The Pentagon plans to make $20 million available for highway improvements around an Alexandria office complex to which 6,400 defense workers will be transferred this year.

State and local officials have feared a traffic catastrophe on Interstate 395 in September, when the influx will begin at the Mark Center complex at the Seminary Road exit. The Virginia congressional delegation has pressed the Pentagon to provide funding for road improvements around military bases in Fairfax and Montgomery counties that will absorb tens of thousands of relocated military workers.

“We welcome this funding, but it’s just a start given the major challenges we face,” said Rep. James P. Moran (D-Va.) “Combined with funding from the commonwealth, the Mark Center is set to receive an infusion of $100 million in an effort to prevent a traffic nightmare.”

Moran again called on the Pentagon to delay the Mark Center relocation — part of the Base Realignment and Closure (BRAC) process — until the highway improvements are completed.

“It’s the height of irresponsibility to push forward on the relocation until the Mark Center is complete — and that includes the roads, public transit and ramps that will service it,” he said.

The 2011 budget on which Congress is expected to vote Tuesday includes $300 million for highway improvements around Fort Belvoir and the National Naval Medical Center in Bethesda.

But plans to expedite building a new car pool and bus ramp from 395 were delayed last week when the Federal Highway Administration required a more thorough environmental impact study. The Virginia Department of Transportation said the $80 million project would be delayed by 18 months.

The Pentagon’s commitment of an additional $20 million, outlined in a letter sent late Wednesday to Capitol Hill, is to be used for ramp improvements, intersections and crosswalks at the Mark Center.

Under its guidelines, the Pentagon is not 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.