From Today I Found Out:
The Origin of the Green, Yellow, and Red Color Scheme for Traffic LightsQuoteThis color scheme derives from a system used by the railroad industry since the 1830s. At this time, railroad companies developed a lighted means to let train engineers know when to stop or go, with different lighted colors representing different actions. They chose red as the color for stop, it is thought, because red has for centuries been used to indicate danger. For the other colors, they chose white as the color for go and green as the color for caution.
FULL ARTICLE HERE (http://markholtz.info/8n)