A self driving truck doesn't have to go 60mph to be as cost efficient as a human driven truck. They could be governed at 45 mph and still come out way ahead and cause far less damage if something does go wrong.
So now you have a greater disparity of vehicle speeds on a typical highway. Having just returned from driving on highways cutting through mountain ranges, I'm acutely aware of the "interesting" driving that results from having vehicles traveling at very different speeds on long uphill grades.
