Rallygirl,
I get stressed out driving as well, especially when people cut in too close, or don't use signals to change lanes, or run through red lights...or read books while they're driving! I don't have a mantra, but what I do now (it has become automatic now) is pray for the driver, first that he gets a divine flash of wisdom and common sense and doesn't get into an accident, and secondly, that everyone else on the road with this crazy driver will also be kept safe (since the accidents can often happen to the people behind these drivers!) It eases my mind to do that and gives me something to focus on besides my desire to rage.