When a metropolitan area adds population, it typically does so in part by adding people to areas that are already developed (infill) and in part by expanding its geographic footprint (sprawl). With the Census Bureau out today with new information on population dynamics in America's cities, I was interested in which effect predominates and where.

So I asked Jed Kolko, an economist with research expertise in housing and urban growth patterns, and he sent me this scatter plot of population growth versus change in density:

The density concept used here is tract-weighted density rather than raw density. Usually you hear about raw density, which takes a metro area's total population and divides it by the total landmass. That's easy to calculate, but it leads to some funny results, like Greater Los Angeles being denser than Greater New York. Instead, this concept of tract-weighted density measures tends to be more accurate. It shows the density of the place where the typical resident of the metro area lives and tells us a lot more about the nature of the urban form.

What we see here, with tract-weighted density, is that the two cities adding the most density are two very low-density metros in North Carolina.

They're followed by the two major metropolitan areas of the Pacific Northwest — further evidence that Oregon and Washington have housing policy regimes that California and the Northeast should try to emulate.

Conversely, we see the metros that are de-densifying at the most rapid clip tend to be experiencing low or negative population growth. These are typically places where the urban core is still hollowing out rather than adding people, while the suburban frontier continues to grow.

Last but by no means least, with the state of Texas accounting for a huge share of America's overall population growth, it's interesting to look at the divergent fates of its metro areas — Austin and Dallas are sprawling, but San Antonio and Houston are getting denser.

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