According to George Wuerthner,
Developed Land- Despite all the hand wringing over sprawl and urbanization, only 66 million acres are considered developed lands. This amounts to 3 percent of the land area in the U.S., yet this small land base is home to 75 percent of the population.This is truly remarkable. God Bless America and Happy New Year!
Wuerthner, George. The Truth About Land Use in the United States
Found here. http://www.westernwatersheds.org/watmess/watmess_2002/2002html_summer/article6.htm