I think as Marvin has pointed out, his driving skills go out the window when he's moderately stressed. He didn't mean to run over Michael Snyder, he's just not good in confrontational situations and when hes driving in that situation its dangerous. His fight or flight response kicks into overdrive, same reason he tries to run away from the scene every time he gets in trouble with the law. He's also implied on facebook that when people yell at him "HEY CHRIS!" or something from afar he gets spooked and thats for some reason why he flipped this fool off.
Plus he rarely drives outside of areas he's unfamiliar with, but he did manage to drive to Ohio without incident.
But yeah, don't JULAAAY him when he's in the driver seat.