Why are traffic lights red, yellow and green?
Quick Answer
Red, green and yellow were borrowed from railway signals, which chose red for danger because it is the most visible colour and has the longest wavelength.
The Full Story
Railway signalling in the 1800s established red for stop and green for go. Red was chosen for danger because it has the longest wavelength of visible light, making it visible from the farthest distance and least affected by atmospheric scattering. Early railways used white for go, but after a lens fell out and a white starlight caused an accident, green replaced it. Yellow was added as a caution colour because it sits between red and green on the spectrum and is highly visible. The first electric traffic signal, using red and green, was installed in Cleveland, Ohio, in 1914. Yellow was added to traffic lights in the 1920s.
Key Facts
YouTube Angle
Suggested video title for this topic:
"From railway signals to city streets — why these three colours"