On 2 occasions while driving on a flat straight Interstate Highway using the adaptive cruise control, with no vehicle in front of me, the vehicle has suddenly tried to stop. Pressing the Cancel button for the adaptive cruise control would not shut the system Off. When the accelerator was depressed to attempt to get the car moving forward, it continued to try to stop the vehicle even harder. Pressing the brake pedal also did not disengage the adaptive cruise control. I had to allow the vehicle to creep off the highway, then Shutdown the vehicle and then Restart to resume driving.