I used to have about a 50 mile commute (almost all highway) to and from work each day. There were a few very attractive people in the office (men and women) that I used to fantasize about. Often on the long drive home I would unzip and bring Mr Stiffy out for some fun. There were a couple of occasions where a truckdriver saw my show, and once when I know a truckdriver was working his own tool while watching me. Unfortunately I wasn't able to see into his truck, but I did get a very friendly wave before he sped off.
Yes, I do it too. Women don't watch, but every guy who looked in and saw me, kept watching. Just make sure you aren't parked, a guy I know was arrested for masturbating in a food store parking lot. A girl gathering up the carts looked in and saw what he was doing, and next thing he was on his way to jail.