See Weather.com: Tornado Safety - Cars Versus Ditches: A Controversy
The NWS recommends that if you are being overtaken in your car by a tornado, then you should get out of the car and into a nearby building or ditch. The new American Red Cross recommendation is that if no building is available, stay in the car - get out of the car and into a ditch only as a last resort. Crouch down with your seat belt on and your head below the windshield level.