http://www33.brinkster.com/iiiii/trfclt/
The world’s first three-color, four-direction, elec. traffic lamp, was installed at the intersection of Woodward Ave. and Fort Street, Detroit, Michigan in October, 1920. It was designed by Superintendent (then inspector) William L. Potts of the Signal Bureau, Detroit Police Department. Basic design remains practically unchanged today. The signal remained in use until 1924 and became a part of the world’s first synchronized signal system. This system extended from Jefferson to Adams on Woodward Avenue and was controlled manually from a tower at Woodward and Michigan.
museum archives, Henry Ford Museum & Greenfield Village (as of Jan 29 2003)
The meanings of the colored lights were essentially the same as today. Green meant “go”; red meant “stop”; and yellow (amber) meant “clear the intersection” (Mueller 1970). An analogous color scheme had been used by the railroads, where as early as 1899, the New York, New Haven and Hartford Railroad introduced a system wherein red, yellow, and green meant “stop”, “caution”, and “all clear” respectively (Brignano 1981).