Not gonna lie... I used to habit once in a while while working to walk in Manhattan of just tapping a fender when someone did this shit. Until a guy got out of a livery cab and tried with an NYPD camera right there (he even pointed to it as evidence of me damaging his car - with an open palm - OK, dude), then proceeded to try and pummel me. Fortunately I was a runner at the time.

It did remind me though, just like in a car itself, you don't know what the person in the other car is thinking.