The Decline in Transit-Sustaining Densities in U.S. Cities, 1910–2000

In this paper, Shlomo Angel, Alejandro Blei, Jason Parent, and Daniel A. Civco track the changes in transit-sustaining density in 20 U.S. cities from 1910 to 2000. They find a continuous decline in population density, which can pose a challenge to the investment in public bus service. Their observation is based on three measurements: (1) change in average population density within U.S. census tracts; (2) change in the share of metropolitan areas that can sustain public transit; and (3) change in the share of transit-sustaining urban population.

Starting in 1950, average tract density declined for 17 of the 20 cities. Only Los Angeles experienced a density increase between 1940 and 2000, with a tract density of 29.2 persons per hectare in 2000—the highest among all the cities in the sample. Despite the declining trend, the authors found that the rate of density decrease has slowed down between 1980 and 2000. The share of metropolitan areas that can sustain transit also has declined substantially over time. More than half of all urban land had a population density ranging from 0 to 10 persons per hectare in 2000. Less than 10 percent of urban land had a population density that could support public bus service.

The average share of transit-sustaining urban population increased between 1910 and 1920 and then declined, with the rate of decline increasing in 1930 and decreasing in 1980 on. Nearly a quarter of the U.S. urban population lived at a density of less than 10 persons per hectare in 2000. In view of the declining trend of density historically, the authors recommend demystifying urban areas by limiting fragmented urban development. Besides setting growth boundaries, city officials should (1) remove restrictions on higher-density development; (2) allow the subdivision of homes; (3) provide special incentives for building on small lots; and (4) encourage apartment house construction. They also propose extending the transit-sustaining measurement metric from population density to other indicators, such as open spaces and level of fragmentation, to fully account for dispersed urban spatial structures in assessing the feasibility of public transit development.

This paper was presented at the Lincoln Institute’s Land Policy Conference of 2010 and is Chapter 8 of the book Climate Change and Land Policies.