The first electric traffic light was installed in Cleveland Ohio, at the intersection of Euclid and East 105th Street on August 5th, 1914. It employed red and green lights and "stop" and "move" directives. It was manually controlled, and the design was patented by James Hoge.
Was it the first traffic light? Well, that depends on what aspects you're considering. As early as 1868, a "traffic semaphore" was installed in London to control horse-drawn traffic and protect pedestrians near the British House of Commons. This device was manually operated and featured red and green lights (a gas lamp with red and green lenses) and signs directing the traffic.
The first automated street control device was patented by Earnest Sirrine of Chicago which used the non-illuminated words "stop" and "go".
A device using red and green electric lights was invented by Lester Wire of Salt Lake City in 1910, but he did not apply for a patent.
The red-amber-green pattern that is so familiar to us today was adopted from the color system used by the railroads, and made its appearance in Detroit in 1920. William Potts, a Detroit policeman, was the inventor of the (unpatented) device, which he constructed from a $37 sign flasher. After his retirement from the police force, Potts became an analyst for the Automobile Club of Michigan.
Photo Credit: Davide Guglielmo, http://www.broken-arts.com
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