Traffic and safety colours

From rails to roads

Early British railways used red for danger. For a time, white meant clear and green meant caution. That proved risky, since a white light could be any lamp, and a broken red lens showed white. Green became clear and yellow became caution. The first road signal, a gas-lit lamp with red and green, was set up outside the Houses of Parliament in London in 1868 and soon exploded, injuring the policeman working it. Electric signals appeared in American cities in the 1910s, and a police officer in Detroit added the amber light in 1920.

An international code

Road signs and signals were harmonised by a United Nations convention signed in Vienna in 1968, which most of Europe and many Asian and African countries follow. India uses the same basic scheme. There are differences. Japan's go lights are a distinctly bluish green, and the Japanese word for the go signal is ao, the old term that covers blue and green. Red sits at the top or left of a signal so that position carries the message as well as colour, which matters for the many drivers with red-green colour blindness.

Safety colours

The standard known as ISO 3864 pairs each colour with a shape: a red circle with a slash for prohibition, a yellow triangle for warning, a blue circle for a mandatory action, a green rectangle for exits and first aid. High-visibility clothing uses fluorescent yellow-green, orange or red with reflective strips, and the yellow-green is close to the peak of the eye's daytime sensitivity. American school buses have been a standard yellow since 1939, chosen because black lettering on it is easy to read in the half light of early morning.

Colours on this page

Sources and further reading

• Traffic light on Wikipedia
• GOV.UK: The Highway Code, light signals controlling traffic
• Health and Safety Executive: Safety signs and signals (L64)
• Health and Safety Executive: High visibility clothing
• US Occupational Safety and Health Administration: Safety color code for marking physical hazards