Rush hour and breakdowns
I can just imaging the scenario where multiple robo-lorries are on the motorway in rush hour and one in the slow lane "sees" a breakdown or accident ahead in the same lane. There no chance to pull out because lanes 2 and 3 are also full. So robo-lorries slows down and eventually comes to a halt. As do all the other robo-lorries behind it. It can't pull out because there is moving traffic passing continuously in the next lane.
Now, think back to the cyclist incident with the Google robo-car where it couldn't tell if the cyclist, stopped but balancing, hence wobbling, so the Google robo-car refused to move.
I have wondered if all these new motorway upgrades with concrete divider walls might have been designed with three divider walls so automated or semi-automated lorries could use them as "super guided bus" lanes, one lane in each direction with exits/entrances at junctions. The obvious problems are that joining or leaving lorries have to cross all the traffic lanes, Using the "slow" lane side would have joining/exiting cars having to cross the guided lorry lane. the upside is segregation of lorries and cars and minimal (if any!) AI needed for the trucks.