People on the autobahn are generally courteous, signal when changing lanes and so on. I guess you'd have to be at such speeds, but it's also part of the German national character. Furthermore, it's a highway so everyone is driving fast and the velocity differences, which cause most of the danger, are actually rather low most of the time. I think the highway may possibly be the safest and easiest environment for automated driving.
I agree that the Autobahn is a well-ordered place to drive but, where there is no speed limit, there are some large velocity differentials: the car in the outside lane may be doing 210kmh or more, but the lorry in the inside is probably doing about half that. That's a little alarming when someone pulls out to overtake the lorry on a two lane carriageway.