First Tri-Color, Four-Directional Traffic Signal, 1920

Detroit traffic officer William Potts created this three-color, four-way traffic signal in 1920. Previous illuminated signals used only red and green lights. The addition of an amber "caution" light made driving safer and the three-color signal became the standard by the mid-1930s. This first traffic light was installed at Woodward Avenue and Fort Street in Detroit, Michigan.