Two national estimates of pmt are available; they differ in coverage, methodology, and other factors. The pmt data presented in section 2 of this report come from the Bureau of Transportation Statistics (BTS) publication, National Transportation Statistics (NTS). BTS compiles these data for NTS annually, using primarily 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 pmt has differing strengths and weaknesses, as discussed in the Data Source and Accuracy Statements for table 1-34 in NTS, available at http://www.bts.gov/.
Section 5, Variables Influencing Traveling Behavior, presents 2001 pmt data from the 2001 National Household Travel Survey, jointly conducted by BTS and the Federal Highway Administration. As survey data, they are collected using a single methodology. This provides a coherence and comparability not available with NTS data. However, these data are not collected annually, making them unsuitable for year-to-year trend analyses. Another difference between NTS and NHTS data is the extent of their coverage among modes. It can be expected, then, that because of methodological and coverage issues NTS and NHTS data will differ.