This Is Why Traffic Lights Are Red, Yellow and Green

There's a perfectly good explanation for those familiar traffic light colors.

The idea that red means stop and green means go affects more than just traffic lights and red light cameras. We have been taught from a young age that red means danger, while green means safety. But why were those particular colors chosen for traffic lights in the first place?

For something we have to look at every day, why couldn’t they have been prettier colors, like magenta and turquoise? You’re about to find out.

When Were the First Traffic Lights Created?

The first traffic signals in the United States were installed in the 1910s. Worried about accidents with more travelers on the road, towns and cities installed traffic towers to help the flow of cars. Officers manned the towers, using whistles and red, green and yellow lights to indicate to drivers when they should stop and go.

Then, in 1920, William Potts created the first tricolor, four-directional traffic light to help drivers stay safe at intersections. The first went up in Detroit, Michigan, at the corner of Woodward Avenue and Fort Street.

Throughout the country, various systems for traffic lights and patterns were in place, with no coordination. Then the Federal Highway Administration created the “Manual on Uniform Traffic Control Devices” in 1935. It set uniform standards for all road signs, pavement markings and traffic signals, requiring red, yellow and green light indicators.

What’s the History Behind the Colors?

Before there were traffic lights for cars, there were traffic signals for trains. At first, railroad companies used red to mean stop, white for go and green for caution.

As you can probably imagine, train conductors ran into a few problems with white meaning go. Bright white could easily be mistaken for stars at night. Railway companies eventually moved to the color green for go.

And because yellow is easily distinguishable from the other colors, it became the standard for trains proceeding with caution. It’s been that way ever since. It became standard for traffic lights as well.

Why Was Red Chosen for Stop?

Red is the color with the longest wavelength. As it travels through air molecules, it’s diffused less than other colors, so it can be seen from a greater distance. Think about how the sun turns red as it sets. Though the human eye is most sensitive to a yellow-green highlighter color — hence the shade of high-visibility safety vests — it can see red from further away.

Yellow has a shorter wavelength than red, but a longer wavelength than green. This means red is visible the furthest away, a helpful advanced warning for slowing or stopping. It’s not clear whether red was chosen based on wavelength, contrast against green, or the natural association of red with blood. It could be a combination of all three!

Believe it or not, yellow was once meant stop. Back in the 1900s, some stop signs were yellow because it was too hard to see a red sign in a poorly lit area. Eventually, with the development of highly reflective materials, red stop signs were born.

Since yellow can be seen well at all times of the day, school zones, some traffic signs and school buses continue to be painted that color.

Reader's Digest
Originally Published on Reader's Digest