Many cars already have dynamic cruise controls, which is basically auto pilot for highway driving. The one which are installed on BMW, brakes when the car in front of you brakes and decreases speed on curves.

as a theoretical problem highway should be much easier because few types of "street agents" can access them (i.e. no bicycles, no pedestrians, no traffic lights)

As far as I know, they already have a pretty good handle on limited access roads.

People use 'highway' to describe all manner of roads, I wonder if the gp is thinking about major 2 lane roads that have higher speeds but still lots of intersections to deal with.