For the guy who spit on you: Tell your wife to carry a fire extinguisher or pepper spray and spray them next time.
If you hot hit them or runover them you will have criminal and civil liabilities. But if you wife fears for her life she can use the pepper spray and you yourself can use the extinguisher.
I have been flipped off one time with my Hummer, and I followed the girl driving got out in traffic accosted her and made her cry.... ( I am a big intimidating guy).
When someone flips off the hummer they are insulting me. If they hate the car go flip off the dealership not the people who bought it.
SF is the capital of tree huggers and nothing else you can do. BUt I do like the horn idea too
