Two issues to be addressed:
- How are VMT (and gasoline consumption) reacting to high petroleum prices?
- Is gasoline consumption a good proxy for VMT?
The Federal Highway Administration
publishes monthly estimates of vehicle miles travelled (VMT) for the U.S. and individual states, with about a three-month lag time. The most recent release is the September 2006 report. Including archived reports, the available time series covers the interval from January 1990 to latest release.
The two major variations in the VMT time series are
- A long-term secular increase,
- An annual variation with maximum in summer, minimum in winter, and an amplitude of nearly 20%.
The seasonal variation can be taken out by imposing a 12-month running average on the series. But perhaps a better alternative is to directly remove this variation. This the annual term can be estimated by superposing the data for each month (Jan.-Dec.) over the 16 complete years (1990-2005) in the series, and then subtracting the over-all average from each monthly average. Operationally, this is done by using the (natural) logarithm of the data, removing the secular term (giving an average growth rate), and then performing the superposition. The result gives the average deviations month-by-month for the seasonal variation in log VMT, which is labeled percent deviation in this graph:
Log VMT is then seasonally corrected using this curve, and converted back to VMT by applying exp:
