Why is Red the International Color for Stop?

Stop signs originated in Detroit, Michigan in 1915. The first had black letters on a white background and were somewhat smaller than the modern one. In 1924, the sign changed to black on yellow. In 1954 the US Federal Highway Administration (FHA) published the The Manual on Uniform Traffic Control Devices (MUTCD). It was in this manual that the stop sign was standardized as red with white type. The color coding for stop goes a bit further back. Red became a color connected with stop when the first primitive railroad signaling devices were developed in the 1830s and 1840s. Inittialy red meant “stop,” green meant “caution,” and clear (i.e., white) meant “go.” They later figured out that Go as a white lead was problomatic when confused with other lights.