In the majority of cases I think a person has to do something. This doesn't mean that you have to take "Physical Action". Sometimes just remaining calm and remembering a description / number plate etc. is enough.
Few weeks ago I was driving through town, a car pulled out a bit close in front of another, this led to a war of the other car trying to cut up the other one, and so on, until the original "aggrieved" driver properly cut up the one who made the mistake.
I pulled in and stopped too, admittedly partly because I was a little annoyed that their antics had almost involved me (one brakes sharply then swaps lane to pull behind the other, causing me to avoid him), but mainly because I could see that this was getting out of hand and told the pair of them to stop behaving like idiots, that they were both as bad as each other and were just causing everyone else hassle.
The response was generally "nowt to do with you, **** off" as they continued to square up.
I popped back to my car and grabbed my camera and took a photo of both of them and told them that if they carried on, the police would be given it as evidencewhich stopped them in their tracks. I didn't report it to the police as the battery was actually flat in the camera, but they weren't to know that!
One pulled up next to me at traffic lights a few moments later giving it "you took a photo of my car, you haven't got permission". I said "that's right, and I don't need permission"
He sneered and said "well, I'm taking your number plate down then!" Never quite worked out what kind of threat (as it was obviously meant as one) that was supposed to be, so I just asked him if he needed a pen and paper.
Right or wrong for involving myself, I have no doubt from the way it was rapidly escalating that there would have been an assault, it was just a case of who was going to swing the first punch