Why does red mean "stop" at traffic lights?

Answer: Road signals copied railway signals, where red already meant danger

Road signals copied railway signals, where red already meant danger ✓ — The first traffic light, put up outside the Houses of Parliament in London in 1868, was designed by a railway engineer, John Peake Knight, and it simply borrowed the colours railways had used since the 1830s: red for danger, because red already meant blood and fire to people long before there were trains. Red's long wavelength does scatter a little less in haze, which is a bonus for a warning lamp, but it is not why the colour was chosen. Green for go came later, after railways gave up white, which was too easy to confuse with an ordinary lamp.

Red is the brightest colour the human eye can see — How bright a signal looks depends on the lamp, not the colour, and the eye is actually most sensitive to yellow-green light. Red was chosen because it already meant danger in railway signalling, which road signals copied.

Early signal lamps could only be made in red and green — Coloured glass came in every colour in the 1860s. The palette was a convention inherited from railways, not a limit of the technology.