Here's my theory on this. Any rocket scientists out there are welcome to correct me.
I submit there is a low pressure area between the big rig and the TT (5er, whatever) and the open windows allow the air from the far side to enter and equalize the pressure, which would be bringing air in from the shoulder side of the road (assuming the trailer is in the right hand lane).
Now, with the vortex created by the vernal equinox and the correct alignment of the planets within parameters, the air on the starboard side should be cleaner than that on the port side. And with properly mounted screens on all open windows, any insects and small furry animals should be denied access into the trailer via said windows.
In summary, on an average 2,3,4 (or more) lane highway or interstate, there should be no noticible increase in dirt and filth entering the home on wheels.
If this theory is plausible, please notify my 12th grade physics teacher.
My driver's license says my name is "Jeff"
2000 Chevy 2500 4/4 6.0/3.73
2002 FR Sandpiper 30FKSS
DD (Darling Dachshund)