interesting.....I didn't think of cars at all (and yet I am a driver, and one with a 35 inch inside leg at that) i thought maybe you got it from kneeling on nastly uneven road surfaces (or gravel in parks) checking to see if drunks are unconcious or actually dead. I assume that you end up kneeling at some point every day, although I suppose you could always avoid the problem by going down on your hunkers.