This question is an exact duplicate of:
Why are danger signal in red, when the eye is most sensitive to yellow-green? You can check luminosity function for more details...
I believe you should have googled first: google hits
Especially the second link very clearly explains the main reason:
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...
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)
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?
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.
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.
A number of people have said that red = danger because of its association with blood, but no one has mentioned that in, say, China, red is the color of fortune and good luck. Brides wear red, not white. So, it's as much a cultural thing as anything else. In European cultures, red was associated with blood, and therefore, danger; and European cultures came to dominate the world in the 19th and 20th centuries.
Thank you for your interest in this question.
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