The lack of distinction between red and green is considered the most common form of colour blindness. Why, then, are these colours internationally used to determine 'Stop' and 'Go' in traffic lights?

The first traffic signals were semaphore signals similar to those seen on railyway lines. The nighttime illumination for those had been red and green to indicate danger (stop) and clear (continue) with yellow meaning caution (prepare to stop, next signal is at danger). The same colors were put into use on roads.

Most (about 75%) of red-green deficient people have only diminished perception, not complete loss, and can distinguish the intense, highly saturated traffic light colors with almost the same ease as fully sighted people. The use of a bluish green hue for many green lights is intended as an additional aid in perception. Even for those who cannot distinguish the colors, safe daytime (but not night) driving is usually possible due to the positioning of the lights; the exception is for some protanopes (who cannot see red at all) because to them, a red light may appear to be off and brake lights can be hard to notice.

https://en.wikipedia.org/wiki/History_of_traffic_lights
https://en.wikipedia.org/wiki/Color_blindness