electric Company. But it has recorded the world's first red-yellow-green color, four directions of traffic lights was put into use in October 1920.