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Why Does Red Mean “Stop” and Green Mean “Go”? The History of Stoplights

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A traffic light in Stockholm in 1953. (Olle Karlsson/Wikimedia Commons)

Automatic Stoplights

In 1920, a police officer named William Potts developed the four-direction traffic signal, which was installed in Detroit at the intersection of Woodward Avenue and Fort Street. Potts’s invention was well received, but it still required a person to observe the traffic and flip the switches to turn on the colored lights, so as the ’20s progressed, clever inventors worked on developing automatic traffic signals. One inventor tried making a signal that changed with sound, featuring a sign instructing drivers to honk their horns as they approached the intersection, but some cars didn’t have horns, sometimes two cars honked at the same time, and mischievous children liked to blow whistles and bang pans to change the signal.


In the end, the most efficient method of automating traffic signals was to set them on a timer. The drawback to this system, of course, is that sometimes cars have to sit and wait for the light to turn green when there were no other cars in the opposite direction, but it was the best solution. By 1935, the Federal Highway Administration officially adopted traffic signals using red, yellow, and green as the standard for all of the nation’s roads. 


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