Flipping the bird is only communication, and it is protected speech. As a response to harassment that has already begun, I don't see how anyone could call it a provocation. As a response to physical threat, I wouldn't even see it as an escalation. From the legalistic point of view, flipping someone the bird just isn't an issue.

Is it a good idea? Oh, maybe not. I remember one time a guy honked at me, and I turned around and made that gesture for his benefit. He looked deeply offended. I guess he had not realized that I would would be offended by his honking; but I'm pretty sure he got that message. In that case, I'm sure the bird was an appropriate response. I'm sure there are myriad circumstances where it the correct response, just as there are situations where it is not.

But aside from all that... it's a done deal. Nutjob car driver did this, OP did that, and we read about it here. I don't see the point in second guessing him now.