I would have given the guy the bird, shouted at him like I was insane, and then followed him until he got to wherever he was going just to make him nervous. (I'd probably also throw in some "creative" driving while following the guy.) OK, maybe I wouldn't have followed him, but I definitely would have yelled and given the guy the bird.
On a slightly unrelated note, you seem to be attracting some, uh, interesting strangers lately eakers; first the judgemental moralist and now screaming man at the stoplight.
Zenmervolt