The problem is not necessarily "obstacles made of stone." Think about lanes in the countryside where you only see grass and the possibility of a ditch to either side... I guess that is really complicated for a self-driving car to manage in each and every case as well as human does.
Yes, walls are easily located. Holes not so much. Emptiness is quite difficult to see.
Yes, walls are easily located. Holes not so much. Emptiness is quite difficult to see.