I experience 1 to 4 phantom brakes everyday, on my rather long daily commute (~115mi round trip). They're evenly split between 2 lane highways (FSDb active) and interstate(Reg AP w/ auto lane change, NoAP annoys me too much). They are almost always a 2-5MPH slow down, but I can't be sure they wouldn't be a hard brake as my foot is always over the accelerator ready to goose it, as mentioned here by others. An 'instant' hard brake is pretty rare for me though.