A long-running conundrum in American urbanism is that the urban area with the highest population density is Los Angeles, rather than New York. Los Angeles is extremely auto-oriented, with a commute modal split that’s only 5% public transit, same as the US average, and doesn’t feel dense the way New York or even Washington or Chicago or Boston is. In the last 15 years there have been some attempts to get around this, chiefly the notion of weighted or perceived density, which divides the region into small cells (such as census tracts) and averaged their density weighted by population and not area. However, even then, Los Angeles near-ties San Francisco for second densest in the US, New York being by far the densest; curiously, already in 2008, Chris Bradford pointed out that for American metro areas, the transit modal split was more strongly correlated with the ratio of weighted to standard density than with absolute weighted density.
DW Rowlands at Brookings steps into this debate by talking more explicitly about where the density is. She uses slightly different definitions of density, so that by the standard measure Los Angeles is second to New York, but this doesn’t change the independent variable enough to matter: Los Angeles’s non-car commute modal split still underperforms any measure of density. Instead of looking at population density, she looks at the question of activity centers. Those centers are a way to formalize what I tried to do informally by trying to define central business districts, or perhaps my attempts to draw 100 km^2 city centers and count the job share there (100 km^2 is because my French data is so coarse it’s the most convenient for comparisons to Paris and La Défense).
By Rowlands’ more formal definition, Los Angeles is notably weaker-centered than comparanda like Boston and Washington. Conversely, while I think of Los Angeles as not having any mass transit because I compare it with other large cities, even just large American cities, Brookings compares the region with all American metropolitan areas, and there, Los Angeles overperforms the median – the US-wide 5% modal split includes New York in the average so right off the bat the non-New York average is around 3%, and this falls further when one throws away secondary transit cities like Washington as well. So Los Angeles performs fairly close to what one would expect from activity center density.
But curiously, Chicago registers as weaker-centered than Los Angeles. I suspect this is an issue of different definitions of activity centers. Chicago’s urban layout is such that a majority of Loop-bound commutes are done by rail and a supermajority of all other commutes are done by car; the overall activity center density matters less than the raw share of jobs that are in a narrow city center. Normally, the two measures – activity center density and central business district share of jobs – correlate: Los Angeles has by all accounts a weak center – the central 100 km^2, which include decidedly residential Westside areas, have around 700,000 jobs, and this weakness exists at all levels. Chicago is different: its 100 km^2 blob is uninspiring, but at the scale of the Loop, the job density is very high – it’s just that outside the Loop, there’s very little centralization.