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2. Association of American Railroads, 1988–2011. Railroad Facts. AAR Washington, DC.
3. Australian Bureau of Statistics, 2000–2010. Survey of Motor Vehicle Use. ABS, Canberra.
4. The Transportation Industries, 1889–1946; A Study of Output, Employment, and Productivity;Barger,1951
5. Second IMO GHG Study 2009;Buhaug,2009