put car in PARK ,got out to check mail box 20 feet away [car door left open]. When retuning to car it began to slowly move backward.I ran around the back and tried to get in but was knocked down by the open door.I rolled away from the approaching wheel with minor scrapes. The car continued backing across the street,hit a pole then toward a group of people.I was able to alert them and all got out of the way. I have been working with GM to find out if this model,this transmission,or only this car has this slip from Park to Reverse problem.