Why Are Traffic Lights Red, Green and Yellow?

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Traffic lights are such a common part of our daily lives that we rarely give them a second thought. We just do what they tell us to. This wasn’t always the case, however. In 1917, when the first interconnected traffic signal was installed in Salt Lake City, motorists thought it was some sort of prank and simply ignored it. Police officers had to be dispatched to make sure drivers understood the lights and obeyed them.

Traffic signals actually date back to the 1860s, when they were introduced in London to direct horse carriage traffic. Unfortunately, the inventor didn’t have access to a reliable source of electricity, so he relied on the same gas that lit the city’s lamps after dark. This led to explosive results when a leak combined with an open flame. The resulting injuries pushed back widespread use of traffic signals until electric power became common.

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Traffic Light

The lights most of us are familiar with are the classic three-aspect models. However, in other countries as well as many large American cities, multiple signals are the norm, with lights for city busses and other specialized vehicles.

One question that comes up fairly often is what led to the standard color scheme, with red on top, yellow (or amber) in the center, and green on the bottom? The answer hearkens back to the early days of rail travel, when workers needed a way to warn trains that there was danger ahead.

RED: They chose red to mean “stop!” because of its allusions to blood.

GREEN: Green was originally the symbol for caution, and a clear, or white, light meant it was safe to proceed. The problem with this arrangement is that engineers frequently mistook the light of the moon or stars for the “all clear” signal. This led to tragic derailments and train collisions.

YELLOW: The green color switched over to mean all-clear (with the ‘G’ in green for ‘Go’), and the yellow was made a caution, so that even in the event of confusion with the moon, the engineer would slow down, rather than speed up. And that, my friends, is how it all began.

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Photo Credit: Horia Varlan / Flickr

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