After significant research done by the American Red Cross, national emergency management professionals in conjunction with the National Weather Service have agreed to update the tornado safety rules for what to do when you are in your vehicle. Traditionally, people have been encouraged to leave their vehicles and get into a ditch.
However, the new rule stresses that if you are in a vehicle and can drive away (the opposite direction) to do it! If you are unable to drive away (ex: divided highways), get into a sturdy shelter (building). As a last resort, you should pull your car over to the side of the road, keep your seatbelt on, and place your head below the windows in your car until the tornado has passed. If these are not options, only then would you exit your car and get into a ditch.