The data in this report come from a variety of sources, principally from the Bureau of Transportation Statistics (BTS) and other operating administrations of the U.S. Department of Transportation. However, other sources are federal government agencies, such as the U.S. Census Bureau, the Bureau of Economic Analysis, the U.S. Environmental Protection Agency, the U.S. Coast Guard, and the Energy Information Administration. To supplement government sources, the report occasionally uses data and information from trade associations, such as the Association of American Railroads and the American Public Transportation Association. Data from any of these sources may be subject to omissions and errors in reporting, recording, and processing. Sample data are subject to sampling variability. Documents cited as sources in this report often provide detailed information about definitions, methodologies, and statistical reliability.
To be consistent, when trend data are used in this report they are shown, if possible, for at least a 10-year period. Because of the differing availability of data among all the indicators included, it has not been possible to use the same span of 10 years for each indicator without sacrificing timeliness. Instead, the data span a decade up to the year of the most recent data available when this report was prepared. There are some instances where less than 10 years of data are presented-either because the data are not comparable over the period or are not available.
Source information in the report details where BTS obtained data used (e.g., from a printed document, website, or by direct communication with an individual). The same data BTS obtained from websites and used in this report may not be available to readers because of frequent changes in such postings. However, the day and month of the BTS download is included in the source information, along with the website address (url) at that time.