I’ve been thinking a lot about where subway extensions can go in New York. One of the appendices we’re likely to include down the line in the Transit Costs Project is a proposal for what New York could do if its construction costs were more reasonable, and this means having to think about plausible extensions. Leaving aside regional rail and systematic investments for now, this may roughly be it:
The full-size image (warning: 52 MB) can be found here.
The costs depicted are about twice as high as what I wrote in 2019 with Nordic costs as the baseline, because nominal Nordic costs have doubled since then, partly due to updating price levels from the early 2010s to the early 2020s, but mostly because of the real cost explosion in the Nordic countries. These costs are about $200 million/km in outlying areas, $300 million/km in Manhattan or across water, somewhat less than $100 million/km above ground or in an open trench, and higher than $300 million/km when reconstruction of existing tunnel complexes is proposed; everything is rounded to the nearest $100 million, which creates some rounding artifacts for short extensions that cancel one another out.
But the precise map is not what I think is the most interesting. The point is to build to the frontier of the cost per rider that is acceptable in American cities today, so by definition the marginal line for inclusion on the map, such as the D extension to Gun Hill Road to meet with the 2 train, is also socioeconomically marginal. What I think is more interesting is how important transit-oriented development is for the prospects of lines beyond the most obvious ones (Second Avenue Subway Phase 2, 125th Street, Utica, Nostrand, IBX, and maybe also the 7 to College Point).
The current land use in New York is largely frozen from the middle of the 20th century; the 1961 zoning law was the watershed. Since then, change has been slow, in contrast with rapid redevelopment in places that have chosen a pro-growth path. If the pace of change stays slow, then fewer lines are viable; if the city instead chooses not to keep anti-developmental neighborhood interests in the loop, then more are.
This, in turn, feeds into growth plans. Nordic and Italian planning bundles the question of where the regional housing growth goes with where the subway goes. (Our other positive case study, Turkey, works differently; the answer to both questions is “everywhere.”) This means that subway service goes to areas where substantial quantities of transit-oriented development will be permitted and built, often in negotiations with NIMBY municipalities that would rather just get the infrastructure without the housing; in Stockholm the scale involved is tens of thousands of units per tranche of Nya Tunnelbanan.
In the case of New York, this affects the shape of the map above more than anything. The 6 extension to Coop City is likely good either way, but the other radial extensions in the Bronx are more questionable and depend on where new housing in the borough will be built. The same is true in Queens: more housing in Northeast Queens may argue even in favor of further lines not depicted on the map, for example extending the 7 even further.