Myth: When a tornado approaches your home, the best thing to do is to get in a car and drive away
The official directive from the NWS, past and present, has been for house-dwellers in the path of a tornado to shelter in place rather than risk an escape by vehicle. This is a result of several factors and statistics. An interior room inside of a well-built frame house (especially one with a basement) provides a reasonable degree of protection from tornadoes rated F0-F2, approximately 85% of all tornadoes. This means that in an average scenario, even a direct hit (while highly damaging to the walls and roof) is unlikely to destroy the house enough to severely injure or kill a well-sheltered occupant.
By comparison, a vehicle encountering all but the weakest tornadoes would immediately have the windows shattered (subjecting the occupant to lethal flying debris), and be quickly flipped by the ground-level winds. With stronger tornadoes, the vehicle could also taken airborne and thrown a considerable distance. Since most individuals are untrained at visually identifying subtle severe weather phenomena, they stand an excellent chance of accidentally driving into additional severe weather (including tornadoes) while blindly trying to escape. Flash flooding, *******ial rain, severe thunderstorms, hail, and debris on the roadway could easily cause nearly impassable driving conditions, which, aggravated by driver panic, might well result in a dangerous and preventable accident. The disorganized peril of such a situation would be magnified greatly if all the residents of a warned area felt the need to flee by car.
If a person spots a nearby tornado while driving, the official NWS directive has always been to abandon the car and shelter in a ditch or culvert. No data has been produced on the efficacy of this method, but unscientific analysis of storm chaser video shows that no driver has ever actually been filmed abandoning their car for a ditch. The colloquial opinion at the NSSL currently states that a tornado encountered while driving can be successfully fled from at right angles (90-degree) from its direction of apparent movement. If an encounter is too sudden to flee from, the advice is still to shelter in the ditch rather than the car.
Thanks Wikipedia