The Real Reason Traffic Lights Are Red, Yellow and Green

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

The idea that red means stop and green means go affects more than just traffic lights. We’ve been taught from a young age that red means danger, while green means safety. But why were those particular colours chosen for traffic lights in the first place? For something we have to look at every day, why couldn’t they have been prettier colours, like magenta and turquoise? You’re about to find out.

When were the first traffic lights created?

The first traffic lights in North America were installed because of an increase in travellers on the road in the 1920s. Worried about accidents, 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 tri-colour, four-directional traffic signal. It helped drivers stay safe at intersections. The very first four-directional traffic light was installed at Woodward Avenue and Fort Street in Detroit, Michigan.

What’s the history behind the colours?

It’s important to know that before there were traffic lights for cars, there were traffic signals for trains. At first, railroad companies used red to mean stop, white to mean go and green to mean caution.

As you can probably imagine, train conductors ran into a few problems with the colour white meaning go—bright white could easily be mistaken for stars at night, with train conductors thinking they were all clear when they really weren’t. Railway companies eventually moved to the colour green for go. And because it’s easily distinguishable from the other colours, yellow became the standard for indicating when trains should proceed with caution. It’s been that way ever since.

When traffic lights were put up, it became standard for them as well.

Why was red chosen for stop?

Red is the colour with the longest wavelength; that means that as it travels through air molecules, it gets diffused less than other colours, so it can be seen from a greater distance. For a real-world example, think about how the light turns red as the sun sets. Even though the human eye is most sensitive to a yellow-green highlighter colour (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 that red is visible the furthest away, yellow in the middle and green the least distance away—a helpful advanced warning for needing to slow or stop. But this could be a coincidence. Red meaning stop originated with train warning lights, and it’s not clear whether that was chosen based on wavelength, contrast against green nature or natural association of red with things like blood. It could be a combination of all three!

Believe it or not, yellow was once used to mean stop, at least as far as signage goes. 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, highly reflective materials were developed, and 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 the colour.

Next time you’re impatiently waiting at a traffic light, don’t get mad; be thankful that traffic lights have come a long way.

Next, find out why cars have gas tanks on different sides.

Reader's Digest
Originally Published on Reader's Digest