I used to find myself (and still do, on occasion) responding to the other person's anger by becoming angry myself, which would escalate the situation. What I've learned to do, when cut off in traffic, or yelled at by someone in a rage, is to take a few deep breaths, and tell myself that they must be under a lot of stress, they're rushing someone ill to the hospital, etc. It seems to tick a box in my head that enables me to feel sorry for the person, rather than irritated, so I'm able to move along and get past the incident. I do recommend calling the police and reporting the vehicle and driver, though, if the person presents an obvious continued problem, such as weaving in and out of traffic excessively, cutting people off, etc.