Having been spat on, hit with unknown liquids, hit with bottles and cans, hit with a ball of ice, and threatened by all manner of psychopaths I have decided that a little shouting or a few cruel words out of a car window ( you usually can't understand them anyway) aren't that big of a deal. If it happens I just like to smile and wave, or ignore them.
Most of them don't get physical or get out of their car so why encourage it by flipping the bird? I had a friend knocked into a parked car after he flipped a driver off who then circled back to assault him. Hit him with his car and just drove away.
There was a guy in our road club who was a martial arts expert, even spent time in the Guiness Book for some kicking record. Anyway 4 or 5 of them were riding along and an idiot started screaming and threatening them. He pulled his car over and pulled out a cane and started swinging it at them as they rode by. Martial art guy disarmed the fool and put his ass on the street and held him until the cops got there. So there, a happy ending, for once.