You're better off NOT cumming unless you're in the right place. It's better to find a place to stop the car and do it that way. I have cum while driving, but it's hard to keep your composure well enough to drive so when you're as aroused as a man can possibly get. See my other comment here for (other) details.
I do it all the time too. I'm a very horny man to begin with and the frustrations of traffic and driving in general are too much for me to keep it in my pants; sorry. I've only cum a few times while actually driving. Generally after I've gotten myself pretty worked up, I'll find a reasonably safe place to pull over, park the car, pull my pants down and shoot my sperm into a paper towel in an incredible orgasm. Sometimes I get out of the car and have cum behind my car if there's a wall. I've also done it into bushes depending on where I am.