The Depart of Transportation’s Federal Highway Commission has released the latest report on Traffic Volume Trends (PDF format), data through May. The lead observation is that travel on all roads and streets declined by -1.9% (-5.0 billion vehicle miles) for May 2011 as compared with May 2010.
Here is a chart that illustrates this data series from its inception in 1970. I’m plotting the “Moving 12-Month Total on ALL Roads,” as the DOT terms it. See Figure 1 in the PDF report, which charts the data from 1986. My start date is 1971 because I’m encorporating all the available data from the DOT spreadsheets (see link).
Over the weekend, Bill McBride at Calculated Risk analysed this data series and points out that the rolling 12-month miles driven contracted from its all-time high for 39 months during the stagflation of the late 1970s to early 1980s, a double-dip recession era. The current dip has lasted for 42 months and counting — a new record.
The Population-Adjusted Reality
Total Miles Driven, however, is one of those metrics that must be adjusted for population growth to provide the most revealing analysis, especially if we’re trying to understand the historical context. We can do a quick per-capita adjustment of the data using an appropriate population group as the divisor. I use the Bureau of labour Statistics’ Civilian Noninstitutional Population Age 16 and Over (FRED series CNP16OV). The next chart incorporates that adjustment with the growth shown on the vertical axis as the per cent change from 1971.
Clearly, when we adjust for population growth, the Miles-Driven metric takes on a darker look. The nominal 39-month dip that began in May 1979 grows to 61 months, slightly more than five years. The trough was a 6% decline from the previous peak.
The population-adjusted all-time high dates from June 2005. That’s 71 months — nearly six years. And since the latest data is the lowest reading since the all-time high, the best we can hope for is that May “might” have been the trough.