I don't do it very often, but when I do it is generally not done in anger. Most motorists are simply ignorant. They think of themselves as law-abiding good citizens, so I play to their personal story and give them the wording and statute number for the laws they unwittingly violated. I try to stress the reason for the law and do a lot of smiling. Most times they respond positively and appreciate the information. Either that, or they realize I'll leave sooner if they say thank you.
I live in far too small a city to expect anonymity. I expect that most of the people I encounter on any given day will be people that I have encountered before and will encounter again. It's not going to do me much good to scream at them.