Why are traffic lights red, amber and green?

At a glance

Colour limits

CIE S 004/E:2001 Colours of light signals

US meanings

MUTCD 11th Edition (Final Rule 19 December 2023)

UK sequence

Red → red and amber → green → amber → red

First signal

London, December 1868 (gas-lit, red and green)

From railway lamps to street corners

The first road signal was installed outside the Houses of Parliament in London in December 1868: a gas-lit lantern showing red or green at night, operated by a police officer, modelled on railway signals. Electric signals arrived in the United States in the 1910s; one installed in Cleveland, Ohio, in 1914 is often cited as the first. In 1920 a Detroit police officer, William Potts, designed a four-way signal with a third, amber aspect to warn that the light was about to change. The red–amber–green arrangement, with red at the top of vertical heads, became the international norm and was written into the 1968 Vienna Convention on Road Signs and Signals, which many countries follow.

Specifying a colour that must be recognised

A signal colour has to be identified, not just seen, often at a distance, in rain or fog and against a busy background. CIE S 004 defines allowable chromaticity regions for red, yellow, green, blue and white signal lights used in road, rail, sea and air transport. The regions are drawn so that each colour stays distinct from its neighbours for viewers with normal colour vision and, as far as possible, for people with common colour-vision deficiencies — one reason traffic greens tend to be bluish rather than yellowish, placing them further from red and amber. The standard is explicit that it sets colours only, not meanings; those come from each mode's own rules.

LED signals produce their colour directly rather than by filtering white light, which makes them more consistent from unit to unit; the chromaticity regions still apply to them exactly as they did to filtered lamps.

Same colours, different sequences

In the UK, the sequence is red, then red and amber together, then green, then amber alone, then red. Red-and-amber still means stop; it tells drivers the green is imminent. Most US signals go from green to yellow to red and straight back to green, and the Manual on Uniform Traffic Control Devices sets the national meanings of each indication. Other countries use flashing green before amber, or flashing amber for 'proceed with caution' at quiet times. The colours travel well between countries; the phases do not. That is why the meaning of a signal comes from the traffic law and highway code where you are driving, not from the colour alone.

Amber, orange or yellow

The middle light has several names. British and many Commonwealth sources call it amber; American federal documents call it yellow; Australian usage often says orange. They describe the same chromaticity region, which sits between the hue people call yellow and the one they call orange, and which is deliberately placed well away from red. The naming variation is a useful reminder that colour words carve up the spectrum differently from language to language, while the signal specification itself is written in chromaticity coordinates rather than words.

The data

How to check it yourself

1. For the meaning of an aspect, read the highway code or traffic law of the country you are in.
2. For signal equipment colour, check which chromaticity specification the road authority's procurement cites (often derived from CIE S 004).
3. If you have a colour-vision deficiency, rely on position — red at the top or left — as well as colour.

What the evidence says

Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.

• StandardStrong evidence

  CIE S 004/E:2001 specifies allowable colours for steady and flashing signal lights in road, rail, sea and air transport and does not specify the meanings attached to them.

  Source: CIE S 004/E:2001 Colours of light signals

• FactModerate evidence

  The first traffic signals, showing red or green at night, were installed outside the Houses of Parliament in London on 10 December 1868; an electric signal was installed in Cleveland in 1914, and William Potts created a three-colour signal in Detroit in 1920.

  Source: Traffic signals: a brief history; Traffic light (Wikipedia)

• StandardStrong evidence

  In Great Britain, red and red-and-amber both mean stop, green means go on if the way is clear, and amber alone means stop at the stop line unless it is unsafe to do so.

  Source: The Highway Code: Light signals controlling traffic

• FactStrong evidence

  The US MUTCD 11th Edition was adopted by a Final Rule published on 19 December 2023.

  Source: Manual on Uniform Traffic Control Devices (MUTCD)

• Colourwise interpretationLimited evidence

  Traffic-signal greens are typically bluish to increase their separation from red and amber for drivers with red–green colour-vision deficiency.

  Based on: Inferred from the purpose of CIE S 004's colour regions (recognition by observers including those with colour-vision deficiency) and from the geometry of red–green confusion lines; not a statement from a cited source.

  Caveat: Colourwise did not locate a primary source that states this design rationale directly.

Related questions

Why is the middle traffic light called amber in the UK and yellow in the US?

Different conventions of naming the same signal colour, which sits between yellow and orange. British and Commonwealth usage says amber; US federal documents say yellow.

What does red and amber together mean in the UK?

It still means stop. It warns that green is about to show, so drivers should be ready but must not move until green appears.

Who invented the three-colour traffic light?

A three-colour, four-way signal was designed by William Potts, a Detroit police officer, in 1920. Earlier signals in London (1868) and Cleveland (1914) used two colours.

Related topics

Go further

Sources

• CIE S 004/E:2001 Colours of light signals — CIE (International Commission on Illumination) (Official standard or government)
• Traffic signals: a brief history — Washington State University Magazine (Academic)
• Traffic light (Wikipedia) — Wikipedia (Reference work)
• The Highway Code: Light signals controlling traffic — GOV.UK (Department for Transport) (Official standard or government)
• Manual on Uniform Traffic Control Devices (MUTCD) — US Federal Highway Administration (Official standard or government)

Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.