To illustrate this, consider a sparsely-populated rural region that has only one big shopping mall. During the day, some of the regional residents work in the mall, and many others shop there. People living outside the region seldom either shop or work there.

The result? During working hours, the “population-weighted population density” of the entire region shoots far above its value when the mall is closed.

You might say the whole region becomes more crowded Monday through Friday, but I, for one, would not like to say it becomes more dense.

]]>In addition to the fact that a larger share of growth occured in lower density metros and lower density parts of metros (outer suburbs), there is another factor:

http://www.city-data.com/forum/26347869- post123.html

In most major metros you’ll find both these dynamics. Which is the more prominent — affecting a larger population — determines whether a given metro’s “population-weighted density” is ebbing or growing.

But recognizing coincident contra-trends does not seem to be the point of this blog piece. The author seems more interested in pushing a storyline about inner-city gentrification.

Finally, I’m not impressed by the glossed-over assertion that “population-weighted density” is the one “right way” to discuss land use. I much prefer our usual approach: We clip the urbanized area; then calculate land-weighted density within the urbanized area. If someone thinks “population-weighted density” is preferable, that needs to be explained.

]]>Consider a metro with three areas: a 100 sq mile core with 500,000 people, 300 sq miles of old suburbs with 900,000 people, and 1000 sq miles of sprawlsville with 1,000,000 people. Density is 1714/sm and weighted density is 2583/sm.

By the next census, the core has increased to 101 sq mile (because zoning/permitting makes it hard to expand) and 515,100 people. The old suburbs have increased to 315 sq miles and 976,500 people. Sprawlsville has increased to 1200 sq miles and 1,260,000 people. Density is 1965/sm and weighted density is 2536/sm.

Note that every zone (core, suburb, sprawlsville) got denser and overall city density went up, but weighted density still went down.

]]>http://www.census.gov/population/metro/f iles/CBSA%20Report%20Chapter%203%20Data. xls

First of all, metropolitan area population weighted density is in fact down 6% in 10 years.

Next, a mathematical curiosity. Population weighted densities of the two densest metros (New York and Los Angeles) are down 1.4% and 2.6%. Population weighted density of all other metros combined is down 4.9%. So how is it possible that the density of all of them together is down 6%? The answer is that NY and LA have been growing slower than the rest of the country – their populations grew only 3-4%, and the total population of all other metros grew 12%. When low density metros grow faster than high density metros, overall density goes down. It’s a form of Simpson’s paradox.

Once we exclude NY and LA, the rest of the list is pretty noisy, but one can discern two groups. In MSA’s with initial weighted population densities above 1200 people per square mile, median density change is -3.5% (with no apparent trend). In MSA’s below 1200 people per square mile, median density change is +0.8%. Census has a very broad definition of “MSA” (metropolitan statistical area), which, for example, includes Glens Falls, NY (population 129K). Among “real” MSA’s (populations of 1 million or more), median density change is -3.6%, again with no apparent trend.

]]>There is also at least one effect that is likely stronger than those described above. What did happen, really, between 2000 and 2010? We had a housing bubble, and it led to a construction boom. Some of it took place in city cores, but we saw significant growth of suburbs and exurbs, built out with low density housing. As suburbs grew, they soaked up population growth and attracted some people to move from city cores into suburbs.

According to the census bureau, between 2000 and 2010, the number of apartments in massively multi-unit structures (50 or more units per building, city core style) increased by 5%, from 6.13 million to 6.47 million.

At the same time, the number of single-family detached houses in the country increased by 16%, from 69.9 to 81.1 million.

]]>housing units per square mile in 1940; 36 in 2010. ]]>