I find that having a camera in your car is very helpful, if people steal your space, get a little angsty etc, whip out your camera and just take a photo. Make sure you take the time to get a really good one, fiddle with some settings, zoom in close. Then ignore them.
The conventional response to this tends to be that they go a little white and back down very quickly. Any impulse to do you violence quickly fades when they know you have an excellent mugshot for follow up purposes (be they revenge, police, internet humiliation or whatever else their now slightly off balance mind can muster)