In college I had a jackin buddy, and (besides going to nude beaches) we would sometimes park places where there were hot chicks about, and spend an hour or two masturbating in his van and enjoying the sites....
Later, every weekend my gf and I would go to a cabin in the country, taking separate cars because we left and went different directions. We'd follow each other up there. Once she pulled over to pee, and told me she'd just had a huge orgasm, and I learned the she often jilled while driving (JWD?). That got me fucking hot, and I played with my boner all the way to the cabin and jumped on her when we got there.
After that, we both played all the way up, sometimes cumming, more often just staying hot until we exploded at the cabin (or on the ground in front of the cabin). A couple of times we signaled each other to pull over, and finished each other off by the side of the road. It was EXTREMELY hot, especially since she was very shy about people seeing, and so it made her even hotter.
I drive a lot for work, often on boring interstate highways, and love to keep my boner trim and stiff for the whole journey. Often, when I'm ready to blow, I'll pull into a restaurant parking lot and shoot while watching some sexy tourists tug on their shorts when they get out of their car.
PS I'm always careful to give first priority to the road...