But it also will make those distant suburbs more accessible, and hence more appealing for residential and other development. Families in such areas are likely to travel over longer distances, and by auto, for most needs, compared to closer in peers, thus increasing overall travel. And many riders of the new line are likely to be diverted from other transit services, reducing their revenues, and leading to a reduction of service on those lines. So the overall effect can include many negative impacts, as well as positive ones. Obviously the same arguments apply to highways.
Transit is chosen here to drive home the point that there are almost no unambiguously good options.
Almost all actions improving transport can actually have negative consequences. The lack of growth of transit travel indeed the data show a slight decline is amazing. This is very surprising given the four-fold growth in travel overall, and the massive investments in transit over the four decades covered, including the Center City Tunnel, a virtually entirely new fleet of subway cars, buses, and regional rail trains, the new Airport High Speed Line, and rebuilding of the Frankford Elevated Line, among others. One obvious implication of this is that we must be much smarter about transit investments in the future if they are to capture a larger share of trips. Note the many assumptions that were necessary.