DRIVER PARKED 2011 HONDA FIT IN DRIVEWAY. IGNITION OFF CAR IN 1ST GEAR. AFTER ~ 4 MIN. CAR BEGAN SLOWLY ROLLING BACKWARD DOWN DRIVEWAY, ROLLING OVER CURB AND STOPPED UPON IMPACT W. TREE. IMPACT CAUSED DAMAGE TO LEFT REAR BUMPER, CRUSHED LEFT REAR BRAKE LIGHT AND WHEEL WELL. LEFT SIDE PANELS PUSHED FORWARD, SHIFTING LEFT SIDE FRAME OF CAR TOWARD FRONT OF CAR. INSPECTING CAR AT SITE OF IMPACT, DRIVER VERIFIED IGNITION REMAINED OFF AND GEAR SHIFT REMAINED IN 1ST GEAR POSITION. CAR WAS NOT DRIVEABLE, REQUIRING CAR TO BE TOWED FROM SITE. THANKFULLY NEIGHBORHOOD CHILDREN WERE NOT PLAYING IN PATH OF CAR AND CAR DID NOT ENTER MAJOR INTERSECTION WHERE IT WOULD LIKELY HAVE CAUSED A SERIOUS IF NOT FATAL ACCIDENT.