What is it about getting behind the wheel of a car that turns ordinary decent human beings (assuming they are, of course) and turns them into ignorant, arrogant pigs!? Don't get me wrong. I drive too (if the journey is over 10 miles) and I'm far from perfect but I don't sound my horn at people who just happen to be running on the road (especially when there is no footpath) or cyclists (yes, I do that too). Nor do I lean out of the window to make rude gestures or scream at runners or cyclists.
Incidently, when I get this sort of abuse on my bike I often catch the offending car at the next set of lights, lightly tap on the window and ask 'sorry, were you trying to attract my attention?' That works quite well (I'm 6'3") as the poor morons can't quite belive that the pile of junk they paid so much for is not always the fastest way through town.