I need to jerk off when I'm in the car fairly frequently myself, so I have paper towels handy to shoot my sperm into (it's a lot) right there so it never gets onto my clothing or upholstery. For comfort and more pleasure I always have my pants all the way down while doing it, so it never gets in my pubic, thigh or stomach hair either. The towel makes it easy for me to wipe up my dick when I'm done.