The World's First "Stoplight" Was Born in Detroit — Invented by a Cop From the Thumb

That red-yellow-green traffic light at the corner you stopped at this morning? Its design is almost exactly the same as the one a Detroit police officer named William L. Potts installed at the corner of Woodward and Michigan Avenues in October 1920. And it was the first of its kind in the world.

There had been earlier traffic signals — a gas-powered red/green one outside the Houses of Parliament in London in 1868 (it exploded), and red/green-only signals already in use in some U.S. cities. But “red means stop, green means go” wasn’t enough. Cars rolled up at full speed, the light flipped, and drivers had no time to brake. Crashes were constant.

Potts, who had a background in electrical engineering, was watching the chaos and borrowed an idea from the railroads — which already used three-color signals — and added a third color in the middle: amber. With about $37 worth of railroad lights, wire, and electrical controls, he built a four-directional tower that gave drivers a “get ready to stop” warning. It worked beautifully. Within a year, Detroit had installed 15 of them. By the mid-1930s, three-color signals were standard worldwide.

Here’s the great Michigan kicker: William Potts was born in Bad Axe, in the Thumb — and he never patented his invention. The Detroit Police Department donated the original 1920 signal tower decades later to the Henry Ford Museum, where it’s on display today.