As a trucker sitting up high, most people can't see whats going on so you can get away with a lot. I have got myself off a few times while driving. Masturbation is actually an effective fatigue management strategy as it increases your heart rate (although much safer when performed in the privacy of a sleeper). I had a few sessions where I came so hard that my eyes almost closed which could have gotten nasty quickly. Nowadays there that many cameras inside and outside the truck that it's not possible.
I once looked down and caught a guy with his cock out. He seemed unaware or unconcerned that truckers could potenially see him. I was far less curious then than I am now, so had no real desire to offer him a hand (or more) but these days...