How about something a bit less hostile such as dropping a postcard-sized pamphlet stating the rules of the road in the offending car's open window or having stickers made that also state the same thing that you can slap on the car's side or rear windows? Maybe a sticker saying "I don't give a shyte about cyclists!" Or I almost killed a biker rider today!"

Or you can always u-lock them. I had a friend who rode a motorcycle and always carried a small pipe with him. He told me it was for bashin in someone's taillights if they cut him off or flicked a cig out the window that would hit him, etc.

My name is progre-ss and I don't endorse this post. Well, not really.