As I mentioned last time, I have a couple of problems with Elizabeth Kneebone's study of "job sprawl." The main one is that there is less to the study than might appear at first glance.
But there is a very interesting phenomenon lurking in her data that deserves some discussion
We should expect large metropolitan areas to have more jobs in the outer rings than small metropolitan areas. That's just a function of size. While scanning her data, though, I noticed that large metropolitan areas invariably have more jobs in their central cores as well. Because Kneebone uses the same land area for all central cores -- the area within a 3-mile radius of the CBD -- this means that large metropolitan areas have a higher job density in their central cores than small metropolitan areas.
I decided to test this a little more rigorously. I regressed central-core job densities on the number of metropolitan area jobs within 35 miles of the CBD. (Actually, I used logs, but I don't want to get bogged down in the technicalities.) I expected there to be an association between the two, but I was surprised by the tightness of the fit:
The correlation is .89 and the R-squared is .79. This means, roughly speaking, that 80% of the variation in central-core job density can be explained by the number of jobs in the metropolitan area. Doubling the number of metropolitan area jobs tends to increase the central-core job density by about 68%. (The slope of the trend line is 0.75.)
Think about this a second. This data set includes small metropolitan areas and large metropolitan areas. Young cities and old cities. Sunbelt cities and Northeastern cities. Western cities and Midwestern cities. Cities with manufacturing economies and cities with information economies. Transit-oriented cities and autocentric cities. Dense cities and sparse cities. And yet central-core job densities grow at roughly the same rate across all types of cities; city form or structure doesn't matter much.*
Why? One explanation might be that as cities add jobs, they invariably need more lawyers, insurance companies, banks and other support jobs. This is true regardless of the type of economy or the distribution of jobs within the metropolitan area. Perhaps this "invariant" (too strong?) shows that these service jobs benefit from a central location regardless of how the city grows outside the central core. In other words, this might be evidence that certain professions believe there are benefits to agglomeration. Just a theory.
Or perhaps I'm making too big a deal about this. In any event, I think this is the interesting story and one I'd like to see Kneebone or some other smart person address.
*Detroit and New York City are outliers here. Detroit is an outlier because its central city is emptying out at an alarming rate. New York is always an outlier because of Manhattan's extreme density. Los Angeles is somewhat of an outlier but I think that's a function of Kneebone's narrow definition of CBD's; Los Angeles has some fairly old, dense concentrations of jobs (Long Beach, for example), that she excludes from the central-city core.
Update: Spreadsheet on Google Docs.
