I wouldn't consider flipping someone off to be road rage. Many people go around flipping people off all the time, and it seems that act helps them release some tension, as does screaming at people. I can ignore that type of behavior, but when someone becomes aggressive with following too closely, pulling in front of a car and slowing down or slamming on the brakes, or coming into my lane (or whichever lane the rage-recipient is in), that crosses the line into road rage, because it becomes dangerous, and presents an actual threat.