And the carriers are desperate to increase fuel economy too, as it's eating them alive. They're equipping their trucks with small generators to power the sleeper compartment without running the main engine, and many truckstops are now equipped with umbilicles that don't require the trucks to idle or generate their own power at all; some just supply HVAC and have a couple of power outlets; others interface directly with the cabs' systems.
Unfortunately, the things that are best for efficiency are also more dangerous. Multiple trailers, more weight, more volume, more cargo in a single trip. The Aussies have it down fairly well with their road trains, but there's so little traffic on the roads they run those on that it's not nearly as dangerous there as it would be here.
We would do well to improve our rail system, and to use tractor trailers for regional and local delivery, last-mile as it were. Rail is a lot more efficient than tractor trailers are.