There's a lot of churn where the air comes back together behind the trailer. Useless fact #913 - Most of the problems with truck aerodynamics are right there behind the trailer. There's no good way to put the air back together without a device that would greatly hinder the offloading and backing up processes.
Not to sound like a wet blanket, but I never cared for that zone too much, in the event a 4 foot length of "flying alligator" (recap) comes loose, it could sting a little bit. I've seen those flying recaps push 3" angle iron, 2 inch fender mounting pipe, and light boxes around like they're made of balsa wood.
