In New York, a large fraction of employment clusters in a rectangle bounded roughly by 59th Street, 2nd Avenue, 42nd Street, and 9th Avenue. Although it’s a commonplace that New York employment is centralized around Manhattan, in reality most of Manhattan is residential, and employment is concentrated in a few square kilometers in the heart of Midtown. This is where the subway lines converge from all directions – elsewhere there simply isn’t enough capacity. Of course it wasn’t always like this: Manhattan’s population in the 1890s was the same as it is today, and it was clustered toward the southern third of the island, but employment was relatively evenly distributed in the downtown area. What has happened since then is that New York became a transit city.

There’s a strong correlation between the form of a city and the mix of transportation options people use. This extends well beyond density, but the principle is the same. Transit is at its best at high intensity, because this is what supports high-frequency service. Cars are the opposite: even on a normal urban street, a car alone will beat any rapid transit line, but every additional car will slow down the road dramatically, so that at even the moderate intensity of an edge city gridlock ensues.

Although usually this principle is stated in terms of density, it’s equally true for work centralization. The pedestrian city and the bus city will be dense all over, and feature high job density scattered across neighborhoods: walking is too slow for the transit city pattern to emerge, and buses have too little capacity. But dedicated rapid transit wants to serve an area right next to the stations, and once a network is built, a CBD grows around the central area. This CBD is typically small, just a few square kilometers. Even vaguely CBD-ish locations, such as Penn Station, are too far, as one commonly quoted figure about work locations demonstrates. The CBD isn’t even large enough to encompass all of the 34h-59th Street strip that the tourist guidebooks define as Midtown. The subway lines only form a tight mesh in a subset of that general area.

The job density of such a CBD is measured in hundreds of thousands per square kilometers, requiring many high-rise towers, several of which are supertall. In contrast, most of New York’s residences are mid-rise, and Tokyo’s are low- and mid-rise; their residential densities in the low tens of thousands per square kilometer are high enough that they are considered the epitome of density, but their CBDs are an order of magnitude denser.

Of the major transit cities of the world, Paris is the only one that’s resisted this trend with its height limit, but instead a transit-like CBD started out in La Défense, and the same pattern that comes from the subway in New York or Tokyo or the L in Chicago emerges with the RER. Of course, Paris maintains very high residential density, but its job distribution is more in line with that of a bus city – employment is dense all over, and the Downtown Paris employment density peak is less pronounced than in comparable transit city downtowns.

This does not mean a transit city needs to have empty trains going in the reverse-peak direction, as Cap’n Transit, Jarrett Walker, and others charge. A transit city will have job destinations outside the CBD, growing around rapid transit junctions: for example, Tokyo has Shinjuku, Shibuya, and Ikebukuro, all of which are so replete with high-rises it’s hard easy to forget they’re secondary job centers. While there is still a pronounced peak direction, people rely on transit so much that they take it for regular errands, supporting very high off-peak frequency by the standards of trains with drivers.

New York has something similar in Downtown Brooklyn, Jamaica, and Long Island City, but the modal split of those job destinations is much less favorable to transit – 50% in Downtown Brooklyn and Long Island City and 30% in Jamaica, according to a study of New York’s secondary job centers that I can no longer find. This is a general feature of many old American cities: the core looks like a transit city, but beyond it is a car-centric city, filled with edge cities and edgeless cities. Because the layout beyond the core is car-centric, the off-peak and reverse-peak traffic that supports high all-day bidirectional frequency on the Tokyo rail network, or for that matter on most New York City Subway lines, does not exist. The preference of American commuter rail agencies for peak-only service comes partly from an operating model that makes it impossible to run frequent off- and reverse-peak service, but also from a job distribution that makes the market for such runs small even under the best industry practice.

A corollary of this fact is that the multipolarity of other cities, for example Los Angeles, is not an asset. It would be an asset if those job centers were intense and could be easily served by transit; in reality, they have moderate intensity, nothing like that of the secondary centers of Tokyo or even New York, and serving many of them requires digging new subway lines. Burbank, on the legacy Metrolink network, could make a reasonable site for a transit-oriented secondary center, if commuter rail operations were modernized and local transit lines were extended to it; the Westside and Santa Monica do not, and the hope is that the investment in the Subway to the Sea could enable them to grow to reasonable size.

The key here is that the reason Shinjuku, Ikebukuro, and Shibuya are as transit-oriented as Central Tokyo is that they historically arose as connection points between the Yamanote Line and the private railroads. In particular, they already had rapid transit fanning out from multiple directions when they became major job centers. But Tokyo’s transit development history is peculiar; most other cities did not have large electrified rapid transit systems terminating at the edge of the urban core prior to building local subway lines.

A second corollary then is a strategy that sought to make New York a more transit-oriented city would treat centralization differently. It should turn the secondary centers into transit nodes in their own right, with tails extending as far out as reasonably possible. Jamaica already has some of the infrastructure, but it’s used poorly because of antiquated LIRR practices; the same can’t be said of Flushing, so a priority should be to build reasonable-quality transit from multiple directions, connecting Flushing with College Point and Jamaica and modernizing the LIRR so that it could connect it with Bayside.

A point that many people writing about this neglect (with pleasant exceptions like Cap’n Transit, the Streetsblog crowd, and Paul Barter) is that this requires both the carrot of more transit and the stick of less parking. In any case it’s hard to create high job densities when much of the land is used for parking. But on top of that parking mandates make it difficult for transit to be competitive when it’s expected to include railyards and depots in its budget and roads are not.

But what a transit city doesn’t need is job dispersal. The importance of creating secondary centers is strictly as alternatives to auto-oriented edge cities and edgeless cities, since whatever happens, not all jobs will be in the CBD. A large city with rapid transit connecting to all major neighborhoods will automatically have high transportation capacity. Rapid transit is good at transporting tens of thousands of people in one direction in the peak hour; let it do what it’s good at.