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The first crude traffic light was a manually operated gas lantern installed in London around 1868. The signal consisted of two colors -- red meant "stop," while green meant "caution." One day the lantern exploded and injured the policeman who was operating it. It was painfully obvious a more efficient traffic signal was needed. In 1920 Police Officer William Potts addressed the chaotic traffic of Detroit, Michigan, by co-opting the red, amber, and green automatic traffic light system used on railroads. Around the same time, the prolific African American inventor Garrett Morgan devised and patented the precursor to the automatic traffic lights we use today.
Apparently the colors of red and green were standard in the electrical industry at the time. The makers of railroad signals and traffic lights simply adopted the familiar color scheme. We can really only speculate as to why these colors were originally used in the industry. An About.com entry on color symbolism points out that different colors evoke different emotions in humans. Red generally symbolizes danger or warning, making it a good choice for "stop." Green is calming to us, probably because it is prolific
in nature and a sign of health and abundance, so it might have lent itself to mean "go." Curiously, yellow or amber, is associated with warmth, happiness, and the sun. Despite these connotations, it was chosen to symbolize "caution." Perhaps because it contrasts well with green and red, and is highly visible without being too harsh on the eyes of motorists. Today, yellow is commonly used as a warning symbol for both traffic signs and alert systems.
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