Lets not dress up 'car slapping' as some sort of road safety device, its an ego driven means of showing dis-pleasure at another road user mis-deads.

I let most things go, I drive a car, I ride a motorbike, I make mistakes. I might be able to muster a "tut" or a shake of my head at careless driving, I am a reasonable person. However, when reckless driving endangers me, then I will always hit/slap the car, they need to know that they have done wrong, they need to realise that endangering a cyclist is not worth a few metres or seconds gained, and, in the main, the drivers aknowledge and accept their fault.