Slap a car the wrong way and you could end up with a broken hand. I've slapped a few cars. Most of the time it was to push off to keep my distance. I did slap the hood of a very slow moving car that was holding up a line of cars along an admittadly senic street with beautiful historic houses on one side and an equally beautiful vista of the river and nearby islands on the other side. I passed the line and slapped the uberslow car on the hood as I past to get his attention.