Quantifying and decomposing the uncertainty in appraisal value of travel time savings

This paper is concerned with computing interval estimates for appraisal values of travel time savings (VTTS) for non-work journeys. The paper has important conclusions relating to the benefits, in terms of uncertainty in appraisal VTTS, of resampling the base year VTTS and improving on the precision of the estimate of the GDP elasticity in the uprating equation. Importantly it is shown that the interval widths increase dramatically as VTTS is forecast further into the future. This has a significant modelling implication in that it is the uncertainty associated with the process of uprating base estimates of VTTS which results in the large interval width, rather than that associated with the base year VTTS estimate. This in turn implies a need to regularly resample the base VTTS, so as to avoid excessive temporal extrapolation of the base VTTS.