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.
William L. Potts, a Detroit police inspector, is universally accepted as the originator of the first four-way traffic signal.
Note: The Red Tape Editor arbitrarily designated the 15th since the article only says October 1920.
For more information, see Sheldon Moyer, Mr. Trafficlight, Motor News, March 1947, pp.14-15, 27.