I think people (myself included) have a mental model of the other driver that seems to default to the worst possible interpretation of their motives.

If I was in a grocery store and someone "cut me off" and forced me to slow down because they misjudged the timing, I would think nothing of it. I certainly wouldn't make an obscene gesture and shout at them.

Also, when we are driving, we're in a pretty high state of anxiety; just as a baseline.

I find it amazing, that, when I'm driving, and some knucklehead does something that almost makes me crash (and thus, maybe kill me), I get incandescent with rage, but, five minutes later, I've all but forgotten the incident.

I could easily see myself fanning that rage into something that could result in self-destructive road rage.