Measuring Inaccuracy in Travel Demand Forecasting: Methodological Considerations Regarding Ramp Up and Sampling

Transportation Research Part A: Policy and Practice, vol. 39, no. 6, pp. 522-530

9 Pages Posted: 23 Mar 2013 Last revised: 28 Jan 2014

See all articles by Bent Flyvbjerg

Bent Flyvbjerg

University of Oxford - Said Business School

Date Written: July 1, 2005

Abstract

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.

Keywords: Travel demand forecasting, Inaccuracy of travel forecasts, Demand ramp up, Sampling for measuring inaccuracy, Bias

Suggested Citation

Flyvbjerg, Bent, Measuring Inaccuracy in Travel Demand Forecasting: Methodological Considerations Regarding Ramp Up and Sampling (July 1, 2005). Transportation Research Part A: Policy and Practice, vol. 39, no. 6, pp. 522-530. Available at SSRN: https://ssrn.com/abstract=2238017

Bent Flyvbjerg (Contact Author)

University of Oxford - Said Business School ( email )

Park End Street
Oxford, OX1 1HP
Great Britain

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