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The White House. Office of Management and Budget, Washington, DC. Obtained from:
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Bureau of Economic Analysis. National Income and Product Accounts Tables, U.S. Department of Commerce, Washington, DC. Obtained from: http://www.bea.gov/iTable/iTable.cfm?ReqID=9&step=1, as of November 2011.

U.S. Census Bureau, State and Local Government Finances, U.S. Department of Commerce, Washington, D.C. Annual issues. Obtained from: http://www.census.gov/govs/estimate/, as of October 2011.

U.S. Environmental Protection Agency. Planning, Budget, and Results, USEPA, Washington, DC. Obtained from: http://www.epa.gov/ocfo/par/index.htm, as of November 2011.

Federal Aviation Administration. Compliance Activity Tracking System (CATS): Airport Financial Reports, U.S. Department of Transportation, Washington, DC. Obtained from: http://cats.airports.faa.gov/Reports/reports.cfm, as of November 2011.

Federal Aviation Administration (FAA), Budget, U.S. Department of Transportation, Annual issues, Washington DC. Obtained from: http://www.faa.gov/about/budget/, as of November
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Federal Highway Administration. Highway Statistics Publications, U.S. Department of Transportation, Annual issues, Washington, DC. Obtained from: http://www.fhwa.dot.gov/policy/ohpi/hss/hsspubs.htm, as of November 2011.

Federal Transit Administration. National Transit Database (NTD), U.S. Department of Transportation, Annual issues, Washington, DC. Obtained from: http://www.ntdprogram.gov/ntdprogram/, as of November 2011.

Surface Transportation Board Statistics of Class I Freight Railroads, Annual issues, U.S. Department of Transportation, Washington, DC. Obtained from: http://www.stb.dot.gov/econdata.nsf/66a333195e0491c885256e82005ad319?OpenView, as of November 2011.