[Washington, District of Columbia] : U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, 2013
[Washington, District of Columbia] : U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, 2011
[Washington, District of Columbia] : U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, 2011
[Washington, District of Columbia] : U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, 2011