One part of me says "way to go"! Great work there on some instant feedback.

Another part knows that you may have guaranteed that the police are not on your side in this any longer, should the need arise to enlist their aid.

I may have simply sat next to them at the light while I called the cops, with their plate clearly in sight. Oh, and made it obvious that I was snapping a picture as well. Cell phones with cameras make that sort of thing so easy now.

The keys out of the ignition suggestion is awesome! I don't know how one would actually pull that off. I can't imagine extracting someone's keys while they're fighting to defend themselves and their property.