His proposal was accepted and, on December 10, 1868, the system was put in place at the junction of Great George and Bridge Street in London, near Parliament. At night, his system used the red and green colors for stop and go. In the daytime, this semaphore method used an arm or arms that could be raised or lowered by a police officer, notifying carriages when they should stop when the arm(s) stuck out sideways. A railway manager and engineer named John Peake Knight, who specialized in designing signaling systems for the British railway, approached the Metropolitan Police with the idea of using a semaphore/lighted system for road traffic. So how did this system transfer to the road? In London, England in 1865 there was a growing concern over the amount of horse-drawn traffic causing danger to pedestrians trying to cross the roads. Thus, the railroad decided to change it so the green light meant go and a caution “yellow” was chosen, primarily because the color is so distinct from the other two colors used.
This ended with a train running a “stop” signal and crashing into another train. For instance, in an incident in 1914 a red lens fell out of its holder leaving the white light behind it exposed. The choice of a white light for go turned out to cause a lot of problems. For the other colors, they chose white as the color for go and green as the color for caution. They chose red as the color for stop, it is thought, because red has for centuries been used to indicate danger.
At this time, railroad companies developed a lighted means to let train engineers know when to stop or go, with different lighted colors representing different actions. While some of the specifics have been lost to history, it is known that this color scheme derives from a system used by the railroad industry since the 1830s. Today I found out the origin of the green, yellow, and red color scheme for traffic lights.
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