I got kinda tired of having people honk the crap out of their horns at me.
Just made up some little laminated cards with magnetic tape. I'll post one on the next car who blows by, honking away. Here's the text

Dear Motorist:

Based on your lengthy honk earlier, I am led to believe that you were resentful of having to travel behind myself, a slow commuter cyclist. I do understand that the 5 seconds which you lost traveling behind me were very precious to you, and am sorry that they were wasted. However, I think you should know that I and other commuter cyclists are extremely resentful of the fact that automobiles such as yours are responsible for filling the atmosphere with outrageous amounts of carbon dioxide, inducing global warming, and effectively destroying the planet for all of our children.



They're easy to make, I suggest trying it.