For consistency, most trend indicator data are shown over 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. 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.
With a few exceptions, trend data involving costs have been converted to 1996 chained (“real”) dollars to eliminate the effect of inflation over time. Appendix B provides both 1996 chained dollar and current dollar value tables. Throughout the report, results of percent calculations have been rounded up or down, as appropriate, to a whole number. Average annual percentage change calculations have been made using a logarithmic formula to account for compounding over time.
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. Sampling data are subject to sampling variability. Documents cited as sources in this report often provide detailed information about definitions, methodologies, and statistical reliability.
1 The formula is: Average annual rate = Exp [(LnY–LnX)/ (n–m)] –1, where Y is the end year value; X is the initial year value, n is the end year, and m is the initial year.