I've just posted a bad driver thread, so I'll add something here, too. On the same trip yesterday there was a lady who blew her horn as a thanks because I waved her past us when she couldn't see. I think we always get a friendly wave or honk every time it's very clear to the driver that we are trying to help. On Thursday we were out riding and a car was waiting behind us to pass. He couldn't see much, but due to the long peloton we saw that everything was clear, so we waved him past us - the front rider clearly sprinted forwards to check the road and then waved back. We got a friendly honk back.

Most drivers are really nice, but the rest of them are so dangerous - I sometimes get most of the cardiac training from being freaked out by mad drivers!