From an old man's point of view:
When I was much younger, almost every time I could, I would unzip, pull my cock out - it seemed to be hard most of the time back then, and just slowly jack on it. Most times, I would not even try to cum, but sometimes, I would see a sexy girl walking and it was off to the races. There were a few times when I would have a condom on so I could cum without making a mess, but most times if I reached the point of cumming, I would just find a place to pull over and finish up in my hand or sometimes I would simply pull my foreskin over the head, hold it closed and cum inside my foreskin - then clean up.
While in high school, I had a friend who introduced me to male/male sex, mostly mutual jacking off. Several times, he and I would be driving and he would reach over, begin to feel me up, get my dick hard, take it out and jack me off. Then he would do himself. Once we went to the Greyhound Bus station in Houston, went inside to the men's room, found a gay guy and he got in the car with us. He sucked my friend off first while I was driving, and of course I had my dick out jacking on it. Then when my friend shot his load, he started driving and the gay guy sucked me off till I shot.
In later years, I have not done much jacking while driving, but recently, very early in the morning, I have done it some - not to the point of cumming, but just enjoying the feeling of driving and jacking. The area I drive through sometimes has girls (prostitutes) walking. I keep hoping to see one while I am jacking and drive up to her and see if whe will masturbate while I jackoff.
One final thought. I have thought how difficult it would be to explain it if while driving and jacking, I had a wreck and cut my dick off - not that would be bad, so guys be careful.