In our popular imagination, America is the land of wide-open spaces.

But in reality, much of it is more densely populated than Europe.

Two-thirds of the U.S. population lives on less than 20% of the privately owned land.

With an expected population of 400 million by 2040, America is morphing into an economic system composed of 23 megapolitan areas that will dominate the nation’s economy by midcentury.

These megapolitan areas are networks of metropolitan areas sharing common economic, landscape, social, and cultural characteristics.

In Megapolitan America: A New Vision For Understanding America’s Metropolitan Geography, authors Arthur C. Nelson and Robert E. Lange detail the rise of megapolitan areas and how they will change how America plans.

For instance, in an area comparable in size to France and the low countries of the Netherlands and Belgium — considered among the world’s most densely settled — America’s megapolitan areas are already home to more than 2.5 times as many people.

Indeed, with only 18 percent of the contiguous 48 states’ land base, America’s megapolitan areas are more densely settled than Europe as a whole or the United Kingdom.

Megapolitan America goes into spectacular demographic, economic, and social detail in mapping the dramatic-and surprisingly optimistic-shifts ahead.

While earlier generations heeded the call to “go west,” people today will more likely “go to town,” heading for the urban hubs that are redefining the American experience.

Arthur C. Nelson is Presidential Progressor of City and Metropolitan Planning at the University of Utah where he is also the founding director of the Metropolitan Research Center, adjunct professor of finance in the David Eccles School of Business, and the founding co-director of the Master’s of Real Estate Development Program.

Robert E. Lang is professor of sociology at the University of Nevada, Las Vegas, the director of Bookings Mountain West and a nonresident senior fellow of the Brookings Institution.