Where does the Yellow Light come from?

Having stop and go lights correspond nicely to what a traffic policeman might signal to motorists. But where does the yellow light come from? RoadTrafficSigns.com is here to provide you with the answer. The inventor was William Potts, a Detroit policeman, who modified railway signal equipment into a traffic signal with the first yellow light, at the corner of Woodward Avenue and Michigan Avenue, Detroit. This happened in 1920.
Traffic lights are descended from 19th-century railway signals, which traditionally had three positions: stop, proceed with caution, and all clear. These would be done with red, yellow, and green lamps, or, in the alternative, with semaphore signals. The first traffic light, installed in London in 1868, was actually a modified railway semaphore signal. (That traffic light lasted a few weeks before it malfunctioned and blew up, injuring the policeman charged with operating it.)
Early traffic lights all drew on the railway experience. In 1912, law school dropout turned policeman Lester Wire, of Salt Lake City, invented the first traffic light. Several other inventors, working in parallel, reached the same conclusion simultaneously in a process of convergent evolution. Potts’ contribution was to adopt the railway “proceed with caution” signal and to turn it into the modern yellow light.