I'd probbaly just take a leak in an empty mountain dew can if driving alone.
You'll piss your pants I promise. Unless you got a 12 incher flacid, how are you gonna shake your prick driving 75 miles an hour down the road, while trying to fit the head into that little hole?
Not to mention that you'll likely easily fill the can, with more to spare.
You need to either
A. Piss your pants or
B. Quit being a lazy fuck and pull over the vehicle for a couple minutes and pee on the side of the road.