I've jacked off many times in the car, tho in retrospect it IS kinda risky. Once I did it when returning a moving van that I had just finished emptying into my new abode. It was a hot summer day, I was 31, shirtless, and horny. Shot my load all over the driver's floor. I suppose, out of courtesy, I wiped it up before I got out, but I"m not actually sure.
In the 80's, I had a car with a sunroof and I used to jack off driving on the CT highway. Most truckers' passengers were cool with it: laughs and thumbs-up as it oughtta be between guys, but one maniac swerved his truck towards me to scare me. That worked. I stopped doing it after that. What kind of fucked up dude would actually try to injure or kill someone just because he was beating off in his car??
Nowadays, with cell phones, it seems incredibly risky ; (