Why did red and green get selected as the signal colors to mean “stop” and “go”?
One of the best references on questions about the history of traffic control devices such as traffic signals is a 1971 ITE publication entitled “Traffic Devices—Historical Aspects Thereof.” To summarize information from that publication: Traffic signals descended from the “semaphores” used by the railroads. The earliest train tracks were laid in the horse-and-wagon roads of England. The laws of England required the locomotives to be accompanied by a flagman who walked in front of the train to warn other vehicles and horses, and the flagman was required to carry red flags and red lanterns for this purpose. Red was a color associated with “danger.” (This may be associated with red being the color of blood.) After the tracks were taken off the roads in England and put on separate alignments, the railroads developed systems of semaphores to give directions of stop, go, or proceed with caution to train engineers. The earliest semaphores often used either red letters or a red background on
