The Pentagon agreed Tuesday to provide almost $270 million for improvements around Fort Belvoir in Fairfax County and the new Walter Reed National Military Medical Center in Bethesda, yielding to pressure from elected officials who complained that mass transfers to the two facilities could worsen gridlock.

The Defense Department said it would fund a $180 million project to widen Route 1 to six lanes near Fort Belvoir and provide $88.9 million for intersection improvements and a new underpass in Bethesda.

“This is one we’ve been working on for a long time, so I’m pleased this day has arrived,” said Rep. Chris Van Hollen (D-Md.), who joined Maryland’s two senators and congressional colleagues from Northern Virginia in lobbying for the funding relief.

The fight for federal help came about because the Pentagon’s guidelines don’t require it to support transportation enhancements when it transfers thousands of defense personnel unless the changes would cause congestion to double.

While that might be a reasonable standard for military facilities in relatively rural locations, in a region already beset with some of the worst congestion in the nation, traffic likely would reach complete gridlock well before its volume doubled.

The transfers — under the Base Realignment and Closure plan — were made more problematic because they involved tens of thousands of workers who already lived in the region, making it more likely that they would drive to their new offices. People relocating to the region might find housing closer to their jobs.

In addition to doubling the workforce at Belvoir, relocating about 2,500 people to Bethesda and anticipating an additional 571,000 patient visits at the Bethesda facility, the Pentagon is moving 6,400 workers to the Mark Center complex on Seminary Road in Alexandria. Virginia is investing $80 million in state funds to build an off-ramp from the high-occupancy vehicle lanes of Interstate 395 to help accommodate the expected traffic, a project that won’t be completed before 2013.

Although the efforts in all three locations are expected to provide relief in the long term, the construction may snarl traffic as the newly transferred workers flood the roads.

Four major intersections near the Bethesda hospital along Rockville Pike, Connecticut Avenue, Cedar Lane, Jones Bridge Road and Old Georgetown Road will become construction zones, and a $40 million underpass will be created beneath Maryland 355. With funding now ensured, the State Highway Administration will proceed with planning and environmental reviews. Construction schedules will be finalized after the money is received, the state said.

“Last April, thanks largely to the tireless efforts of Congressman Chris Van Hollen and Senators Barbara Mikulski and Ben Cardin, Congress established a fund for certain BRAC-related transportation programs,” said Montgomery County Executive Isiah Leggett (D). “Today’s announcement is the culmination of their hard work.”

In addition to widening Route 1, the Belvoir project will include sidewalks and bicycle paths and accommodations for future transit. Construction is expected to start in 2013 with possible completion in 2015.

Route 1 near Fort Belvoir is four lanes wide and carries 56,000 vehicles a day. Between 2006 and 2008, state records show, there were 294 crashes on the 3.4-mile section between Telegraph Road and Mount Vernon Memorial Highway, including 13 injuries and one death. In 2011, there were two more fatalities.

“Widening Route 1 is critical to ensuring a viable transportation network in the Fort Belvoir area,” said Virginia Gov. Robert F. McDonnell (R). “Commuters are stuck in long traffic queues on Route 1 that has only worsened with the additional traffic generated by BRAC changes.”

He credited Rep. James P. Moran Jr. (D) and other members of the Northern Virginia congressional delegation with persuading the Pentagon’s Office of Economic Adjustment to bend its guidelines and fund the project.

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