Why Do Traffic Lights Use Red, Yellow, and Green?

Road traffic inherited its most important colors from the railways.

Before automobiles crowded city streets, railway companies already needed signals that train drivers could recognize quickly from a distance. Red had a long association with danger and was adopted as the instruction to stop. Green eventually came to mean that the track ahead was clear. Yellow warned that a driver should proceed cautiously or prepare to stop.

Early railway systems were not completely standardized. Some used white to mean clear, but this created a dangerous possibility. If a colored lens broke or fell away, the exposed lamp could appear white and falsely tell a driver to continue. Green became the safer clear signal, while yellow took the warning role.

The first traffic signal installed outside Parliament in London in 1868 used railway style arms and red and green gas lamps. It was manually operated and lasted less than a month before leaking gas caused an explosion.

Electric road signals appeared during the early twentieth century. James Hoge’s system in Cleveland borrowed red and green from railway practice. In 1920, Detroit police officer William Potts developed a four direction signal that added yellow between them, giving drivers time to react before the light turned red.

The colors now appear so natural that they seem to carry universal meanings. In reality, their modern language was developed through decades of railway experience, then transferred from tracks to intersections when cities needed machines to negotiate the arrival of the automobile.