Congress has moved to limit parking at the controversial Mark Center office complex in Alexandria, forcing the Army to reevaluate the traffic impact as it transfers 6,400 workers to the region’s most congested corridor.

The parking cap was included in a defense appropriation bill moving toward final passage Friday. It would limit the Army to the use of 2,000 parking spaces until the Pentagon can prove that gridlock has not occurred at intersections surrounding the complex, which sits just off Interstate 395 at the Seminary Road exit.

The I-395 corridor recently was singled out in a study that said it was the most congested place in what already had been established as the nation’s most congested region. There is no public transportation to the Mark Center, and cars trying to exit at Seminary Road began backing up onto the interstate not long after the transfers began this summer.

“This is progress, but I don’t want to leave anybody with the false impression,” said U.S. Sen. Mark Warner (D-Va.). “There’s still going to be congestion around I-395 and the Mark Center.”

Warner said the parking space cap would force the Army to encourage people to work from home and to stagger work hours. Warner said he would invite Defense Secretary Leon E. Panetta to join him on a rush-hour drive through the area in March after the transfers are complete.

The Northern Virginia delegation in Congress has been at the forefront in challenging the Army’s transfer plan as ill-conceived and based on questionable traffic data compiled by a Pentagon consultant.

“The situation threatened over 200,000 commuters” who use I-395 daily, said U.S. Rep. James P. Moran (D-Va.), calling the legislation the best available option to address a bad situation. “It was the only means available to lessen the impact. For those up here who still needed convincing after five studies on the Mark Center, the [inspector general’s] report was the final nail in the coffin, hammering home the gross negligence that went into all aspects of this flawed decision.”

The report by the Pentagon’s inspector general found that the Army had used badly flawed data in compiling traffic impact projections that it used to defend the planned transfers, most of them involving people already living in the region who would be more prone to drive to work rather than relocate close to their new jobs.

“They cooked the books,” Warner said.

The inspector general recommended that the Army do a new traffic study. The Army responded with a point-by-point rebuttal of the inspector general’s findings. It said that conducting a new traffic study would be a pointless exercise that would not “provide additional solutions to past or existing traffic issues.”

The Army said it was cooperating with state and local officials, and providing more than $20 million for traffic improvements.

The transfers result from the decision of the 2005 Base Realignment and Closure Commission to relocate thousands of Washington area defense workers, with major expansions at Fort Meade, Fort Belvoir and the National Naval Medical Center in Bethesda.

In addition to capping the number of available parking spaces at 2,000 until the Army does another study, the measures included in the appropriations bill contained other provisions related to the Mark Center transfers. One key provision changes the way the Pentagon determines whether it is responsible for traffic improvements around a base to which it transfers personnel.

It eliminates a current provision that says the Pentagon must provide financial assistance only if the traffic surrounding the base doubles as a result of the transfers, an impossible standard in a congested area such as Alexandria.

With nearly 5,000 workers scheduled to be in place by year’s end, and the rest arriving early in 2012, commissioning a new traffic study would not alleviate the expected massive congestion.

Construction on one of the key improvements — a new carpool and bus ramp from I-395 to Seminary Road — was delayed in April when the Federal Highway Administration required a more thorough environmental impact study. VDOT said the $80 million project would be delayed by 18 months.

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