Next up, they will hit up on the idea of dedicated roads for truck convoys; also, a single engine to pull them all; and finally, the creation of roads with steel rails to put the trucks on.
I've seen UPS and FedEx semis pulling 3 short trailers on interstates. I assume they're only limited by current regulations on weight and length.
If we get the self-driving thing figured out, I expect that last-mile freight will use fewer actual trucks than are used today. Rather, the typical robofreighter will be quite small. Every time I receive a truck freight delivery, the trailer is basically empty. They use a big tractor-trailer rig because they have to hire a driver anyway, and sometimes they have enough freight to fill the trailer.