Erik, you ought to make up some fake stationery on your computer from a ficticious law firm. Send the guy a letter that says your lawyer will be suing his ass. Make up all sorts of legal mumbo jumbo that will get his panties all in a bunch. Then forget about it.
Alternately, you can inform the State Patrol about the incident. They will send him a nice letter saying that he shouldn't be mean to people on the road...