# Origin of Red, Yellow, and Green lights. **URL:** \ **Category:** Factual Questions\ **Created:** [April 18, 2003, 2:48am UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477 "2003-04-18T02:48:12Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![moejuck](https://avatars.discourse-cdn.com/v4/letter/m/57b2e6/32.png) [@moejuck](https://boards.straightdope.com/u/moejuck)\ **Post date:** [April 18, 2003, 2:48am UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/1 "2003-04-18T02:48:12Z")
A simple question: Why does red mean “stop”, yellow to “slow down”, and green “go”? In other words, what is the origin of our current traffic light. ---
**Author:** ![Ice\_Wolf](https://avatars.discourse-cdn.com/v4/letter/i/dfb087/32.png) [@Ice\_Wolf](https://boards.straightdope.com/u/Ice_Wolf)\ **Post date:** [April 18, 2003, 3:33am UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/2 "2003-04-18T03:33:49Z")
It would appear that traffic lights copied those of railway signals. [This site](http://www.geocities.com/signalcollector/history.htm) has that green originally meant “caution” in the 19th century experiments. According to [this page](http://www.ci.nyc.ny.us/html/nypd/html/transportation/newpage5.html), the confusion in America was from city to city. > [@](#): > > a red light in New York City meant traffic in all directions had to stop. An amber light meant cross-town traffic would have to stop so that north and southbound traffic could pass. And a green light would stop north and south bound lanes of traffic so that cross-town traffic could proceed. The difficulty in understanding this confusing color sequence was compounded by neighboring towns using another system, the “railroad signal designation” sequence of red for stop, green for go, and amber for slow. It came as no surprise that out-of-town drivers headed north on Madison Avenue were not thrilled to receive a summons for “passing a green light!” [Here’s Cecil’s take on it.](http://www.straightdope.com/classics/a2_152.html) ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [April 18, 2003, 11:44am UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/3 "2003-04-18T11:44:04Z")
Traffic lights are descended from railroad semifores (sp?). Red is the color of blood, so it’s always been the first choice for warning signs. Green and yellow were kinda pulled out the air. Originally the colors were red and white (no yellow). But if the red lens broke, the train’s engineer would think he was getting a “go ahead” signal instead of a “stop” signal, and there would be a wreck. So they switched to green for “go.” I dunno why they chose green instead of blue or yellow; green lenses were probably cheaper. Yellow was added later, and was chosen because it looks nice between green and red. ---
**Author:** ![stuyguy](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@stuyguy](https://boards.straightdope.com/u/stuyguy)\ **Post date:** [April 18, 2003, 2:09pm UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/4 "2003-04-18T14:09:34Z")
I read that green was chosen for “GO” because experiments showed it could be seen from the farthest distance. I’ll try to dig up a cite. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [April 21, 2003, 2:59pm UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/5 "2003-04-21T14:59:20Z")
Red, green and yellow (=red+green) are the most distinct 3 colors you can have without using blue light. Blue light can’t be used because eyes focus it differently, and less distinctly, so that at a distance the blue is lost and the light would appear one of the other colors. ---
**Author:** ![everton](https://avatars.discourse-cdn.com/v4/letter/e/ecb155/32.png) [@everton](https://boards.straightdope.com/u/everton)\ **Post date:** [April 21, 2003, 5:59pm UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/6 "2003-04-21T17:59:40Z")
> [@](#): > > \*Originally posted by stuyguy \* > \*\*I read that green was chosen for “GO” because experiments showed it could be seen from the farthest distance. > > I’ll try to dig up a cite. \*\* I think you’ll find that that was why red was chosen, not green. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [April 21, 2003, 6:06pm UTC](https://boards.straightdope.com/t/origin-of-red-yellow-and-green-lights/169477/7 "2003-04-21T18:06:49Z")
There’s a lengthy discussion of this in, of all places, Marvin Harris’ book **Cultural Materialism** , his book on his philosophy of anthropology. It comes up because Harris wants to discredit those who tried to see some logical progression between the colors. In fact, over time, quite a few colors have been used in railroad lighting and streetlights before the systems were standardized, with blue sometimes meaning “go”.