Forecasting the impossible: The status quo of estimating traffic flows with static traffic assignment and the future of dynamic traffic assignment

Abstract

Roadway expansion proposals are evaluated primarily with travel time metrics including vehicle hours traveled (VHT) and vehicle hours of delay (VHD). Travel time metrics have been criticized for ignoring other travel modes, over-emphasizing mobility over accessibility, and failing to account for economic externalities. However, there is an even more fundamental problem. The travel time metrics are inaccurate because they rely on Static Traffic Assignment (STA), a 40-year old approach that routinely forecasts unfeasible future traffic flows that exceed capacity. Basing metrics on these impossible volumes produces invalid results. The common practice of exporting link volumes or subarea trip tables to microsimulation fails to address the STA problem because the unrealistically high STA traffic forecasts are forced onto a capacity-constrained network. Inaccurate travel time modeling helps to explain why so many roadway projects fail to deliver promised travel time savings. Replacing STA with Dynamic Traffic Assignment (DTA) produces more realistic metrics. A case study from the Portland Maine region is presented where STA and DTA are compared with the same inputs. The DTA model fits base year count traffic count much better. The DTA model produces more much lower and more realistic estimates of congestion relief from freeway widening.