Why red, green and yellow??

From:  "Marc"

Why do traffic lights use red, green and yellow for the respective meanings?

TB did some research and found this: 

The very first traffic light was a revolving gas lantern with red and green lights installed in a London intersection in 1868. This was before the autos came about. They used red because it was the color of blood which for thousands of years denoted danger. The colors green and yellow were chosen by the trial and error method. Green was originally selected as CAUTION and clear was chosen as GO but in certain lighting conditions they were mixed up after the fact causing several fatal accidents.
This failure prompted the railroad engineers to alter their color selections to red for stop, green for go, and yellow for caution. Traffic engineers, either lacking in ingenuity or a work ethic, scurried off with this system of color coding, and instituted the very first electric stoplight in Cleveland, Ohio in 1914. The first signal did not include the color yellow for caution, but that was later added within a few years. Railroad engineers, not traffic engineers, should be credited for the lives saved in the interim, by their system of coding warning signals red, yellow, and green.

Catt gave us this information: 

This is a guess, but Human emotions towards the colors would spark reactions Red=Danger-Stop. Green=Good-Go Amber=Calming but also danger-Caution making the movements instinctive

light came up with this simple answer: 

they are colors that we can easy see and tell apart.

Edvado concluded and gave us this: 

Start with the middle, which is actually "amber" the color that is the exact center of the visible light spectrum. It is as far away from Red and Green as possible.