In DC, I've heard it's illegal to honk your car horn except for safety reasons. If true, that's a revenue source for the city far more lucrative than parking violations.
Do all the people who've been behind me at intersections who start honking really want me to run over the pedestrians in the crosswalk to make my turn (oh -- maybe they're off duty DC bus drivers?).
I've started telling cabbies this: First, I give my destination, then I say, "and if you honk for anything other than safety reasons, there will be no tip." So far, it's worked.