The road system of the U.S. is one of the largest human constructions on our planet.
The road system of the United States now surpasses four million miles in combined length, plus numerous additional service roads and off-road driving trails. The registered roads and their rights-of-way occupy about 1 percent of the land of the United States (roughly the area of South Carolina).
The map below color codes distance from “major roads”.
Areas of lowest distance to road appear in blue. Areas of longest distance are represented in shades of red.
Climate would be the key factor, as severe and long winters kept human habitation from becoming particularly dense beyond the southern coast.