Why are traffic lights
red, yellow and green?

Traffic lights use red, yellow and green because road signals were copied from railway signals, and because each colour has a perceptual job.

Table: What each traffic-light colour does and why (history from railway and signal sources; perception from vision science)

First road signal, Westminster, London

9 Dec 1868

First electric traffic signal, Cleveland, Ohio

5 Aug 1914

Men of Northern European descent with red-green colour deficiency

1 in 12

Vienna Convention standardises road signals

1968

The colours came from the railways because the railways had already solved the problem of telling moving vehicles when to stop. Early British railways used red for danger, white for all clear, and green for caution. White turned out to be a dangerous choice: a driver could mistake a street lamp, a house window or a star for "clear". By the late 19th century green had taken over as "clear", and yellow later became the caution colour on distant signals.

John Peake Knight, the railway engineer behind the 1868 Westminster signal, simply moved this railway system to the road: arms raised for stop, lowered for caution, with a red and green gas lamp for night use¹. After the explosion, London went back to police officers directing traffic by hand for decades.

Electric signals came from the United States, where cars multiplied in the 1910s. On 5 August 1914 the American Traffic Signal Company installed an electric signal designed by James Hoge at Euclid Avenue and East 105th Street in Cleveland, Ohio. It had red and green lamps, the words "stop" and "move", and a buzzer that sounded before the colours changed, a forerunner of the amber phase².

Around 1920, Detroit police officer William Potts built one of the first signals with all three colours, adding amber as a warning. In 1923 the inventor Garrett Morgan patented a T-shaped manual signal with a third position that stopped traffic in every direction, giving people time to clear the junction³. Morgan is often wrongly called the inventor of the traffic light; his contribution was the all-stop interval.

Red works for stop partly because of history and partly because of how light and eyes behave.

Red has the longest wavelength of visible light, roughly 620 to 750 nanometres. Very small particles in the air scatter short wavelengths more than long ones (the same Rayleigh scattering that makes the sky blue), so red light carries slightly better through fine haze and dust. In thick fog the droplets are large enough to scatter all colours almost equally, so this advantage should not be overstated.

Red is rare in the background. Streets are mostly grey, brown and green; a bright red point is easy to pick out, a case of the von Restorff effect, where the item that differs from its surroundings is the one we notice.

Amber is close to peak sensitivity. In daylight, the human eye is most sensitive to yellow-green light around 555 nm. Amber, around 590 nm, sits close to that peak, so a caution light looks very bright for the power it uses.

Colour-blind drivers read traffic lights by position as much as by colour. Red-green colour vision deficiency affects about 1 in 12 men and 1 in 200 women of Northern European ancestry, according to the US National Library of Medicine⁴; rates are somewhat lower in many other populations, but it is common everywhere⁵.

Signals are designed with this in mind. The colours always appear in the same order, red at the top or left, so the lit position carries the meaning. And signal green is deliberately a bluish green rather than a yellowish one, because for many people with red-green deficiency, yellowish greens and reds look alike while a bluish green looks different.

Japan takes this further for a linguistic reason. In Japanese the word ao historically covered both blue and green, and the go light has always been called ao. Since 1973 the government has specified the bluest green that international standards allow, so many Japanese lights look turquoise⁶.

The colours became universal through international agreement. The 1968 Vienna Convention on Road Signs and Signals, signed by dozens of countries, standardised the meaning of red, amber and green and the order of lights⁷. Details still differ: the UK shows red and amber together before green; many countries, including India, add countdown timers; and some flash green before it changes. The core code is the same everywhere.

Traffic lights are the world's most successful colour code, and they work because they never depend on colour alone. Apps often do.

• Pair colour with a second cue — an error should have an icon and text, not only a red border; WCAG's "Use of Color" criterion requires it⁸.
• Keep positions consistent — like red on top, put destructive and primary actions in the same place on every screen.
• Use red sparingly — if everything is red, nothing stops anyone; reserve it for real danger or errors.
• Check your greens and reds together — test success and error states with a colour-blindness simulator.
• Respect learned meanings — red means stop or error worldwide; using it for "go" fights decades of training.

Our accessibility checklist covers colour contrast and colour-independent cues in detail, and a mobile app UX audit tests them with real users.

Historical dates come from contemporary reporting on the 1868 London signal, the Cleveland Police Museum's history of the 1914 signal and Garrett Morgan's 1923 patent. Colour-vision figures come from the US National Library of Medicine and the National Eye Institute; Japan's 1973 standard from published explanations of the ao colour term. The physics of scattering and eye sensitivity is standard optics; wavelengths in the illustration are approximate. Last reviewed October 2026.