AFTER BACKING OUT OF GARAGE, I PLACED THE VEHICLE IN PARK WITH ENGINE RUNNING WHILE I WALKED BACK INTO GARAGE. NOTE VEHICLE WAS PARKED ON A SLOPE. EMERGENCY BRAKE WAS NOT SET. WHILE IN GARAGE, I NOTICED VEHICLE WAS ROLLING DOWN DRIVEWAY. I WAS KNOCKED DOWN BY DRIVERS SIDE DOOR WHILE TRYING TO GET IN TO APPLY BRAKE. THE VEHICLE THEN ROLLED OVER MY MAILBOX, ACROSS STREET, OVER NEIGHBOR'S MAILBOX AND STOPPED WHEN IT HIT A TREE. MY WIFE WAS IN CAR SO KNOWS THAT REAR PARKING SENSORS NEVER CHIMED TO INDICATE IT WAS ABOUT TO HIT SOMETHING WHICH I BELIEVE CONFIRMS THAT THE CAR WASN'T STILL IN REVERSE AFTER BACKING OUT OF GARAGE. THE AIR BAGS WERE NOT DEPLOYED. I BELIEVE THAT VEHICLE SLIPPED OUT OF PARK. IT WAS NOT MOVING WHEN I FIRST GOT OUT. MY INJURIES FROM BEING KNOCKED TO THE GROUND AND HERS FROM BEING IN THE DRIVERS SEAT BOTH FEEL THEY DIDN'T WARRANT EMERGENCY ROOM TREATMENT BUT WILL GET TREATMENT IF PAIN PERSISTS.