The data in this report come from a variety of sources—principally, from the Bureau of Transportation Statistics and other operating administrations of the Department of Transportation. However, other sources are federal government agencies, such as the U.S. Census Bureau, Bureau of Economic Analysis, U.S. Environmental Protection Agency, U.S. Coast Guard, and 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 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 most recent data available when this report was prepared. (More information about data in the report can be found in the introduction to chapter 2.)
In this summary chapter, full data sources are not provided. However, the data here are from chapter 2, where full citations are given in the text and graphs. Corresponding tables in appendix B also contain source information. The data here are presented in the same order—by topic section—as they appear in chapter 2 enabling easy access to data sources. Also not present in this chapter are complete definitions, which can vary across data sources.