Depending on how much of an idiot the driver is being I either just glance at the drivers seat, or glance and shake my head, while just continuing to ride, it makes them look like they are in the wrong. Of course this only works if you are riding safely and obeying traffic laws yourself. I ride through the Philadelphia ever day so I run into bad drivers constantly. At times I've even had other drivers have blocked the dangerous driver in and give me space to ride (rare but it has occurred on a few occasions, e.g below). Due to the bad riders who flip off everyone and cut people off, etc. I try to give a little wave of thanks to any driver who gives me room to get around a parked car or lets me take my turn at a stop sign. I'm not sure if it helps much but hopefully some drivers come to think that not all cyclists are bad.
also +1 to alienator's "Nothing gets solved by cutting loose on a driver. Doing so, however, could further endanger yourself and other riders."
Of course there are always those "insert expletives" (insert whatever word you like) drivers. Just yesterday I was riding back to my apartment at 8:00pm, in a bike lane, with flashing lights. Some guy behind me railed on his horn to get out of the way. Since I was in the bike lane already I just stayed put, I wasn't sure of anything else to do. Looking back he was probably ticked that he couldn't pass the car in the actual traffic lane by using the bike lane because I was riding the speed of traffic; 24ish mph. Eventually traffic allowed him to get past me and he made sure to swerve into the bike lane coming within two feet of clipping my front wheel (both lanes were clear in front of him I'm pretty sure he did it just because he is a "insert expletives". Of course he reaches a red light 50ft latter, and swerves to stop me from riding up to the light. When the light turned green the taxi in front of him moved so that no car could get by and drove slowly enough so the guy was always stuck behind me. The "insert expletives" driver became rather pissed, but the taxi just sat there in front of him and waved for me to continue on (I gestured gratitude). Of course I laughed and couldn't resist flashing a rather large smile at the "insert expletives" driver as I continued on my way.
5' 9" 145lbs male who is a rower that likes to cycle.
love climbing. VO2 55.9 anaerobic threashold 178bpm aerobic threashold 164bpm.