[Alupis]

> Both gravel and blacktop are lower cost options that can be used in low traffic areas though

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...)

[maxerickson]

Asphalt roads are a type of concrete.

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.

[HeyLaughingBoy]

Here too. One of the gravel roads near me was recently paved and they simply flattened it, added some sort of cement + water spray (as opposed to the usual multiple inches thick layer of concrete) and the asphalt placed on top of that. It is definitely better than gravel, but it's already showing some wear in places.

[vonmoltke]

> And requires an underlying concrete layer to lengthen it's lifetime

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.