The average tract density of all these (U.S.) cities taken together declined in every decade since 1910, from 69.6 persons per hectare in 1910 to 14.6 persons per hectare in 2000, almost a fivefold decline. Fitting an exponential curve to this average density in every decade from 1910 to 2000, we found that the average annual rate of decline for the entire period, assuming a constant rate, was 1.92 percent. Declines in average tract density between any two consecutive censuses were registered in 124 of the 153 observations, or 81 percent of the time.
That is from the new and quite interesting Planet of Cities, by Shlomo Angel. My takeaway is that the Avent-Yglesias push for greater urban density, which I sympathize with, is unlikely to happen on a significant scale. If you are looking for hopeful signs, there is this:
…between 1990 and 2000, six cities in this group registered an increase in average tract density: New York, Washington, Los Angeles, St. Paul, Syracuse, and Nashville. Hence, while average densities in U.S. cities have been in general decline for almost a century, they may now be reaching a plateau and even gradually increasing.
I definitely recommend this book to all those with an interest in urban issues.