For 9 out of 10 rail projects, passenger forecasts are overestimated; the average overestimation is 106%...
Rail forecasts are systematically and significantly overestimated to a degree that indicates intent and not error on the part of rail forecasters and promoters...
…seemingly rational forecasts that underestimate costs and overestimate benefits have long been an established formula for project approval…
Forecasting is here mainly another kind of rent-seeking behavior, resulting in a make-believe world of misrepresentation that makes it extremely difficult to decide which projects deserve undertaking and which do not. The consequence… is that too many projects proceed that should not. We would like to add that many projects don’t proceed that probably should, had they not lost out to projects with “better” misrepresentation…
http://flyvbjerg.plan.aau.dk/Traffic91PRINTJAPA.pdf