A DRIVER AND PASSENGER STOPPED NEAR AN EXIT GATE. THE GATE IS OLD AND TAKES TWO PEOPLE TO CLOSE AND LOCK. AFTER THE DRIVER STOPPED, THE VEHICLE WAS PLACE IN "PARK" BUT LEFT RUNNING, AND THE DRVIER GOT OUT OF THE VEHICLE TO ASSIST IN CLOSING THE GATE. AFTER CLOSING THE GATE, THEY TURNED AROUND AND SAW THE VEHICLE MOVING, IT ROLLED DOWN AN APPROXIMATELY 10 FOOT EMBANKMENT AND ROLLED ABOUT 50 FEET HITTING A PIPE RAIL FENCE WENT OVER THE FENCE AND LANDED 10 FEET DOWN PAST THE PIPE RAIL AND STOPPED ON TOP OF LARGE RIP-RAP. THE VEHICLE WAS NOT DRIVABLE. THIS THE SECOND TIME THIS VEHICLE HAS ROLLED WHILE IN 'PARK' WITHIN THE PAST ELEVEN DAYS.