The theoretical framework of urban and regional economics is built on transportation costs for manufactured goods. But over the twentieth century, the costs of moving these goods have declined by over 90% in real terms, and there is little reason to doubt that this decline will continue....

Over the past century, America changed from a nation of distinct cities separated by farmland, to a place where employment and population density is far more continuous. For some purposes, it makes sense to think of the U.S. as consisting of a number of contiguous megaregions. Using the...