Colourlovers weblog has some brief cultural histories of popular "standard" colors, offering historical reasons for how some colors became defaults for specific meanings: Pink for boys, red for girls; republicans red, democrats blue; white flag for surrender, etc. Here's an entry for "red = stop" (along with "green = go" and "yellow = caution"):

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.

(These are not definitive histories, btw—citations to sources are offered for most, but they're mostly to single websites.)