The 2017 A8L 4.0T is equipped with a engine Start-Stop system that switches off the engine when the car is motionless, to save fuel. The system is supposed to restart the engine immediately when the brake is released. This system malfunctions routinely and refuses to restart, leaving me with an immobile car at stoplights and intersections, putting me and my family at risk. This safety hazard has caused two accidents. In the first incident, I parked in my spot and got out of the car. The Audi rolled backward and collided with the car parked behind (insurance claim filed). In the other incident, I was stopped at an intersection, at an incline. The car failed to restart when I took my foot off the brake and rolled back, colliding with the truck behind me. My rear bumper was damaged, truck was not damaged. The car was inspected multiple times, and problem was reproduced by the Audi dealer. They attempted fixes, but failed to solve the problem. No warning lights or message appeared to alert me to this start-stop system failure.