This picture shows the fans behind the truck not the trailer. Bicycles and drafers can't hang out there since the trailer is there.
So it it behind the truck (and in front of the trailer) or behind the trailer. Either way more passive solutions are in place. I've seen an end cap on the back of the trailer that is rounded to help the air flow back and I've seen moveable or flexible side panels covering the space between the truck and trailer. neither reuqire fans.

The space right behind the cab is a minor aerodynamic issue compared to the huge squared off end of the trailer (where people love to draft). The air between the cab and trailer may benefit from this idea, the question being, is the parasitic load on the engine (as well as increased maintenance costs) going to be low enough to make it a viable option.

For companies that have a fleet of identical trailers, (UPS, Wal-Mart, Fed-Ex) you would think that a simple fairing that matched the trail profiles would be a simpler and cheaper solution.

Another thing that would help truckers (although the safety issue might be too great) would be to have an active cruise-control on long haul trucks so that they could drive in a convoy fashion and save fuel. Systems like those in Mercedes sedans that can keep a steady distance from the truck in front might be worth considering.

RoadRailer trailers would be better yet in my opinion. These are trailers that can be linked together and towed behind a train. Search goodl for 'triple crown service', 'roadrailer', or 'Wabash trailers' to see how they work.

-Lego

Trying to improve a dying mode of transport doesn't make too much sense. Maybe this invention could help truckers hold on for a little longer, but we need investment in rail.

Dying mode of transport? Um, trucking will still be the number one mode of freight delivery 50 years from now, Isaac - we're not moving to flying freight drones anytime soon. Not sure why you would think that's going to change somehow in the short run...

Some BMW car carriers, enclosed, have big domes on the rear of the trailer, supposedly to improve mileage. this seems to be a simple fiberglass part that is light and easy to attach. I've also heard about barn doors or some things that channel air easily around the back. Cheap stuff like this should be used first.

In Italy I saw alot of heavy trucks with plastic or fiberglass cowls under the trailers to better aerodynamics and keep really small Fiats out from under the trailers (getting run over).

Have often wondered by safety conscious America doesn't require something like that b/c it would seem that the fuel savings might pay itself pretty quickly.

I'd like to see trucks get MUCH greener than they are now if they want to continue to share America's highways and atmosphere. I prefer the idea of trains and know they have much more efficient mileage figures but I wonder if the whole truck to train, cross country by rail, train to truck to customer is still more efficient than loading a truck that goes directly to the customer.

One thing that I do like about trains is that they can be wholly electric and we know we can make electricity from a long and diverse list of methods. No local pollution from an electric train, quieter than the interstate near my house and if the power to run them comes from some of the energy sources, no pollution at all.

Trucks will be around forever, Isaac. How in the heck do you think they will replace all trucking routes will rail lines? Do you have any idea how many trillions of dollars that would cost?

looks like an overly elaborate, questionable value solution.

A simpler solution (in theory) is to dimple the last 6-8 inches of the truck or trailer so the boundary layer trips to turbulent and follow the sharp corner end the end of the truck better before separating, thus resulting in a smaller wake.

Don't know how well it'd work in practice

I've seen concepts for making truck more aerodynamic since the 70's. Why hasn't this taken off in a big way? I remember thinking that these truck producers would be the first to produce ultra-low drag vehicles. What happened?

One of the difficulties is that trucks are expected to have a standard floor height to match up to a loading dock, and making it any lower makes if difficult to unload using a forklift, or by tall employees. The rear end of the truck is what it is, unless you resort to something like Jeremiah mentioned. I recall seeing something about turbulators before.

Towns used to be built where the railroads were, this isn't always the case now when small towns boom and grow and aren't exactly on a major railway. A lot of old rail right of ways are now parks and houses, so good luck getting the railroads back without spending lots of money.