Post-Construction Evaluation of Forecast Accuracy

Project summary:

Evaluations of proposed projects and their subsequent performance depend critically on forecasts of demand made well in advance of actual implementation. Depending on the forecasting method used, there may be considerable uncertainty in projections of future benefits or other performance criteria. This study sought to evaluate the accuracy of demand forecasts made for a number of recently completed projects and to estimate the impact of forecast error on post-construction measures of project performance. Data for the study was drawn from completed environmental review documents as well as other official forecasts produced in support of selected projects, combined with recent traffic volume data from Mn/DOT. Results were compared for urban and rural projects as well as for different functional classes. The analysis indicated a general trend of underestimation in roadway traffic forecasts with factors such as highway type, functional classification, and direction playing an influencing role. Roadways with higher volumes and higher functional classifications such as freeways are subject to underestimation compared to lower volume roadways/functional classifications. The comparison of demographic forecasts showed a trend of overestimation while the comparison of travel behavior characteristics indicates a lack of incorporation of fundamental shifts and societal changes. Sources of forecast error were investigated in terms of technical and institutional issues, and the research concluded with several suggestions for managing and possibly reducing uncertainty in project demand forecasts.