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Traffic lights are safety devices that control the flow of traffic on roadways, railways, and waterways by signalling which way the traffic is to proceed and for how long. There is a universal colour code of red for stop or wait, yellow for caution or transition, and green for go, but beyond that signals vary even within a single country. The most common traffic light system has three vertically-stacked lights facing the driver with red on top, yellow or amber in the middle, and green at the bottom. In areas where wind is a factor, the lights are placed horizontally with the ordering of the colours remaining the same when read from left to right.
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