What is this? Solved!

I see these on the road all the time. What are they?


It turns out, this is a Windmill! This is the part that houses the generator, where the blades attach (#3 in this pic).

There was one stopped at a truckstop, so I pulled in and talked to them. They told me they deliver these all over, though I've never actually seen an assembled one anywhere around here (I did see them in Canada all over the place).