What Has Driven Urban Density Changes In The U.S. Over The Last 70 Years?

Living in the West, we've long known that mountains and water supplies impact density, sometimes discouraging sprawl.

Housing unit densities are examined in 56 large urban areas in the United States defined in a consistent manner from 1950 to 2020. 

The mean density declined slightly over this period but this masks tremendous variation across the urban areas. Some of the most dense urban areas at the start experienced large drops, but substantial numbers of areas had increases in density, some large. 

Densities across regions changed dramatically, with mean densities for urban areas in the West rising from only slightly above the South to the highest by 2020, well above the Northeast and the Midwest which were highest in 1950. 

Density and density change are related to the size of the urban area (number of housing units), and change is also related to change in size and (negatively) to density at the start. The effect of potential barriers to expansion on density is investigated, with strong, significant effects of water and mountains on urban area densities.

John R. Ottensmann, "Density of Large Urban Areas in the U.S. and Barriers to Urban Expansion, 1950-2020" SSRN (May 14, 2023).

From the body text:

The list of the urban areas with the highest densities in 2020 is likely to be more of a surprise to many. 

New York remains on the list but in second place, edged out by Las Vegas, both with densities below 2,000 housing units per square mile, below all of the top 6 in 1950. 

San Francisco-Oakland-San Jose retains its position in third place but is followed by Los Angeles and Miami-Fort Lauderdale-West Palm Beach with nearly identical densities, all ranging from 1,834 to 1,859 units per square mile. 

San Diego is the sixth densest urban area with a density about 200 lower. 

New York and San Francisco-Oakland-San Jose were the only areas among the most dense in both years. New York accomplished this in spite of a huge decline by starting with by far the highest density in 1950. San Francisco-Oakland-San Jose experienced a modest drop in density over the period but maintained its position given the overall lower densities in 2020.