The 2011 budget deal struck by Congress last week includes $300 million to expand roads around military bases in Fairfax and Montgomery counties that will absorb tens of thousands of new commuters as part of a seismic relocation of the region’s military workforce.

The money would be used to widen roads and expand intersections that are expected to choke with traffic later this year with the massive shift of military personnel to Fort Belvoir and the National Naval Medical Center in Bethesda.

The width of Route 1 at Fort Belvoir would be increased from four to six lanes, and turning lanes and signal improvements would be made at several intersections adjacent to the Bethesda medical center.

“We’ve been working on this for a long time to make sure our wounded soldiers and their families get the support they need and we mitigate the traffic congestion,” said Rep. Chris Van Hollen (D-Md.).

The potential for a traffic nightmare has put the region’s congressional delegation sharply at odds with the Pentagon, which took little account of rush-hour congestion in 2005 when it shifted thousands of people from locations better served by mass transit to facilities that offered few ­choices but commuting by car as part of its base realignment program, known as BRAC.

Under its guidelines, the Pentagon isn’t required to help pay for transportation improvements surrounding a military base slated for growth unless its actions would cause congestion to double. For a region already burdened with some of the nation’s worst traffic problems, that was an impossible and undesirable standard to meet.

With the bulk of the relocations slated to take place by September, the new funding won’t arrive in time to avert traffic woes. The $300 million is included in the compromise reached to avert a federal shutdown, an agreement that will come to a vote in both houses this week.

“We have worked for more than a year with our colleagues in both chambers, from both states and in both parties, to make sure we could provide this critically important investment in the region’s already overloaded transportation network,” said Sen. Mark R. Warner (D-Va.).

Virginia has spent $400 million on BRAC improvements, and almost $1 billion will be spent in a public-private partnership to build high-occupancy toll lanes on Interstate 95, which will be heavily affected.

“This is a major win for Northern Virginians,” said Rep. James P. Moran (D-Va.). “We can’t have our service members tied up in traffic trying to get to the hospital, nor accept what would otherwise be an interminable delay for the tens of thousands of commuters who must drive by Fort Belvoir each weekday morning.”

The funding doesn’t include money for what is expected to create the region’s biggest traffic snarl, the relocation of 6,400 defense workers to the Mark Center, an Alexandria office complex alongside Interstate 395.

A plan by the Virginia Transportation Department to build a new carpool and bus ramp where I-395 exits at Seminary Road was delayed last week by 18 months when federal officials mandated a more thorough environmental impact study.

“The BRAC funding is a good down payment, but much more will be needed to meet the transportation challenge of 22,000 new employees coming to Fort Belvoir this fall,” said Rep. Gerald E. Connolly (D-Va.). “We still have the larger problem of how we’re going to resolve the gridlock that will inevitably occur on Route 95, the Fairfax County Parkway, and other highways around Belvoir.”

With the Bethesda facility absorbing the Walter Reed Army Medical Center, 2,200 new workers and a half-million hospital visitors a year will be added to the traffic load.

Maryland has invested $135 million in transportation improvements and says it needs an additional $300 million to handle BRAC-related traffic at Bethesda and Fort Meade.

A report in February by the National Academy of Sciences found that the Pentagon should have shouldered some of the burden of 30 major highway and transit projects needed to accommodate traffic at the three installations. It forecast “lengthy queues of stalled traffic” and “a near perfect storm of problems.”