In response to my takedown of Reason, specifically my puzzlement at the estimates of inaccuracy in traffic forecasts, alert reader Morten Skou Nicolaisen sent me several papers on the subject. While there is past research about traffic shortfalls, for example this paper by Flyvbjerg (hosted on a site opposing the Honolulu rapid transit project), Flyvbjerg’s references are papers from twenty years ago, describing mostly subway projects in developing countries, but also rapid transit and light rail projects in the US built in the 1970s and 80s. Unlike Flyvbjerg, who posits that planners are lying, the authors of the papers he references have other theories: currency exchange rate swings, the challenges of underground construction, inaccurate forecasts of future economic growth, outdated traffic models based on postwar road traffic models. See section 6 of Walmsley and Pickett, and sections 3.3 and 4.2 of Fouracre, Allport, and Thomson (see also the range of costs for underground construction in developing countries in section 3.3).
The question is then whether things have improved since 1990. Since the first study to point out to cost overruns and ridership shortfalls in the US was by Pickrell, the question is whether post-Pickrell lines have the same problems, or whether there are better outcomes now, called a Pickrell effect.
The answer, as far as ridership is concerned, is very clearly that ridership shortfalls are no longer a major problem. See recent analysis by Hardy, Doh, Yuan, Zhou, and Button; see specifically figure 1. Cost overruns also seem to be in decline and are no longer big, although a multiple regression analysis finds no Pickrell effect for cost, just for ridership.
In particular, there is no comparison between projects from 30 years ago, most of which are underground, and present-day developed-world high-speed and urban rail lines.