VEHICLE WAS STATIONARY (PARKED), ENGINE RUNNING, AIR CONDITIONING ON, NO ONE IN DRIVER. SEAT, OCCUPIED BY SEAT BELTED ADULT PASSENGER WITH ARM RESTS DOWN, FOR ABOUT 10 MINUTES. VEHICLE WAS PARKED IN A PRIVATE PARKING LOT WITH A SLIGHT INCLINE, SLOPING DOWN TO & PERPENDICULAR TO ROADWAY. PASSENGER REACHED OVER & SHUT OFF THE IGNITION & THE VEHICLE IMMEDIATELY BEGAN TO ROLL BACKWARDS TOWARDS ROADWAY APPROXIMATELY 30 FEET BEHIND. ROADWAY IS A 2 LANE HEAVILY TRAVELED HIGHWAY WITH TYPICAL SPEEDS OF 60 MPH, POSTED SPEED OF 50 MPH. PASSENGER WAS FACING EMINENT DEATH & MAYHEM. PASSENGER QUICKLY PUSHED THE SHIFTER BUT IT COULD MOVE NO HIGHER, & THEN GRABBED STEERING WHEEL (POWER STEERING, NO POWER) TURNING IT. THIS CAUSED THE VEHICLE TO TURN, AVOIDING ENTERING THE ROADWAY, & STRIKING AN UNOCCUPIED STATIONARY (PARKED) VEHICLE; COMING TO A STOP. THE PASSENGER BELIEVES THAT THE VEHICLE SHOULD NOT HAVE ROLLED BACKWARDS OR IN ANY OTHER DIRECTION WHEN THE IGNITION WAS SWITCHED OFF.