Car was stopped at the top of a hill. Driver put the car in park and exited to talk to a security guard to raise a security barrier. Car began to roll down a hill and didn't stop until it hit a metal security gate 250 yards away. Car was totaled as result of collision with gate. No one was hurt. It would appear that the transmission/engine did not stay in park and began to roll and that none of the other safety features stopped the car. There were no passengers in the car at the time of the accident and no airbags went off.