I PARKED AT THE TOP OF MY DRIVEWAY AT A SLIGHT INCLINE POSITION WITH THE EMERGENCY BRAKE ENGAGED AND THE CAR WAS IN SECOND GEAR (MANUAL TRANSMISSION). IT WAS IN A STATIONARY POSITION AT THAT TIME. MY FAMILY EXITED THE VEHICLE AND WE WENT INSIDE OUR HOME. APPROXIMATELY A HALF OUR LATER I NOTICE MY CAR ROLLED DOWN MY DRIVEWAY AND CRASHED IN BETWEEN TWO TREES ON MY PROPERTY. EITHER THE EMERGENCY BRAKE FAILED OR THE CAR'S TRANSMISSION FAILED WHILE PARKED. THE CAR WAS FOUND AT THE BOTTOM OF MY PROPERTY STILL IN GEAR AND HAD THE EMERGENCY BRAKE IN THE ENGAGED POSITION.