Ex-post evaluations of demand forecast accuracy: A literature review

Travel demand forecasts play a crucial role in the preparation of decision support to policy makers in the field of transport planning. The results feed directly into impact appraisals such as cost benefit analyses and environmental impact assessments, which are mandatory for large public works projects in many countries. Over the last couple of decades there has been an increasing attention to the lack of demand forecast accuracy, but since data availability for comprehensive ex- post appraisals is problematic, such studies are still relatively rare. The present paper presents a review of the largest ex-post studies of demand forecast accuracy for transport infrastructure projects. The focus is twofold; to provide an overview of observed levels of demand forecast inaccuracy and to explore the primary explanations offered for the observed inaccuracy. Inaccuracy in the form of both bias and imprecision remains problematic for road, rail and toll projects alike, but the explanatory causes are difficult to pinpoint. Systematic ex-post evaluations and improved archiving protocols are suggested as necessary steps to improve the learning aspect of ex-post evaluations, as all studies in the review suffer from severe lack of data availability.