Note the drag reducing flaps on the roof. I believe they might also help keep the interior comfy while you are traveling down the road. http://www.big-boys.com/articles/coolbus.html
The flaps probably allow enough air to escape so it doesn't flip over backward. I saw the crew build a wheelie wagon out of an old ambulance on Monster Garage once. It takes a lot of weight in the back to get it up in the air like that.