I usually point at their license plate too, although on my way home I had to point at the solidly red traffic light. I start riding after waiting at MY red light and this woman in a benz suv comes through the intersection and jams on her brakes/horn. I pointed at her red light and she yelled at me. Awesome.

Reminds me of when I nearly creamed a pedestrian who walked out into traffic and proceeded to tell me to watch where I'm going. Riiiiight.