come on man, lets live and let live.
It does depend on how wild you get.
I have had a few slow wanking sessions whilst driving, when feeling really horny on the motorway or a quite road, mainly when dark. Have only shot my load twice, one intensionally the other by accident.
The first time, I had a 4 x 4 so was quite high up, I got horny and was rubbing my cock, down my suit trouser leg, as I had seen this hot chick going past in a low cut top, her tits and body were fantastic, I paid a little too much attention to my cock and blew my load right down my leg, through my trousers and on the underside of my dash.
The second time, realising the mess I had to clean up last time, I again was getting horny on way from a meeting in London on my way upto Manchester for my overnight stay and meeting the next day, got a bit bored so started stroking with my cock out, (this time in a normal saloon car), this trucker was going past and beeped his horn along side me (he was a Italian (found out later) in a left hand drive so was right along side me). I realised that he must have noticed me wanking, he stayed along side, his seat was quite high as I could look up and see at least 3 inches of his cock above the window.
Not sure how big that made him, it depends how high his seat was or could go, but looked tanned and throbbing. He stayed along side me and wanked off and shot a big load in the air. This spurred me on and I reached for a handkerchief in my pocket before I blew my load. The handkerchief held some of it but a lot dripped back on my trousers. (how I loved that spunk stained pair of trousers) .... NO I'm Joking.!
Since I have only stroked just for that nice feeling, whilst in the car ... cumming too messy. | 
I thought it was great so i had to blow my load when I got home.
