Something else to consider, if the guy is unarmed and he does break your side glass, just retreat to the other side of the car.
Jayman, this tactic is kind of questionable considering how quickly a weapon (firearm or other) can be put into play and used offensively. By the time you realize you're under attack, and try to move to the other side, you're dead and he owns you IF that's his intention.
Of course, you're probably better off driving off if that's possible, but if you're stuck in traffic with a maniac breaking in, that's as close to a lethal threat as you can get IMO.
In NC where I live you are authorized to use lethal force if someone's breaking into your occupied car.