I have to agree with frequentstroker and weinerman, and think it could be dangerous to cum when actually driving. To me, its a pretty pleasant feeling just having a boner. So its good just to have your boner out and to be able to touch the head or underside, whatever, to keep it up. For me, fingertips around the rim of the head is enough to do it. Particularly nice in the summer (shorts weather), when you can tug your shorts down enough to have your balls out as well.
If you're going to cum, its safer to pull over. Also, if you are going to jack off in the car, try putting a condom on, just rolling it back to cover the head of your cock. All the same great feelings, but much easier cleanup. No cum on the upholstery, your clothes, etc., and easy to carry a few with you.