The signals would work like this: Red - Stop Yellow - Get ready for lights ... or humans through the junction. Once the light turns white, humans simply follow the car in front of them, which ...

behave differently to the green ones and the yellow ones and the red ones. They come out at a shallower angle, and that's why you get the full spectrum of colours from the white light of the sun.

You can use a prism to split (or disperse) white light into a spectrum of colours: red, orange, yellow, green, blue, indigo and violet. Dispersed means that the colours are separated out.