The Size of U.S. Metropolitan Areas

Metropolitan areas—unions of nearby built-up locations within which people travel on a day-to-day basis among places of residence, employment, and consumption—serve as a fundamental unit of economic analysis. But existing delineations of U.S. metro areas—including metropolitan Core-Based Statistical Areas (CBSAs), Urbanized Areas, and Commuting Zones—stray far from this conception. We develop a flexible algorithm that uses commuting flows among U.S. census tracts in 2000 to match varied interpretations of our metropolitan conception. Under a baseline parameterization that balances encompassing commuting flows and excluding sparsely settled land, our Kernel-Based Metropolitan Areas (KBMAs) capture almost all of the population and employment of metropolitan CBSAs but only a small portion of their land area. The population of the baseline KBMAs is distributed more diffusely than the population of existing delineations, suggesting KBMAs better match the geographic scope of agglomeration. But as with existing delineations, population becomes more bunched across the largest KBMAs. Settled land area expands proportionately with population across small KBMAs but increasingly less than proportionately as population becomes larger, suggesting the existence of centripetal forces that temper the geographic expansion of metropolitan areas.