I've jerked off in my car dozens of times....none recently. Did it when I was younger and willing to take foolish risks. I liked to finish what I started with willing observers. When I found women engaging with me and wanting to see more, they usually did the jerk-off gesture with their hand. I obliged by raising my hips and giving them a full view of my cum shot - which often landed on my face. This shocked them a bit but they seemed to enjoy the activity as I licked the cum off my fingers.
Also watched my wife finger her pussy for passerbys in high SUVs or trucks. She would expose her whole pussy since she would slide her skirt to the floor. She exposed her healthy 36C's too. It was fun to watch.