Why Is Red Stop and Green Go?

(Last Updated On: August 23, 2022)

Colors are pretty useful, they serve as nice shorthands for things. Among the most well-known color shorthands is probably “red means stop” and “green means go.” You see it everywhere on stop signs, traffic lights, or children’s games. While the whole red/green thing might seem like common sense, we probably don’t pay much thought to why our traffic lights look the way they do. Now it’s cooking though–why is red stop and green go?

Thank Railroads for Your Traffic Lights

Early traffic lights trace their way back to the 1830s. Specifically they go back to the railroad industry. Railroad engineers were figuring out a way to tell trains to stop or go. Red meaning stop was easy for them to settle on; red has always been the color for danger. It’s well documented that red is a solid danger cue for people anyway, so settling on red was easy. 

Engineers chose white for go and green for caution, which turned out to be rather problematic. Not the green-for-caution bit–but the white-for-go bit. That’s because the way the lights were designed was with white lights and a color filter placed over them. You can see where this might become a problem. Allegedly in the 1910s, a red filter fell off of the light behind it–leaving the light white and not red. Which caused a collision because a train that was supposed to stop instead didn’t. 

It’s said that green was moved to the “go” slot, and yellow was adopted for caution because it was distinct from the already used red and green. When signals made their way to roads, they just picked up what was already working for railways.

Traffic Lights

Once cars started getting put on the road–especially in post-WWI America–people figured out we needed to organize our roads quickly. Even when horse-drawn carriages were still being used, Londoners in the 1860s wanted to start managing the right of way. They started by translating the railway semaphore to the roads–which was quickly adopted in both the UK and the US.

The first traffic light was installed in 1914, in Ohio. The first four-way traffic light was installed in Detroit in 1920. Early traffic lights apparently didn’t have a yellow signal–the officer managing the signal just blew a whistle to tell people the light would change. The yellow signal was adopted because… well people didn’t listen. Like the semaphore, these early traffic lights were operated by humans–and it wasn’t until 1922 that traffic lights were just switched over to timers. The red/yellow/green would become an international standard in 1968.


See if you know which cities have the worst traffic here.

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About Kyler 727 Articles
Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.