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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

Deriving Elasticities of Demand From Pivot-Point Transport Models

Elasticities in transport are often used to answer questions such as: - ‘What happens with the transported volume if the price of the transport volume increases?’ - ‘If the price of a mode increases, how does that affect the volume transported by other modes?’ Changes in the price of a transport mode or transport chain (expressed in generalized costs, travel distance, travel time or impedance) lead to changes in the volume transported by that mode as well as other competing modes. A price elasticity of demand expresses the ratio of the percentage change in the volume versus the percentage change in the price. The impact of a change in price on the volume of a transport mode itself is called ‘own’ elasticity, while the impact on the volume of other modes is called ‘cross’ elasticity. The elasticities are often derived from the results of transport models. Besides giving an impression of the potential impacts of changes in price on the volume, elasticities are often used to validate models against sources in the literature or other models. Recently, Rijkswaterstaat has completed an update of the Dutch strategic freight transport model BasGoed. An analysis of the elasticities was part of the overall analysis which looked into the robustness of the model. The elasticities however gave counter-intuitive results. Further examination showed that it was the pivot-point method used in the freight transport model that caused a change in sign in the elasticities for some combinations of mode and commodity. The pivot-point method uses the estimated model only to produce changes (at the OD level) between the base year and a future year. These changes are then combined with a representation of the base year flows, which comes in the form of a base matrix, derived (as much as possible) from other data sources than those used for model estimation. Pivoting can be done separately at the end of each model stage (e.g. generation, distribution, mode choice) or a single time just before network assignment. An important aspect for an effective pivot-point application is the absolute and relative difference between the synthetic (modelled) matrices and the ‘observed’ base matrices. With the recent update of Basgoed the zoning system has become more detailed, which results in more origins and destinations without any freight transport volumes. This observed pattern is more difficult to predict in a model, leading to bigger differences between the synthetic- and observed base matrix and an ineffective pivot. To increase the problem, these differences between the matrices are not equal for the different modes, which influences the cross-elasticities even more. It leads to the problem that elasticities from the intermediate synthetic results do not match with the final results. The paper will address this problem of counter-intuitive elasticities from models that use a pivot-point method. An analysis of synthetic and observed matrices will be provided and a comparison of synthetic elasticities and elasticities after pivot will be given. The authors present possible causes of the elasticity problem, and some potential solutions will be provided in the paper. The BasGoed model serves as a case to illustrate the authors problem and the solutions. One of the conclusions that the authors reach is that further spatial disaggregation of models with more detailed base data also requires more specific choice models.

  • Record URL:
  • Supplemental Notes:
    • Abstract used by permission of Association for European Transport.
  • Corporate Authors:

    Association for European Transport (AET)

    1 Vernon Mews, Vernon Street, West Kensington
    London W14 0RL,    
  • Authors:
    • Kiel, Jan
    • de Bok, Michiel
    • Wesselink, Bart
    • van den Berg, Monique
    • de Jong, Gerard
  • Conference:
  • Publication Date: 2019

Language

  • English

Media Info

  • Media Type: Digital/other
  • Pagination: 17p
  • Monograph Title: European Transport Conference 2019

Subject/Index Terms

Filing Info

  • Accession Number: 01749318
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Aug 27 2020 11:08AM