Stop signals are usually red because the color stands out against most natural backgrounds.

The longer version

Red sits at the long-wavelength end of the spectrum, so it scatters least in haze, fog, and rain and carries furthest through dirty air. It also contrasts strongly against foliage, soil, and sky, which are dominated by greens, browns, and blues. Red also triggers strong associations with blood and fire, which likely reinforced the choice.

The convention began with railways, which used red for danger long before road traffic existed, partly because early red lenses were reliable to produce. Green was adopted as the opposite signal, and amber as a warning. Red-green signalling caused trouble for colour-deficient drivers, which is why position and shape carry the same meaning.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.