PARKED CAR ON RELATIVELY FLAT DRIVEWAY (MAYBE 3 DEGREE GRADE). DID NOT PUT CAR IN GEAR BUT DID ENGAGE THE PARKING BRAKE. ABOUT 15-20 MINUTES LATER, CAR WAS SEEN ROLLING DOWN/ACROSS THE DRIVEWAY AND OVER AN EMBANKMENT. IT CAME TO REST AGAINST A TREE AND INCURRED SIGNIFICANT DAMAGE TO THE BODY.