The question sounds somewhat paradoxical so I'll explain. In case of ships: recent trends of overcapacity and cut throat competition (and a few years ago: expensive fuel) lead to slow steaming:
So I started to wonder about possibility of similar trend in autonomous cargo transport cars - saving fuel in case of overcapacity would lead to reducing the transport speed for purely economic reasons.
no great breakthrough in engine technology
self driving car would have a comparable price to a model with driver
no regulatory limits influencing the choice
cars can have smaller engine for start, if that seems like a good business
the vehicle has choice of either driving slowly or just arrive early just wait until later
What's the minimum speed for self driving cargo cars, below which savings from reduced air friction would be offset by other kinds of friction or by inefficient engine operation?
Bonus question: any idea what speed would be a minimum speed (optimized for lowest energy consumption) for a self driving electrified rail?
(I actually think that near future with futuristic, self driving... and often very slow cars is a nice twist, which is technically speaking quite rational)