Monday, February 19, 2007

Why does green mean go?

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.


JIM LAMB said...

One of the interesting things that only use old farts would remember is that the red on top configuration didn't become a universal standard until quite late. There was a convention in the fifties that aimed at getting all the state vehicle codes together so that what was required in one state wouldn't get you a ticket in another state. That was before congress got into the business in a big way of passing laws that were better left on the table.
Many of the cops and local politicians were Irish immigrants or first generation and could not abide the idea of King William's orange (red or amber) over the Green of St. Patrick, so many smaller cities had the green on top.
As time went on the automatic sensors that turned the light green as a car approached were installed.
When forced to change to the current standard, many cities just changed the lenses, so in a lot of towns the lights would turn red as you came up to the intersection.
Since this took money and effort to change, this problem lasted into the sixties in some locations.
One last insult to the Downstate Protestants.

Madmax said...

Red is used because the color has a very large wavelength in the color spectrum. It can travel much farther. Along the same lines of why the sky is blue and sunsets are orange reddish.