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 pmt data in this section of the report
are from the Bureau of Transportation Statistics (BTS)
publication, National Transportation Statistics (NTS). BTS
compiles these data for NTS, 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, as discussed in the Data
Source and Accuracy Statements in appendix C, available at
http://www.bts.gov/. BTS updates NTS