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 study found that 95 percent of forecasts were “accurate to within half a lane” and that forecast accuracy is generally improving. The average difference between forecasted traffic volumes and actual volumes is around 17 percent, but it was closer to 28 percent in the 1990s. Some of the best models today are off by only 14 percent on average, while some of the worst performing ones are around 32 percent.