I agree that you should have stopped, if you were unable to safely stop I'm curious to know why. That said, the guy was out of line and you are justified in feeling threatened.
I'd suggest you don't flip the bird from now on for two reasons:
1) You never know what someone else on the road is hopped-up on and packing.
2) It gives us all a bad name.
I've been tempted more than a few times and know the anger that comes from almost being killed, but lowering yourself proves nothing and doesn't get the message across.
If only
whuffie was real.