But were they clean?

My mother always used to tell to make sure I wore clean underwear in case I was hit by car. I never fully understood the logic. Do clean knickers somehow protect you from the car? And to be honest if I'm going to get hit by a car it's entirely possible that my underwear would not remain clean very long. Either way it's got to be better than fainting from your vibrating knickers. Poor woman. Very funny though.