Traffic Light

A traffic light is a signaling device positioned at road intersections and pedestrian crossings that uses colored lights to tell drivers and pedestrians when to stop, prepare to stop or go. The English railway signaling engineer John Peake Knight designed the first traffic signal, a manually operated semaphore installed at Parliament Square in London in December 1868 that showed a red or green gas light at night, though it was withdrawn after exploding on 2 January 1869. The American police officer Lester Wire developed the first electric traffic light in 1912 in Salt Lake City, Utah, and a similar device was installed in August 1914 at East 105th Street and Euclid Avenue in Cleveland, Ohio; the American inventor William Potts then built the first four-way, three-color traffic light in Detroit, Michigan, in 1920, adding an amber caution light to the existing red and green, and had automated versions installed across the city by 1921.