Often the division of responsibilities within MPOs relegates travel forecasting to technical experts while use of model output for policy and plan development is in the hands of planners and policy makers. This division of labor can lead to unintended consequences in the decision-making arena. As noted by the Committee for Determination of the State of the Practice in Metropolitan Area Travel Forecasting in the Transportation Research Board’s Special Report 288, Metropolitan Travel Forecasting – Current Practice and Future Direction, “There are many sources of error and uncertainty in travel demand forecasting, but end users of most travel forecasts would not be aware of these limitations” (page 85). Also, “In the committee’s experience, agencies have reported future-year facility volumes on the basis of data taken directly from the model outputs. Unless the models have been carefully restructured or estimated with the objective of addressing such issues, the resulting forecasts may not be valid” (page 75).
Planners are engaged in increasingly complex decision-making analyses relying on the output of ever-more sophisticated modeling tools, yet they often possess only a cursory understanding of travel forecasting models and their inherent assumptions, biases, and limitations. This heightens the risk that the modeling process or its output will be unintentionally misrepresented or misapplied, with unfortunate consequences for resulting decisions and investments.
Transportation planners do not need to be able to develop a travel demand model or conduct a traffic forecast, but they do need a solid understanding of key modeling fundamentals that go beyond recitation of the four-step process. This is true whether the model is developed in-house by agency staff or is contracted to a consultant or outside agency. Planners often act as the liaison between policy makers, the public, and modelers. A stronger foundation in the appropriate applications of forecasting models and the ability to effectively communicate this will strengthen that planning role.
This report identifies and describes assumptions, applications, and limitations that transportation planners should understand about both conventional and advanced forecasting models and processes. It should help planning practitioners to better understand the forecasting process, increase their ability to interpret and apply model output effectively, and help them better communicate this aspect of the transportation planning process to policy makers. Transportation planners should be able to (1) ask and answer critical questions about their agencies’ models and model development processes; (2) to understand how robust or sensitive the outputs are, why that matters; and (3) incorporate that knowledge into planning and programming decision-making processes.