Why are traffic lights red, yellow, and green?

Railroad engineers first used traffic signals in the 1830s to direct trains. The earliest form of this signal system used a clear (standard/white) color to indicate "proceed" and a green color meaning "caution." Due to the fact that clear lamps were used in street lights and households, train conductors would frequently interpret them to mean "go," even if the real traffic signal shined green. Needless to say, this misinterpretation caused problems. On their second attempt, railroad engineers created a traffic signal with the colors red, yellow, and green. They selected a red color to indicate "stop" because they thought it intuitively elicited feelings of danger, perhaps because blood is red. Yellow was chosen to mean "caution" because it is close to red yet still distinguishable from it. Lastly, green was chosen rather haphazardly to indicate "proceed." Most likely, the green color was carried over from the original traffic signal design for simplicity's sake and the fact that it was not pervasive in streetlamps and households.

The first traffic signal to be installed on a road was in London in 1868. This gas-powered lantern had a red light meaning "stop" and a green light meaning "caution." Next came the creation of the electric traffic light, which also only had a red and green light. The yellow "caution" light was not carried over from railroad to road right away; rather, a buzzer on the traffic light would indicate that a signal change was about to occur. In 1920, however, William Potts reintroduced yellow lights in his creation of the first electric, three-color stop light in Detroit, MI. From there, green indicated "proceed," yellow "caution," and red "stop." The electric stoplight spread rapidly throughout the United States and then the world, keeping the same three signature colors.