When I was in college, I had a girlfriend that loved to give me blowjobs while I was driving...and my car was a little two seat Spitfire convertible. Had a few truck drivers get distracted and honk their horns at us. I generally got my hand up her dress with my free hand, and she really liked the mutual exchanges. But she was always covered, while she really got off on having my cock fully erect and out in public, so it was up to me to keep us on the highway, and to give us some privacy by keeping up or keeping down the speed. She was really excited by giving me handjobs too, since that was pretty risky. She was also really into trying to get me to cum really hard so it got out over the car door (didn't always work). I learned to wear a shirt that was long enough to cover me in an emergency, but then, when all that is going on, it's really hard to concentrate!