Sure, the people who frequently use those roads are the ones who will know them best and be able to predict their condition, but your stopping/maneuvering power is so much worse on gravel that assuming an average speed of anything greater than, maybe even near 50 mph seems pretty reckless.
Both gravel and blacktop are lower cost options that can be used in low traffic areas though. There are certainly large stretches of very low traffic high maintenance roads all over Iowa.
Blacktop is asphalt (what roads are normally made of). And requires an underlying concrete layer to lengthen it's lifetime (asphalt is a lot "softer" than concrete and more susceptible to ground movement, erosion by the elements and friction from tires, etc...)
In my area, paved, minor county roads certainly don't have a Portland cement concrete base layer, when they are paving they might add some gravel during the surface preparation and grading, but that is about it. As you say, the ground movement shows through, and they are more susceptible to erosion.
Depends on the underlying soil. In north Texas it's common when asphalt is laid to pour a Portland cement concrete base, though many roads and highways are just straight concrete. In south Florida, the roadbed is usually just packed limestone, because the land has a rock layer no more than 50 feet down and the sandy soil is pretty incompressible, unlike the constantly expanding and contracting clay in Texas.
At least one can hope so!
[1] https://en.wikipedia.org/wiki/Washboarding [2] https://en.wikipedia.org/wiki/The_Wages_of_Fear . The protagonist drove a load of nitroglycerin over washboard by going fast enough to float over the bumps.