William Potts created the first four-way, three-color traffic light in Detroit, Michigan. His 1920 design brought red, amber, and green signals together so traffic in four directions could be coordinated.
Potts was Detroit’s superintendent of signals and understood the limits of earlier two-color systems. Red and green lights could stop and release traffic, but they gave drivers no clear warning that a change was imminent. Potts added an amber phase between green and red, allowing drivers time to prepare to stop.
The first four-way, three-color signal tower was installed at Woodward and Michigan Avenues in Detroit in October 1920. In 1921, Potts installed automatic four-way, three-color lights in 15 Detroit towers. His work helped establish the basic sequence still recognisable at road intersections.
The question is sometimes confused with Garrett Morgan, who patented a manually operated three-position signal in 1923. Morgan’s patent was important, but Potts is credited with the earlier four-way, three-color traffic light.