Nexus group (Pavithra Parthasarathi and David Levinson) recently completed a study for MnDOT on the
Post-Construction Evaluation of Forecast Accuracy. A version of this was recently presented at TRB.
The net is, in the Twin Cities, freeway traffic was underestimated while non-freeway traffic was overestimated. The reasons are many.
This research evaluates the accuracy of demand forecasts using a sample of recently-completed projects in Minnesota and identiﬁes the factors inﬂuencing the inaccuracy in forecasts. The forecast trafﬁc data for this study is drawn from Environmental Impact Statements (EIS), Transportation Analysis Reports (TAR) and other forecast reports produced by the Minnesota Department of Transportation (Mn/DOT) with a horizon forecast year of 2010 or earlier. The actual trafﬁc data is compiled from the database of trafﬁc counts maintained by the Ofﬁce of Transportation Data and Analysis at Mn/DOT. Based on recent research on forecast accuracy, the inaccuracy of traffic forecasts is estimated as a ratio of the forecast trafﬁc to the actual trafﬁc. The estimation of forecast inaccuracy also involves a comparison of the socioeconomic and demographic assumptions, the assumed networks to the actual in-place networks and other travel behavior assumptions that went into generating the trafﬁc forecasts against actual conditions. The analysis indicates 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 and lower functional classifications. The comparison of demographic forecasts shows a trend of overestimation while the comparison of travel behavior characteristics indicates a lack of incorporation of fundamental shifts and societal changes.