William Potts

For the U.S. Army general, see William E. Potts.

William Potts (May 1883 – 1947), a Detroit police officer, is credited with inventing the modern, three-lens traffic light in Detroit in 1920 (the two-lens, red/green traffic signal was invented in London in 1868).

William Potts was born in Bad Axe, Michigan. The 1900 census lists Potts as 17 years old and a police officer. By 1910, he was married to Grace (Baker) Potts, and they subsequently had 4 children. By 1920, Potts had become the 'superintendent, signal person police' for the city of Detroit. Charged with finding a way to control traffic and with the old system of police directing traffic increasing outmoded (there were some two-color signals, with green and red lights already), Potts invented a 'yellow' or 'amber' light which removed the need for police to switch the signal.

The first four-way traffic signal tower in the world was located at the Woodward and Michigan Avenue intersection in October 1920. The tower was manually operated and had 12 lamps, three in each direction. In December 1920, signals were added along Woodward Avenue at Grand River, State, Fort and Congress, but all were controlled from the manual tower at Woodward and Michigan.

Then in 1921, Potts, who was a Superintendent of Signals for the Police Department, installed his first automatic signal light system in 15 towers. During the next 25 years the location and operation of traffic lights throughout the world changed considerably, but the original principle developed by Potts remained. The last traffic tower in Detroit was removed from East Jefferson and Grand Boulevard in 1930. The traffic light system has been modernized with corner posts, hanging center lights and mast arm extensions.

Twenty-six years later, there were more than 600 signalized intersections in Detroit with more than 11,000 lamps in service. Throughout the United States, the American Automobile Association reports, there are more than 36,000 signalized intersections.

According to Potts, the idea for red, green and amber lights was first used by railroads and his first automatic timing device at Woodward and Michigan was built from a sign flasher that cost $37.

“There was a definite growing need for traffic control, or as they say, ‘Necessity is the mother of invention,’” Potts related. “We couldn’t coordinate men at different intersections with a manual system, but we could coordinate automatic electric signals.”

Potts held no patents on the device, but is generally regarded as its inventor. In 1924 the traffic signal device was being made commercially, and in 1928 a legal dispute arose between manufacturers. The United States District Court of Appeals in Chicago handed down a decision in which Potts was credited with constructing and initiating the use of the first four-way traffic signal.

The first traffic lights had only four-inch lenses and the shell was wood with a tin cover. An original lamp was presented to Henry Ford’s collection of Americana at the Edison Institute, Greenfield Village, in 1938.

Potts retired from the police force in 1939 and joined the Safety and Traffic Department at Automobile Club of Michigan. He was initially recognized only locally for his invention and died in obscurity.

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