A road is a thoroughfare, route, or way on land between two places, which typically has been paved or otherwise improved to allow travel by some conveyance, including a horse, cart, or motor vehicle. Roads consist of one, or sometimes two, roadways (British English: carriageways) each with one or more lanes and also any associated sidewalks and tree lawns. Roads that are available for use by the public may be referred to as public roads or highways.
- Gravel roads cost $3k per mile to maintain, and paved roads cost $4k/mile to maintain in iowa. But this appears to hide the initial installation cost of a road, from $60k to $400k per mile. NB: it appears that gravel is cheaper year to year, as otherwise all county roads would be paved assuming that financing were not an issue. Elsewhere the article states 'Generally speaking, gravel roads will actually cost more to maintain than paved roads' - that is consistent. High initial cost, lower maintenance for paved. For $400k/mile and 20 year life, that is still $20k/mile. So the conclusion appears clear that gravel is cheaper over life than paved. 
- This study in Maine 30 years ago shows $8k/mile for gravel road maintenance, and $6k/mile for paved. Overall, only low-traffic gravel is more cost effective than paved. 
- Question remains - why are most roads in the country not paved? Is it a financing issue?
According to the Federal Highway Administration 2008 statistics, there are 2,734,102 miles of paved public roads in the United Sates, with an additional 1,324,245 miles of unpaved public roads. Of these, about 50k miles are interstate highways.
For the highways, there are about 264 acres covered per mile of road (see Acreage of Highways. For the total length of roads in the USA, that is about 13M acres. The total land mass of the usa is about 2.4 billion acres. Thus, highways are 0.5% of the land mass of the USA.
- The Section of the Construction Sets page on infastructure (out of date, needs major rework once "needed" ]