Discouraging, but far from unexpected.

In an excellent post, blogger Kevin Drum summarizes the dilemma of making transit work in sparsely populated suburbs, using data from Joel Garreau’s Edge City.

According to Garreau, the “floor-to-area” ratio (or FAR)—the ratio of the floor space of buildings to the land on which they sit—is highly predictive of transportation conditions. At an average ratio of 1.0 (that is, where the total floorspace of buildings, including multi-story buildings, is equal to the total land area), traffic jams become a major political issue. As a result, in sprawling suburbs and “edge cities” at the fringes of a metro area, the FAR rarely exceeds 1.5.

The problem is that transit doesn’t start to become cost effective until the FAR reaches 2.0.

So there’s a big gap between the density where local traffic gets backed up, and the density where alternatives to the car become economically feasible.

This suggests that the best way to create viable alternatives to the car isn’t to try to gradually increase densities in sparsely populated suburbs. Residents who depend on their cars will start to complain of the traffic, and create political pressure to halt new development. Instead, the best strategy may be to concentrate development—both jobs and houses—in downtown areas, where the FAR already typically exceeds 5.0, and where transit and walking are already viable means of tranportation.

Garreau (via Drum) also cites an interesting statistic: for every $1 in tax receipts from a residential subdivision, as much as $1.22 goes out to pay for schools and other infrastructure. But for every $1 in tax receipts from commercial real estate, only about $.40 goes out, mostly in road expenditures. This is one reason why many suburbs bend over backwards to attract commercial development of any kind—and, perhaps, why so many wind up with the big box retail stores that are the hallmark of suburban sprawl.