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Why are danger signal in red, when the eye is most sensitive to yellow-green? You can check luminosity function for more details...

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Beware that there probably does not exist a concrete answer to this question and that most will be speculative. –  santa claus Mar 27 '13 at 8:07
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Essentially a duplicate of physics.stackexchange.com/q/52581/2451 –  Qmechanic Mar 27 '13 at 12:13
    
I'm surprised this was asked here instead of on: ux.stackexchange.com –  Dan Neely Mar 27 '13 at 13:38
    
The answers are very much physics related so I think Kazi made the right choice to post it here. –  Michiel Mar 27 '13 at 17:57
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4 Answers

I believe you should have googled first: google hits

Especially the second link very clearly explains the main reason:

The primary reason why the color red is used for danger signals is that red light is scattered the least by air molecules. The effect of scattering is inversely related to the fourth power of the wavelength of a color. Therefore blue which has the least wavelength of all the visible radiations is scattered the most and red which has the highest wavelength of all the colors we can see is scattered the least. So red light is able to travel the longest distance through fog, rain, and the alike.

Also, red is a color we inherently perceive as one that is associated with danger. I guess it has been genetically programmed as hot objects are red in color and also red is the color of blood. These reasons make danger signals the most effective when they are painted in red.

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Yes, It's true...

We know that our eyes have three types of cone cells - S (short), M (medium) & L (large). The naming is done in order to differentiate the cells from "which cell absorbs which color". S to Blue, M to Green and L to red. The peak wavelength of L is 564 nm, yellowish-green. The peak of M is 534 nm, bluish-green. The peak of S is 420 nm, bluish-violet.

This can also be predicted from the responsivity curves. At some comparatively higher magnitude, both M & L cells are sensitive to this 564 nm. So, our eye is most sensitive to yellowish-green. You're right. There's one more thing to notice the the Luminosity function is based on two types of vision. Photopic and scotopic. Photopic vision is when you have brighter conditions (sufficient enough) for eye vision. Hence, the sensitivity towards green is only on well-lit conditions (cone-cells). While on the other hand, scotopic vision is during the night-time when the rod-cells are highly sensitive to even dim lights. Both cone & rod cells are sensitive to green (but, different wavelengths).

Have a look at the Purkinje effect for more stories on greenish...


Why Red?

Okay, enough with all those sensitivities. Now, to Physics. The reason behind the red-theme for traffic lights is nothing more than just Rayleigh scattering. The amount of scattering is inversely proportional to the fourth power of the light's wavelength. $S\sim1/\lambda^4$

Red, having the higher wavelength is the least scattered. So, it can travel longer distances without appreciable loss by scattering than any other colored lights. Our eyes may seem to be sensitive when the green light is nearer to us (probably, a km). But, red can reach us more effectively. And so, we can spot it very easily at farther distances. (All circle the same thing)

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My hypothesis is that the use of red for danger signals is because we instinctively recognize red things as potentially dangerous. Aposemitism (Wikipedia) is the use of bright colors by prey to signal the presence of secondary defense mechanisms, such as toxicity. Aposematism only functions as a defense mechanism because predators, such as humans, instinctively recognize and avoid brightly-colored markings.

Take a look at the yellow-banded poison dart frog, and the granular poison frog. Does their coloration look familiar?

Yellow-banded poison dart frog, granular poison frog, and warning signs

Of course, this kind of "pushes back" the question.

Q: Why are danger signs red or yellow?

A: Aposematism has caused these colors to be instinctively associated with danger.

Q: Why are poisonous frogs red, when the eye is most sensitive to yellow-green?

A: Some are yellow, but green would blend in too much with foliage.

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I believe that is because human blood is more red, than yellow-green. We evolved from basic tribe based societies yet kept the same idea, blood means there is danger near by.

When we see blood, it sets off a warning in our minds of unknown dangers lurking about, this is why we associate red with danger.

Now with modern warning lights it's because the color red is used for danger signals is because red light is scattered less by air molecules than other colors.

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As michielm said, in the quote, hot objects are associated with red, yet this is not always the case, somethings will also glow white, but why is red the only danger color? –  user10108 Mar 27 '13 at 6:26
    
Because if something is hot enough to glow white it's probably too late to run away :p (tongue in cheek) –  Thomas Mar 27 '13 at 9:26
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