Vehicle parked faced in in a tight fitting space in a hotel underground parking area. When leaving the knob was in the park position, push button start, vehicle started in the park position, looked behind and the people across from me had just loaded their van. They were now on the side of their van. It was now clear. I then placed the transmission from park to reverse still with the foot on the brake. I then released the brake . The vehicle suddenly and abruptly went in reverse without accelerating but when applying the brake the vehicle would not stop and continued going backwards even though I was firmly now pressing and pumping on the brake pedal to no avail. I yelled at my wife, "it won't stop, it won't stop" It backed up not stopping for approximately 25 feet and struck two vehicles finally coming to rest, The damage to all three vehicles including mine was minor. I reported the accident on private property to my insurance company. I know for a fact that my foot did not push down on the accelerator pedal nor was part of my foot on both the brake and accelerator pedal. which is almost impossible to do. The stop start system had broken three times since purchasing the van. This was done again 2 weeks before this accident. The vehicle is available for inspection. Quite apparent ithis is a safety risk. This occurred 1 day ago and has not been inspected. Accident on private property in Calif. No accident report was taken. No warning lamps, messages etc occurred