Why are traffic lights red, yellow and green?

The colour system for traffic lights was established long ago, before cars were invented, as railway signals. To begin with, the colours were a bit different from those used today. Railway companies adopted red for “stop”, green for “caution” and white for “go”.

Red means “stop”

It’s obvious why they chose red to mean “stop”: we associate it with danger and things that are hot. The red glow of a kitchen hob, for example, tells us it’s switched on and too hot to touch. Red makes us stop in our tracks: it’s the colour of fire and blood. Red also has a longer wavelength than other colours, making it the most visible colour from a distance. Our reactions to this colour-coded warning go back much further than anything man-made. Although animals see colours differently to the way humans do, many animals can see shades of red, and many of them have evolved red colouring or markings to show that they’re poisonous and to warn off predators.

A 2011 study published by the Association for Psychological Science reported the results of an experiment involving rhesus macaque monkeys – whose eyes are sensitive to red, green and blue. The monkeys would accept food from a person wearing blue or green clothing, but in most cases, would not take the food if the person was wearing red. Red seemed to signal danger to the monkeys.

Green means “go”

At first, train companies used white lights to indicate that it was safe for train drivers to proceed. But some drivers found it confusing: they thought that bright white stars low down in the sky were “go” lights in the distance.

So, green was chosen instead. The colour was clear to see and couldn’t be mistaken for any naturally produced light. It was eventually used internationally as the “safe” colour to indicate “go”. Green is the colour of plant life, of leaves and grass, and in many cultures, it symbolizes life.

These two colours, red and green, are used to help us understand the difference between danger and safety, stop and go, and on or off, for systems, machines and appliances. Indeed, in many contexts, red and green signals protect us from accidents every day, without our even realizing it.

Yellow means “be careful”

Yellow contrasts strongly with red and green and is easy to see at all times of the day, in all sorts of light. It has a slightly shorter wavelength than red, but not as short as green. The yellow light was invented as a warning light for traffic in 1920, in Detroit, Michigan, by a policeman called William Potts. It was soon being used all over the world. Nowadays, the three traffic light colours are part of most countries’ traffic regulations – helping to keep us safe.

Info to go...

In Japan, even though “go” lights are blue-green, they are referred to as being “blue”. This is because, for a long time, there was no word for “green” in the Japanese language because green was considered to be a shade of blue.