Who's asking: Henry Lavinder, Mechanicsville, VA
This will sound simple-minded, but as best I can tell, green means "go" because red means "stop."
Red has always been the color of danger. Red is the first color babies see, and the one color every language has a word for. When early railroad engineers needed to put up signal lights along the tracks, red was the obvious choice.
Red meant "stop," and when the light changed, the train could go again. The very first railway signal lights changed from red to clear -- but that didn't work, because the clear light was too hard to see in the daytime, and it was too easy to confuse the clear signal light with other lights at night.
It's an oversimplification to say that green is the opposite of red -- but, roughly and generally and in a non-scientific sense, green is the opposite of red. So when they decided to make the "go" signal a color instead of clear, green was an easy call.
The very first traffic light was installed in London, in 1868, to govern the buggy traffic in front of the British House of Commons. It used red and green lights for "stop" and "go."
The first red-and-green traffic light in the United States was put up in Salt Lake City, in 1912. Red-and-green traffic lights started to catch on by the end of that decade -- San Francisco installed the first traffic light system in 1917. The amber "caution" light was added in 1920 in Detroit, and the design we use now -- red on top, green on the bottom -- was patented in 1923.