The Department of Transportation repeatedly over-estimates vehicle miles traveled.

By now, you may be familiar with any number of variations on this chart, which shows the historic arc of American car consumption with a notable peak around the year 2006. For decades, without fail, Americans drove more collective miles each year than they had the year before. The upward trend was tremendously reliable – until, that is, the middle of the last decade. Our collective "vehicle miles traveled" hit a ceiling just over 3 trillion miles per year, then dipped, and the number has been wavering ever since, prompting much speculation.

That now-familiar line takes on new meaning, though, in this chart, from Eric Sundquist at the State Smart Transportation Initiative, and Tony Dutzik at The Frontier Group. Their latest illustration contrasts our actual VMT with government projections over the last decade forecasting driving trends for the future:

Vehicle miles traveled in trillions as tracked by FHWA’s Travel Volume Trends, compared to projections from the U.S. DOT "Conditions and Performance Report to Congress."

Those projections come from an annual U.S. Department of Transportation "Conditions and Performance" report to Congress. And the DOT bases the estimates on traffic data from local agencies like Metropolitan Planning Organizations.

We've talked with Dutzik before about the alarming difference between these official projections of automobile use and reality as we're witnessing it in the short term. What's startling about this latest chart isn't the difference between that thick black line and the 2010 projection. It's the fact that the government seems to have made the exact same miscalculation repeatedly, and despite growing evidence that something quite different is going on. These projections also aren't wobbling off course years after they were first produced (no one expects a prognosticator to perfectly predict what will happen 10 or 20 years from now). They've turned out to be grossly inaccurate sometimes months after their release.

In total, Dutzik and Sundquist looked at 61 yearly projections from these DOT reports, going back years. And every one of them has turned out to be too high. "For example," Sundquist writes, "the 1999 C&P overshot 2012 reported VMT by more that 22 percent—almost 11 extra states’ worth of driving."

This debate isn't academic. These projections come from a report, Sundquist notes, that is widely referenced by Congress in setting funding levels for highway infrastructure. That means we dramatically overestimate car use as the default setting for federal policy.

This chart suggests that we need to get better at projecting future automobile use, because that's how we're going to get better at funding the future transportation infrastructure that people actually need.

Hat tip to Wonkblog.

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