I always like that look of Argh! Why does the cyclist want to speak to me!?! when I get a driver's attention.
Last night a Honda Civic went past me with no lights on, except I noticed that the headlights were lit, and the brake lights were working. I knew I'd catch up at the queue for the Duddingston crossroads so pulled alongside and got her attention. She was very appreciative that someone had taken the time to let her know her rear lights weren't worknig (and might possibly save her being stopped by the cops at some point).
Spread the goodwill, some of it might rub off...