VEHICLE WAS PARKED FACING UPHILL IN THE DRIVEWAY,THE PARKING BRAKE WAS APPLIED AND THE MANUAL TRANSMIISON WAS IN NEUTRAL. THE CAR WAS PARKED AND LOCKED AT APPROXIMATELY 3:30 PM. AT APPROXIMATELY 7:10 PM THE CAR ROLLED BACKWARDS DOWN THE DRIVE AND ACROSS THE STATE HIGHWAY AND STOPPED AGAINST THE GUARDRAIL, BLOCKING TRAFFIC. MY SON HEARD A COMMOTION AND NOTICING THE CAR, WENT TO MOVE IT OFF OF THE HIGHWAY. HE RETURNED TO THE HOUSE, GOT THE CAR KEY AND PROCEEDED TO DRIVE THE CAR BACK TO THE DRIVEWAY. HE NOTICED THAT THE PARKING BRAKE WAS NOT APPLIED AT THAT TIME. HE VERY RARELY DRIVES THIS CAR AND WOULD NOTICE THAT THE BRAKE WAS NOT ON. *AK