If you've been driving on a very long, very straight, exceedingly boring stretch of road with zero traffic then your reaction times will go down. It's very difficult to stay alert in those situations and there's a frighteningly high level of single vehicle collisions on those sorts of roads caused by a sudden change in conditions or even the angle of the road.

Auto-pilot will make driving a lot more like flying is for pilots: 99% extremely dull, 1% terrifying.