We do this on road trips all the time, everyone screams and slam on brakes (where safe to do so) then piss yourself laughing at the screaming wakers!, funniest one was with a lad called Craig, was driving from Singapore to Kuala Lumpur and just as we arrived at hotel we slammed on screamed, poor little craig woke up screaming and was scrambling to get out of the car!!
One of my mates has an awesome video of him in a hamster ball (Reverse Bungee in a cage), he was absolutely crappin himself, funniest thing I saw in years, even better his girlfriend at side of him was just sat there smiling all the way through, not a peep from her!