The most congested stretch of highway in the most congested region in the country is weeks away from getting more balled up.

That’s the conclusion to be drawn from fresh and compelling data released Friday. Gathered in the spring, the data showed that Interstate 395 in Virginia was the most surefire place in the Washington region to get stuck in traffic during rush hour.

Fast-forward from that cherry blossom season assessment: This summer the Pentagon ignored the pleas of Congress and began the mass transfer of 6,400 defense workers into an Alexandria office complex that sits right next to that already traffic-troubled interstate at Seminary Road. The complex was notable — and quickly became controversial — because there was almost no way to get there other than to drive. There is no Metrorail station and no immediate bus service.

Most of the new workers began arriving in August, and 5,000 of them are to be in place by year’s end as part of the Base Realignment and Closure process. The final 1,400 are coming early next year.

By September, major intersections around the Mark Center complex already were on the brink of what traffic engineers call “failure,” and cars waiting to get off at the Seminary Road exit were backing up into traffic lanes on I-395.

All that new Mark Center traffic came after the time period covered in the new report.

“This has been one of the most congested corridors for years,” said Ron Kirby, transportation director for the Metropolitan Washington Council of Governments, the group made up of regional elected officials that compiled the data, “and now we have found that it is the most congested.”

Northbound morning rush-hour traffic on I-395 was moving at an average of 5 mph when the spring survey was conducted. Southbound traffic in the evening was almost as slow.

Kirby’s organization, known by the acronym COG, has been conducting an aerial photo survey of the region’s traffic since 1993, but he considers this year’s 157-page report more authoritative because it’s been integrated with another potent stream of data. That’s information from Inrix, a private firm that tracks traffic patterns nationwide in real time by using hundreds of thousands of tracking transponders in fleet vehicles.

Bad data

The pot was already roiled this week when a Pentagon inspector general’s report said that the Army had used bad data in a report defending the decision to move the workers, most of them already living in the region, to the Mark Center.

“It appears that they were, in effect, cooking the books in terms of when and how these first traffic studies were conducted,” said Sen. Mark R. Warner (D-Va.). “Anybody who drives in that area . . . knows what incredible congestion we have at this moment. The Army ought to take a timeout in shoving more people into this facility.”

The Army fired back a memo challenging each of the inspector general’s findings and disagreed with the recommendation that a fresh traffic study using more accurate data be done.

“The traffic counts that the Army used to determine the level of traffic were conducted around national holidays, namely July 4 and Memorial Day,” said Rep. James P. Moran (D-Va.), who joined Warner and other members of Congress from Northern Virginia in opposing the move. “And when schools were not in session because, you may know, there’s a large middle school on the other side of 395 and Seminary Road within a stone’s throw.”

Moran said the Army’s traffic figures were 35 percent below the reality, and that it underestimated the number of transferred workers who would arrive by car by 100 percent. Plus, the Army looked at the six intersections closest to the center when “a professional analysis would look at the intersections within a two-mile radius. That would be 63 intersections in the case of the Mark Center,” Moran said.

He said that the number of parking spaces available to center workers should be limited until transportation improvements, including a new off-ramp off I-395 to Seminary Road, are completed.

“The Army created the mess. We’d like for them to come up with solutions themselves,” Moran said. “We do think some steps can be taken to avoid the chaos that’s about to hit us within a couple of months.”

Among them, he said, would be providing shuttle bus service for workers who park in remote lots or garages.

Ranking congestion

The COG report said that the Capital Beltway’s inner loop in Virginia from Georgetown Pike to the George Washington Parkway was the region’s second most-congested stretch of highway during the evening rush hour. It tied with evening rush hour on I-395 southbound in the District.

Others in the top ranks were eastbound I-66 from Leesburg Pike to the Dulles Access Road, the Beltway’s inner loop from I-270 to Connecticut Avenue and the Beltway’s inner loop from the Dulles Toll Road to Chain Bridge Road.

The report said the 2008 opening of the new Woodrow Wilson Bridge had significantly improved the region’s traffic flow. Reductions in congestion also were reported on the eastbound Dulles Toll Road between Centreville Road and Hunter Mill Road in Fairfax; southbound I-295 approaching and beyond the Interstate 50 interchange; and on the Southwest Freeway in the District.