Question

why is red coloured light in used traffic signals

Answer 1

Because the red colour has more wavelength than other colour.

Answer 2

Because red light has lower frequency, that is higher wavelength. So it is gets scattered a lot less(by tiny particles) than blue (or higher frequency lights).

Okay so imagine you have to run in a bumpy ground, going from point A to B.

Imagine running in a zig zag way towards B, you will cover a lot of distance, and run into lots of bumps.

Now imagine you ran in straight path, you will meet bumps, but lot less.

See you can imagine red as long straighter lines (technically waves,yes), and blue going lots of zig zag way. So blue actually like covers more ground.

It's an analogy, but it is good enough. As light waves are complicated( disturbances in electromagnetic fields! ), this explanation is a simplification.

But I hope it helps.

I am assuming you know what scattering is, what waves look like given it's frequency, if you have any doubt, you can ask.