Why are stop signs red?

Evidence suggests the signal colors were borrowed from the railroads, where red meant "stop," green meant "caution," and clear (white) meant "go." These came into use in the 1830s and 40s. Around 1914, a broken red lens caused a locomotive engineer to mistake this for a clear "go" and smash into an oncoming train with disasterous results (another example of Deadly Design). Subsequently, the red/yellow/green signals we use today were adopted; yellow was thought to provide the greatest contrast to the other two colors. Red has been a symbol of danger since the time of the Greeks. The reasons for the use of green have been lost to history.

However, not all stop signs are red.

The Vienna Convention on Road Signs and Signals standardized colors for stop signs are either an octagon with a red background, white border, and the word Stop written in white or a circular shape with a yellow or white background, red border, and the word Stop written in blue, or black inside red inverted triangle. 62 countries, including the UK, have signed the Vienna Convention on Road Signs and Signals.

The US uses the Manual on Uniform Traffic Control Devices (MUTCD), which states that the minimum requirements for stop signs are a 30"x 30" octagon with white lettering and border on a red background. Canda, Australia, New Zealand, and Ireland use road signs based on the MUTCD.

Japan uses an inverted solid red triangle and Zimbabwe uses a disc bearing a black cross.