As reliant on traffic forecasts as transportation agencies are to plan and design road projects, those forecasts are rarely evaluated to see how well they held up after project implementation. A massive new study, outlined in a recent NCHRP report, fills that gap. It compares traffic forecasts from 1,291 projects that opened since 1970 to actual traffic counts. Only 10 percent of the projects opened before 2003.
The U.S. DOT recently released its 2015 Conditions and Performance Report to Congress, describing the current state and future needs of the country’s roads, bridges, and other transportation infrastructure. In this newest C&P report, U.S. DOT recognizes that its past forecasts were too high, adding, “states have tended to underpredict future VMT during periods when actual VMT was growing rapidly and to overpredict at times when actual VMT growth was slowing or declining.”
As VMT decreases, forecasting demand and toll revenues for new projects is becoming increasingly difficult. DOTs should consider three new factors in traffic forecasting: first, how flat-to-declining VMT will affect revenues collected; second, how the presence of untolled parallel roadways will also impact toll revenue; and third, how driver value-of-time plays into roadway choice, also affecting toll collection.