Total passenger-miles of travel (pmt) excludes travel in heavy trucks, by bicycle, by walking, and by boat (including recreational boat). Pmt in heavy trucks is excluded because such travel is assumed to be incidental to the hauling of freight, the main purpose of this travel. Bicycle, pedestrian, and boat travel are excluded because national estimates are not available on an annual basis. The Bureau of Transportation Statistics (BTS) compiles pmt data primarily using mode-by-mode data derived in various ways by BTS and others. For instance, pmt for large air carriers and intercity trains are estimated from ticket sales and trip lengths; for transit, data are reported by transit authorities. Each method used to estimate these data has differing strengths and weaknesses.
BTS has another set of national pmt, the National Household Travel Survey (NHTS), last conducted jointly by BTS and the Federal Highway Administration in 2001 and 2002. As survey data, they are collected using a single methodology. This provides a coherence and comparability not available with data compiled by individual modes. However, the survey data are not collected annually, making them unsuitable for year-to-year trend analyses. Another difference between NHTS data and the data compiled by BTS is the extent of their coverage among modes. It can be expected, then, that because of methodological and coverage issues the two sources of pmt data will differ. In section 4, Variables Influencing Traveling Behavior, the report uses data from the 2001 NHTS.