Huge chunks of the midwest are a paved checkerboard of roads spaced at 1 mile intervals. Many of these get 3-5 cars passing a day, if that. There is no need for a lot of these to be paved, as they serve as field access roads. The houses off these roads are farm houses, and most of the farmers I know don't mind either way if it's a gravel or paved road. Some actually prefer gravel, as in the winter, when it is icy, the gravel can provide better traction, and the county doesn't do a good job plowing.

That's a bit of an exaggeration - there are definitely roads checkerboarded at 1 mile intervals, but most are gravel in between some paved ones.

This may vary state to state. I know that in IL it seems that well over half of them are paved, or tarred gravel. The southern end of IL has less of this tho... but the amount paved is increasing. When I was a kid visiting relatives, I learned to drive on a lot of gravel roads, but heading down there for a funeral not long ago, most of the gravel I learned to drive on is paved now.

Paving rate varies but can be quite high. The one mile grid though is quite prevalent.

It's not so much the roads as the BRIDGES.

Iowa's road map was drawn from afar, before any surveyors could look at it, so there are a lot of bridges keeping the road grid continuous over rivers, creeks and dry streambeds.