The National Model System LMS and the New Regional Models NRM are regularly used in The Netherlands to forecast the national and regional transport volumes and traffic flows on specific network links for a single or a limited number of scenarios. The same models are also used to give the likely impacts of transport infrastructure projects (e.g. new roads, wider roads, new railway lines) and transport policies (e.g. road pricing). All these predictions are point estimates, and, even when produced for several scenarios, do not give insight into the uncertainty margin that exists around these forecasts. The objectives of the project ‘Uncertainty in traffic forecasts’ are: • To develop a methodology to estimate the amount of uncertainty in forecasting for new infrastructure (especially roads). • To implement and test this methodology in two case-studies (using the Dutch National Model System LMS and the New Regional Models NRM respectively). Throughout this report, the authors distinguish between input uncertainty (e.g. on the future incomes) and model uncertainty (including specification error and error due to using parameter estimates instead of the true values). Chapter 2 of this report presents the main outcomes of two literature reviews on quantifying the amount of uncertainty in forecasting with transport models: • Literature review for public projects; • Literature review for public-private partnership (PPP) projects. In these reviews the national and international literature on methods to quantify the amount of uncertainty around the forecasts from transport models was described and assessed. Most relevant papers do not distinguish between public and PPP projects. Therefore we combined both phases into a single literature review on how to quantify uncertainty in transport modelling. In Chapter 3, the method developed for the treatment of input uncertainty as well as for model uncertainty is described. Chapter 4 contains outcomes on uncertainty margins for the LMS, and Chapter 5 for the NRM Noord-Brabant. Finally, in Chapter 6 the conclusions from the project as a whole are listed and recommendations are given. A general discussion on uncertainty in policy modelling (not specifically dealing with transport issues and transport models) can be found in Appendix 1. This chapter is based primarily on Walker et al. (2003), and summarises RAND’s experience on the issue of uncertainty. Appendix 2 contains the short descriptions of the papers and reports on quantifying uncertainty in transport models, as identified in the review of the international literature. It also contains the outcomes of interviews with international experts. In Appendix 3, the analytic method is used to derive the equations for the variance for model uncertainty in the LMS mode-destination and tour frequency models. Detailed outcomes for the LMS and NRM on quantifying uncertainty in traffic forecasts are in Appendix 4. (Author/publisher)
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