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Trapped on an 'Island' of Transit Isolation

with Louis Liss

Looking at job access by transit in metropolitan America certain things emerge that were to be expected in most places; because transit has long been a city-oriented service and a lot of transit agencies operate from the “hub-and-spoke” model inherited from centuries past--that is, the old tradition of running most of the lines into downtowns. From that, cities like Chicago, Boston, and Cincinnati have systems that have almost concentric-looking levels of access from high in the central cities to moderate in the inner suburban fringe, to negligible in the coverage outskirts. As covered in the last post, pockets of good service around routes of higher frequency or rail stations or bus hubs create exceptions to this rule.

On the other end of the spectrum from pockets, “islands” of less transit service to jobs can be harder to explain, and it is easier to look at the behavior of the model (mapping tool here) as a whole to explain it. The model launches random trips from the “weighted center” of each neighborhood--that is, the geographic center moved over towards the densest part of a neighborhood to better represent where most people live.  If transit stops aren’t within a three-quarter mile radius of this point, then they aren’t considered accessible. Even if there are a lot of stops accessible, if it’s only for one or two routes with long waiting times in between trips, then it also drives numbers down. This type of thing is really common towards the outside edges of the transit-covered area where things are more spread out and trips appear to be longer.

Sometimes, like in certain parts of Atlanta, a neighborhood in the midst of a few good transit lines can get a lower score if it only connects to infrequent “shuttle” or “connector” service; even if a rail line is a mile and a half away, if the shuttle only runs on the hour or half hour, there are very few plausible trips to jobs available.  More common on the urban fringe, trips to anywhere will take so long that few will allow access to jobs within the ninety minute mark.  Sometimes, this leads to a job access rating of zero even for a neighborhood even if there is access to transit stop. One of the more interesting quirks of the model comes from the weight of demographic data.  Since we only tracked access in neighborhoods that had “households”--separate from, for example, a neighborhood with a hospital or a prison where there may be people but no permanent homes.  An interesting place that exemplifies this is Austin; the area overall does very well in job access, but certain neighborhoods in the middle of the city appear to have no access, because they are occupied entirely by parks or the Texas Capitol.