Procedures for dealing with optimism bias in transport planning

The Supplementary Green Book Guidance on Optimism Bias (HM Treasury 2003) with reference to the Review of Large Public Procurement in the UK (Mott MacDonald 2002) notes that there is a demonstrated, systematic, tendency for project appraisers to be overly optimistic and that to redress this tendency appraisers should make explicit, empirically based adjustments to the estimates of a project’s costs, benefits, and duration. HM Treasury recommends that these adjustments be based on data from past projects or similar projects elsewhere, and adjusted for the unique characteristics of the project in hand. In the absence of a more specific evidence base, HM Treasury has encouraged departments to collect data to inform future estimates of optimism, and in the meantime use the best available data. In response to this, the Department for Transport (henceforth DfT), has contracted Bent Flyvbjerg in association with COWI to undertake the consultancy assignment "Procedures for dealing with Optimism Bias in Transport Planning". The present Guidance Document is the result of this assignment.