Forecasting inaccuracies: a result of unexpected events, optimism bias, technical problems, or strategic misrepresentation?


  • Petter Naess Norwegian University of Life Sciences
  • Jeppe Astrup Andersen Aalborg University
  • Morten Skou Nicolaisen Aalborg University
  • Arvid Strand Institute of Transport Economics



Forecasting, Inaccuracy, Bias, Modeling


Based on the results from a questionnaire survey and qualitative interviews among different actors involved in traffic forecasting, this paper discusses what evidence can be found in support of competing explanations of forecasting errors. There are indications that technical problems and manipulation, and to a lesser extent optimism bias, may be part of the explanation of observed systematic biases in forecasting. In addition, unexpected events can render the forecasts erroneous, and many respondents and interviewees consider it to be simply not possible to make precise predictions about the future. The results give rise to some critical reflections about the reliability of project evaluations based on traffic forecasts susceptible to several systematic as well as random sources of error.

Author Biographies

Petter Naess, Norwegian University of Life Sciences

Department of Landscape Architecture and Spatial Planning Professor in Planning in Urban Regions, Head of Doctoral School

Jeppe Astrup Andersen, Aalborg University

Part-time lecturer

Morten Skou Nicolaisen, Aalborg University

Department of Development and Planning Assistant Professor

Arvid Strand, Institute of Transport Economics



Andersen, J. A. 2013 The shaping of transport model based knowledge production: An embedded case study of Danish transport modelling practice in a contemporary and historical perspective. Ph.D. thesis, Aalborg University, Aalborg.

Bain, R. 2009. Error and optimism bias in toll road forecasts. Transportation 36(5):469-482.

Bhaskar, R. 2008. A Realist Theory of Science. London and New York: Routledge.

Bradburn, N. M., S. Sudman, and B. Wansink. 2004. Asking Questions: The Definitive Guide to Questionnaire Design – For Market Research, Political Polls, and Social and Health Questionnaires. San Francisco: Jossey-Bass.

Danermark B., M. Ekström, L. Jacobsen, and J. C. Karlsson. 2001. Explaining Society. Critical realism in the social sciences. London and New York: Routledge.

Flyvbjerg, B. 2007. Policy and planning for large-infrastructure projects: problems, causes, cures. Environment and Planning B: Planning and Design 34(4):578–597.

Flyvbjerg, B., N. Bruzelius, and W. Rothengatter. 2003. Megaprojects and risk. An anatomy of ambition. Cambridge: Cambridge University Press.

Flyvbjerg, B., and COWI. 2004. Procedures for dealing with optimism bias in transport planning. Guidance document. London: Department for Transport.

Flyvbjerg, B., M. S. Holm, and S. Buhl. 2002. Underestimating Costs in Public Works Projects: Error or Lie? Journal of the American Planning Association 68(3):279-295.

Flyvbjerg, B., M. S. Holm, and S. Buhl. 2005. How (in)accurate are demand forecasts in public works projects? The case of transportation. Journal of the American Planning Association 71(2):131-146.

Fouracre, P. R., R. J. Allport, and J. M. Thomson. 1990. The Performance and Impact of Rail Mass Transit in Developing Countries. UK Transport and Road Research Laboratory.

Mackie, P., and J. Preston. 1998. Twenty-one sources of error and bias in transport project appraisal. Transport Policy, 5(1):1-7.

NAO. 1988. Department of Transport, Scottish Development Department and Welsh Office: Road Planning. London: National Audit Office.

Newman, P. W. G., and J. R. Kenworthy. 1989. Cities and Automobile Dependence. Aldershot: Gower.

Nicolaisen, M. S. 2012. Forecasts: Fact or Fiction? Uncertainty and Inaccuracy in Transport Project Evaluation. Ph.D. thesis, Aalborg University, Aalborg.

NOU 1999:28. Gardermoprosjektet: evaluering av planlegging og gjennomføring. (The Gardermoen project: evaluation of planning and implementation.) Report from a committee appointed through Royal Resolution of May 15, 1998. Oslo: National government.

Næss, P. 2011. The Third Limfjord Crossing. A case of pessimism bias and knowledge filtering. Transport Reviews 31(2):231-249.

Næss, P., J. Andersen, M. S. Nicolaisen, and A. Strand. 2014. Transport Modelling in the Context of the ‘Predict and Provide’ Paradigm. Forthcoming in European Journal of Transport and Infrastructure Research 14(2).

Næss, P., B. Flyvbjerg, and S. Buhl. 2006. Do road planners produce more 'honest numbers' than rail planners? : an analysis of accuracy in road-traffic forecasts in cities versus peripheral regions. Transport Reviews 26(5):537-555.

Næss, P., and A. Strand. 2012. What kinds of traffic forecasts are possible? Journal of Critical Realism 11(3):277-295.

Næss, P., and A. Strand. 2014. Traffic forecasting at ‘strategic’, ‘tactical’ and ‘operational’ level: A differentiated methodology is necessary. Forthcoming in DISP 50.

Osland, O., and A. Strand. 2008. Evaluating large transport infrastructure projects — a critical-constructive review of the theory of strategic misrepresentation. Presented at the conference ‘Infrastructure Systems and Services: Building Networks for a Brighter Future (INFRA)’, Rotterdam, November 10-12, 2008.

Parthasarathi, P., and D. Levinson. 2010. Post Construction Evaluation of Traffic Forecast Accuracy. Transport Policy 12(6):428–43.

Pickrell, D.H. 1990. Urban Rail Transit Projects: Forecast Versus Actual Ridership and Cost. Urban Mass Transportation Administration.

Tennøy, A. 2003. Bidrar bruk av transportanalyser i byplanleggingen til vekst i biltrafikken? Paper for the conference ‘Traffic days at Aalborg University’, August 25–26, 2003.

Wachs, M. 1989. When planners lie with numbers. Journal of the American Planning Association, 55(4):476-479.

Welde, M., and J. Odeck. 2011. Do Planners Get It Right? The Accuracy of Travel Demand Forecasting in Norway. European Journal of Transport and Infrastructure Research 11(1):80–95.




How to Cite

Naess, P., Andersen, J. A., Nicolaisen, M. S., & Strand, A. (2015). Forecasting inaccuracies: a result of unexpected events, optimism bias, technical problems, or strategic misrepresentation?. Journal of Transport and Land Use, 8(3).