Why the colours on traffic lights were chosen?

We can really only speculate as to why these colors were originally used in the industry. An About.com entry on color symbolism points out that different colors evoke different emotions in humans. Redgenerally symbolizes danger or warning, making it a good choice for "stop." Green is calming to us, probably because it is prolific in nature and a sign of health and abundance, so it might have lent itself to mean "go."

Curiously, yellow or amber, is associated with warmth, happiness, and the sun. Despite these connotations, it was chosen to symbolize "caution." Perhaps because it contrasts well with green and red, and is highly visible without being too harsh on the eyes of motorists. Today, yellow is commonly used as a warning symbol for both traffic signs and alert systems.