Why is red colour used as the stopping light at traffic signals?

• The atmospheric particles scatter the light which travels through them. This reduces the visibility of the signal. 
• So, the colour which is least scattered by the atmospheric particles is selected.
• Red colour has the longest wavelength among the seven colours of white light and therefore it is not scattered easily.

Therefore, stopping light at traffic signals has a red colour as it has a maximum wavelength and least scatter.