Vehicle miles of travel (VMT) are key data for highway planning and management, and a common measure of roadway use. Along with other data, VMT are often used in estimating congestion, air quality, and potential gas tax revenues, and can provide a general measure of the level of the nations economic activity.
Statistical analysis reveals an unexpected drop in the VMT data that began around June 2001 and continues through September 2001. This drop, which is now reflected as a step downward in the underlying trend, cannot be explained by variability in the seasonal behavior of the data. BTS is doing further analysis to be certain what factors may be affecting this change.
|Vehicle Miles Traveled||Sep-00||Sep-01|
|Millions of highway miles||222,635||219,653|
|Vehicle miles percent change from same month previous year||-0.74||-1.34|
NOTE: The current value is compared to the value from the same period in the previous year to account for seasonality.
A trendline has been provided for vehicle miles traveled. The trend has been calculated through a statistical procedure called Structural Modeling, in which the time series under study is decomposed into seasonal, trend and irregular components. For further information on this statistical procedure, see: S.J. Koopman, et al., Structural Time Series Analyser Modeller and Predictor (STAMP), London: Timberlake Consultants Ltd., 2000
SOURCE: U.S. Department of Transportation, Federal Highway Administration, Office of Highway Policy Information, available at http://www.fhwa.dot.gov/ohim/tvtw/tvtpage.htm.