CAR ENGINE RUNNING PARKED IN DRIVEWAY. I WAS IN THE HOUSE. SON WAS ON PORCH. YELLED THAT CAR WAS MOVING. WHEN IT SAW THAT IT WAS BACKING UP I RAN TO TRY TO STOP IT. THE CAR BACKED DOWN MY DRIVEWAY, WHICH WAS FLAT WITH NO INCLINE, TO THE STREET, ACROSS THE STREET AND INTO THE NEIGHBOR'S HOUSE. THE STREET IS QUIET A BIT HIGHER THAN DRIVEWAY BUT IT SEEMED TO PICK UP SPEED WHEN IT WENT OVER IT. I REACHED IT RIGHT AS IT HIT THE NEIGHBORS HOUSE. I CALLED POLICE DEPT. VEHICLE WAS IN PARK THE ENTIRE TIME. I TOOK IT TO NISSAN DEALERSHIP BUT THEY COULD NOT FIND ANYTHING THAT WOULD HAVE CAUSED IT TO DO THAT. IF MY HANDICAP SON HAD BEEN GETTING IN VEHICLE WHEN THIS OCCURRED IT WOULD HAVE CAUSED HIM SERIOUS INJURY AND COULD HAVE CAUSED ANYONE THAT WOULD HAVE BEEN PASSING ON THE STREET OR IN THE NEIGHBORS YARD TO BE INJURED. NEIGHBOR IS AN OLDER MAN AND HE WAS QUITE SHAKEN UP. THE IMPACT SHOOK HIS HOUSE QUITE A BIT. I HAVE BEEN AFRAID TO DRIVE IT. WILL SUPPLY POLICE REPORT WHEN IT IS AVAILABLE FROM THE POLICE DEPT.