Good question. My first thought is that if I were open carrying, I would stay in the car until the police arrived, tell them about the gun, and ask them how they wanted me to proceed. Safest for everybody that way, including you. If you don't leave the car, it would be really hard for the other driver to claim you were threatening him -- for one thing, if you stay in the car, he won't know you're armed.
I'm not a lawyer, but I don't think that having a handgun in the car would change how the accident would be regarded legally.