There are thousands of colors — so why are traffic lights always red, yellow, and green?

A strange thing to notice

Red means stop. Green means go. Yellow means get ready. Almost every country on Earth uses these same three colors on its roads. But we can see thousands of colors. So why these three?

Start with red

Red light has the longest wavelength we can see — about 620 to 750 nanometers. Long waves are harder for tiny air molecules to knock off course, so they scatter less. In fog, in rain, or at night, red stays bright and stays visible farther away than any other color.

For a driver, being seen is everything. The message that must be seen first — STOP — went to red.

Red had a second advantage: it already meant danger. Railways were using red for "stop" more than a hundred years ago, and roads simply borrowed the habit instead of teaching everybody a new one.

What about green and yellow?

Green started out as the backup. The earliest railway signals were: red for stop, green for caution, and white for go.

But white lights were too easy to mix up with other lamps. After some bad accidents, railways changed the rules: green means go, yellow means caution. Cars copied the railways, and we still follow that agreement today.

Here is a little-known story: in 1868 the world's first traffic signal was put up outside the Houses of Parliament in London. It burned gas. In 1869 it exploded and injured the policeman operating it, so the whole thing was taken down. The idea did not come back until electric signals appeared in the United States in 1914.

Try it yourself: make a glass of "fake fog"

Wrap one flashlight in red cellophane and another in blue. Stir a few drops of milk into a glass of water. Turn off the lights and shine both beams in from the side of the glass.

You will see that red pushes much farther, while blue gets scattered and fades almost at once.

Same reason the sky is blue, sunsets are red, and red lights travel the farthest: short wavelengths are the easiest to scatter.