Senate Committee Markup On Highway and Transit Legislation [Getty Images]

Senator Barbara Boxer, a Democrat from California, center, speaks during a Senate Environment and Public Works Committee markup meeting on reauthorization legislation for highway and transit programs in Washington, D.C., U.S., on Thursday, May 15, 2014. The current transportation funding measure, known as the Moving Ahead for Progress in the 21st Century Act (MAP-21), expires at the end of FY 2014 on Sept. 30. Photographer: Andrew Harrer/Bloomberg via Getty Images

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