I WAS DRIVING MY VEHICLE HOME FROM MY SON'S SCHOOL...THERE WERE NO PASSENGERS IN THE CAR. I PULLED INTO MY FLAT DRIVEWAY AND STOPPED AT THE END OF IT TO RETRIEVE MY MAIL FROM MY MAILBOX. AFTER THE VEHICLE WAS COMPLETELY STOPPED, I PUT THE TRANSMISSION INTO PARK; RELEASED MY FOOT FROM THE BRAKE PEDAL; TOOK OFF MY SEATBELT; OPENED THE DOOR; AND GOT OUT. EVERYTHING SEEMED NORMAL AT THIS POINT. HOWEVER, AFTER TAKING ABOUT FIVE STEPS AWAY FROM THE VEHICLE TOWARDS MY MAILBOX, THE VEHICLE SLIPPED OUT OF PARK INTO REVERSE AND STARTED DRIVING BACKWARDS....ALL BY ITSELF. AFRAID THAT THE VEHICLE WOULD RUN INTO ANOTHER CAR, OR A HOUSE, OR WORSE, MY INSTINCT TOLD ME TO TRY AND STOP IT. I RAN TOWARDS THE FRONT OF THE VEHICLE AS IT WAS MOVING BACKWARD AND TRIED TO GET IN (IN ORDER TO PUT THE BRAKE ON). IN MY EFFORT TO DO SO, HOWEVER, THE DOOR OF THE MOVING VEHICLE KNOCKED ME DOWN ONTO THE STREET AND COMPLETELY RAN OVER MY LEFT LEG, BREAKING MY FEMUR BONE IN TWO PLACES. AS I WAS UNABLE TO MOVE, SCREAMING IN SEVERE PAIN, THE VEHICLE CONTINUED BACKING UP ON ITS OWN ONTO MY FRONT LAWN, THROUGH A DITCH, OVER A CABLE T.V. BOX, THROUGH AN EMPTY FIELD, FINALLY STOPPING WHEN IT TRIED TO GO ACROSS A RATHER DEEP DITCH. THE MOTOR, HOWEVER, CONTINUED TO RUN.