Question

why is red light used as traffic signal for stopping the vehicles?

Answer 1

Red color has high wave length of 700nm. It scatters less when compared to other colors. So it is very easy to identify red color. So it is used as traffic signal fir stoping.

Answer 2

Scattering of light waves in air is less for higher wavelngth.  In the visible light spectrum, Red color light has the highest wavelength and lowest frequency.  Hence it is used for traffic signals.  The drivers at a long distance are able to view the red light.
 
  Further the contrast between red and other colors (of vehicles, buildings, sky, road) on reads allows us to see it better.

Scattering of light is the reflection and diversion of light in all directions by various particles suspended in air.  These particles are impurities, dust, smoke and dew.