I recently saw an episode of Mythbusters where they tried this with an Automatic and a Manual transmission. The point was to see if you could stop a car by putting it into reverse. They drove at 50mph and then tried to put the cars into reverse. The Automatic had a built in mechanism that prevented the gearbox going into reverse even when selected unless the car was stationary. On the manual, they could not get it in gear, just ground the gears.