Question

define dispersion of light .how it is related to traffic lights and their functions

Answer 1

splitting of white light into its component colours is called dispresion

Answer 2

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having this common property may be termed dispersive media. Sometimes the term chromatic dispersion is used for specificity.
This most likely started because it's the color with the longest wavelength so it can be seen from a greater distance than other colors. As far as yellow goes, it was used to caution drivers because it has a slightly shorter wavelength than red, but not as short as green.