why are danger signals red in colour?

The major reason has to do with how light gets absorbed in fog,
particulates, haze, etc. We know that light is scattered (and thus attenuates or gets weaker) as it travels through a scattering medium composed of small particles. As it turns out, red light is scattered (is weakened) less than other colours. In fact, of the light spectrum, blue is scattered the most and red the least. Why?

Lord Rayleigh was the first to discover that small particles in air scatter
different light colours with different effectiveness; this effectiveness
being proportional to the inverse of the wavelength to the power 4. Blue
light which has the shortest wavelength is scattered the most and red,
having the longest wavelength, the least. As such, at sunset, when the sun
rays travel the longest distance, more of the red part of the Sun's
spectrum reaches us. The other colors do not make it as well because they
have been mostly scattered out along the way. The same explanation goes
for the color of the sky. Away from the sun, we do not see the direct
sunlight but the scattered light. Small air molecules scatter blue light
more efficiently out of the direct sun rays, and that is what we
see. Thus, the sky appears blue.

In summary, red light weakens the least in travelling in the air. That is
Why stop signs and other critical lights are red.