Theoretically, one doesn't even need directional lanes, anymore. You just have a road, and cars go where they most efficiently fit; lanes won't have any meaning anymore, just like most road signs won't have any purpose. If every car is communicating with every other car, and every car knows where it needs to go and how to get there in the shortest time, it can look like utter chaos to the human eye, and still be entirely safe (assuming we go to great lengths to insure reliable software and hardware).
Of course, I expect it to take decades for us to reach that point, which is really unfortunate. That's a lot of senseless auto collision deaths due to human error while we wait for the future to arrive. If governments were capable of looking forward, at all, they would recognize the massive savings an all AI road represents, both monetary and health, and would be pushing hard for it (both enabling private industry to implement it and guiding public funded research).
