"Analysis of traffic forecasts and actual first year traffic at 210 international road and rail projects concludes that the road forecasts are inaccurate with about half out by over 20%, but not seriously biased up or down. Transit ridership forecasts, by contrast, are grossly inaccurate, heavily biased upward, and clearly manipulated by special interests. The average error of transit ridership is +106%. Recent projects show no improvement over earlier ones.
Very often planners are a large part of the problem however, FHB write. Accurate forecasts do not serve their purposes because they are more interested in using numbers to promote the project, what FHB call the "dark side" of forecasting.
'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.' "
Thanks to Chris Steins