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The White House. Office of Management and Budget, Washington, DC. Obtained from: http://www.whitehouse.gov/omb/, as of May 2014.

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 May 2014.

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/local/, as of May 2014.

U.S. Environmental Protection Agency. Planning, Budget, and Results, USEPA, Washington, DC. Obtained from: http://www2.epa.gov/planandbudget, as of May 2014.

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 May 2014.

Federal Aviation Administration (FAA), Budget, U.S. Department of Transportation, Annual issues, Washington DC. Obtained from: http://www.faa.gov/about/budget/, as of May 2014

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 May 2014.

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 May 2014.

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/FinancialData?OpenView, as of May 2014.