BOX 1-A
Data on Passenger-Miles of Travel
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 on
a quarterly basis; the data in this section are the data available when the
report was prepared and may not be consistent with data available later in NTS
or other sources, as noted in the chapter introduction.
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 for
NTS. However, the survey 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. In section 4 of this chapter, Variables Influencing Traveling Behavior,
the report uses data from the 2001 NHTS.
|
