You should check your sources - where is such sign language against the law? This speech is generally protected in the first amendment of the US constitution.
Regardless, humans are social and territorial creatures. Occasional exchanges are natural and should be expected. In the end its just a pissing match which very rarely progresses to anything further. When given the chance to induce real physical injury to a complete stranger - most psychologicaly healthy indivuals would back down.
To mix it up from time to time, I like to react in unexpected (but generally safe) ways. Maybe I should practice a falling in a convincing manner - it would be an interesting social experiment to see how quickly some change their attitudes.
Threatening drivers, to me, are a helpful reminder that all vehicular traffic is dangerous - regardless of their intent. Who is more dangerous on the road - the driver that notices you or the one that does not?
I like the camera idea, perhaps I will carry a small digital in my pocket. Instead of "discussing" the situation with the driver, take a picture of them and their car.
Edited by maydog - 3/12/12 at 12:24pm