  Project promoters, forecasters, and managers sometimes object to two things
in measuring inaccuracy in travel demand forecasting: (1) using the forecast
made at the time of making the decision to build as the basis for measuring
inaccuracy and (2) using traffic during the first year of operations as the
basis for measurement. This paper presents the case against both objections.
First, if one is interested in learning whether decisions about building
transport infrastructure are based on reliable information, then it is exactly
the traffic forecasted at the time of making the decision to build that is of
interest. Second, although ideally studies should take into account so-called
demand "ramp up" over a period of years, the empirical evidence and practical
considerations do not support this ideal requirement, at least not for large-N
studies. Finally, the paper argues that large samples of inaccuracy in travel
demand forecasts are likely to be conservatively biased, i.e., accuracy in
travel demand forecasts estimated from such samples would likely be higher than
accuracy in travel demand forecasts in the project population. This bias must
be taken into account when interpreting the results from statistical analyses
of inaccuracy in travel demand forecasting.
