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Traffic Lights

Date: Fri Nov 15 14:06:25 1996
Posted by: Deepak Shenoy
Grade level: other
School/Organization: University of Missouri-Columbia Alumni
City: Columbia State/Province: Missouri
Country: India
Area of science: Other
Message ID: 848088385.Ot


Why are the traffic lights Red, Amber and Green ? Why not blue or purple ? On what basis were these colours chosen ? I have heard of two explanations : One that the colour red is the most easily visible even during a foggy night and the other that red-amber-green are the three colours that are most easily discernible even to a person who is colour-blind. Can you clarify ?

You are right that red is most easily seen through a fog, and I think this is why stop lights are red. When light passes through fog, particles in the fog scatter the light away, like the way a pinball will bounce around inside a pinball machine. This scattering is strongly dependent on the color of the light. Blue light is scattered the strongest, and red the least. This means that in a fog, a blue light might be completely invisible to you because all the blue light has been scattered away, while a red light more easily penetrates the fog.

[Oops! I blew it here. The particles in fog-- water vapor-- do not scatter any colors preferentially. If they did, then fog wouldn't look white, it would look blue! I put in that part because I have seen red lights better through fog than green. However, red light actually travels less through water than blue! This is why the oceans are blue; they absorb red light so that only the blue part gets out. I am not positive this also happens in fog, but I will do some research and find out. Until then, ignore the last part of the above paragraph! --The Bad Astronomer, 11/11/98]

I am not sure about the second part of your question, however. I had a friend who was color-blind, and he could only tell when to go at a traffic light by knowing the bottom light was on! He went to another country (Canada?) and the three lights were arranged horizontally, and he had to ask someone which light was red and which was green! I know he did not have a rare form of color blindness, so I think that the actual colors chosen were not based on color blindness. BUT I am not sure; I'm just making a guess. You might try calling your state department of transportation. Someone there might be able to help you.

One more thing: scattering, which I described above, is also why the sky is blue. I have written a longer explanation of this effect and put it on my WWW pages. Click here to see it!

Note added September 23, 1999: Mad Reader Russ Bogel (an industrial controls engineer) sent me a note saying that the colors in traffic lights were most likely chosen because they were already standard for other equipment. Russ says:

In industry back at the turn of the 19/20th century, motors, valves and other electrical industrial equipment was standardized on red for stop, green for go. If an indicator light for a motor was red, that meant it was stopped, green meant it was going. As many people were already familiar with this patter (most notably the traffic light design engineers) it was used for the traffic lights too.

That has the ring of truth to it, but unfortunately only begs the question: why were red and green chosen for those uses? Oh well.

©2008 Phil Plait. All Rights Reserved.

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