The Defense Department has concluded that adding thousands of patients and workers to the new Walter Reed National Military Medical Center in Bethesda has reduced traffic congestion, but outside analysts offer an alternative explanation: measuring worsening gridlock can be misleading.

A Walter Reed study found that as of October, almost every part of Rockville Pike and Jones Bridge Road, which border the medical center and are some of Montgomery County’s worst choke points, saw between 1 percent and 34 percent fewer vehicles during the morning and evening rush hours, compared with 2007. Base officials cite the declining vehicle counts as evidence that their employees’ increasing use of transit, teleworking and flexible schedules has not only cushioned Walter Reed’s traffic impact but helped take vehicles off the road.

But the outside traffic analysts say the true picture of traffic in the area over the past year is more complicated and likely not so rosy. Vehicle counts probably fell, they say, because traffic grew so clogged that fewer motorists even reached the mechanical counters during the allotted times. Local residents say that explanation jibes with their daily misery: growing backups extending into downtown Bethesda, gridlocked intersections, and rush hours that now begin as early as 6 in the morning and 2:30 in the afternoon.

Paul Schonfeld, a University of Maryland transportation engineering professor, hasn’t reviewed the military’s analysis but said the idea that worsening traffic congestion would coincide with fewer vehicles “doesn’t sound particularly reasonable to me.”

Finding ways to accommodate new traffic is a key concern with many military base expansions, as Fort Meade and Fort Belvoir also experienced during the past year under the Pentagon’s base realignment process.

But moving Walter Reed from the District to consolidate operations on the Bethesda campus of the former National Naval Medical Center was considered particularly challenging because of its dense, highly residential location. Even without additional traffic from the base, the surrounding roads were heavily congested commuter routes between the Maryland suburbs and the District and carried local traffic between Bethesda, Rockville, Kensington, Chevy Chase and Silver Spring.

State and local transportation planners rated the intersections around the Bethesda base as “failing” even before 3,600 new employees moved there during the expansion. The influx of personnel increased the base’s total workforce 44 percent, to 11,686 people. More strikingly, patient visits are on pace to double, to an annual 1 million, with most expected to arrive by car.

The explanation of why traffic counts can improve as congestion worsens is a grim lesson in traffic physics. Faced with additional vehicles, intersections already stretched beyond capacity end up with bigger backups. At some point, engineers say, intersections deteriorate into an inefficient mess. Motorists must sit through two or more cycles of a green light, and intersections end up blocked by cross-traffic that can’t move.

In such congestion, the number of vehicles going over a counter during a designated period falls, even though traffic volumes have grown and motorists are moving more slowly.

“The system can get so stressed that if you add anything, it pushes the system into an unstable flow,” said Tim Lomax, senior research engineer for the Texas Transportation Institute, which publishes annual rankings of the most congested urban areas.

Jeff Miller, the base’s transportation program manager, said he has no reason to believe the vehicle counts are skewed. The fact that many base employees work relatively early schedules — in by 7 a.m. and out by 3:45 p.m. — to grab limited parking spaces means they aren’t adding to the later, more traditional commuting peaks, he said. The base also has constrained parking during the expansion, which has forced workers to seek options other than driving, he said.

Miller said an employee survey in November showed that 40 percent of respondents drove alone to work, down from 72 percent in 2007. About 44 percent said they took public transit, up from 11 percent four years earlier, he said. Much of those new transit users take buses, he said.

“I think we’ve been very successful in [encouraging] people toward commuting alternatives,” Miller said. “So at the end of the day, we have fewer people seeking to utilize single-occupancy vehicles to commute.”

Miller agreed with local residents that afternoon traffic has grown noticeably worse on Jones Bridge Road, where base officials tried to divert traffic to relieve Rockville Pike.

So why would the vehicle counts show 8 percent fewer vehicles on that stretch?

“I don’t know how to explain that, other than that some days it appears more congested,” he said.

He said it might reflect the fact that a major hospital exit on Jones Bridge had yet to be improved at the time of the count, discouraging some employees from using the busy road then.

Transportation analysts say a sluggish economy and high gas prices likely have depressed some traffic in the Bethesda area, as they have nationwide since 2008.

Walter Reed’s impacts are being closely monitored because the military has plans for more expansion there, and any effects ripple far beyond what is now the U.S. military’s largest medical facility. The 243-acre campus sits just inside the Capital Beltway, where backups can quickly spread across the region. It is also across Rockville Pike from the National Institutes of Health headquarters, which has 20,500 workers, and just up Rockville Pike from downtown Bethesda’s office buildings and entertainment district. More than 70,000 vehicles use the surrounding roads daily, according to state figures.

Local officials pinned their hopes on people using Metrorail’s Medical Center station across the street, but Metro figures show that average weekday ridership there was up by 265 people last month, compared with June 2011.

Several traffic analysts said they suspect the declining vehicle counts are masking another symptom of worsening congestion, called “peak spreading.” As the commuting rush bogs down, some motorists alter their drive times to avoid the crunch. The “rush” period stretches out, worsening congestion for people who once avoided the peak.

“It’s one massive rush hour period now,” said Ilaya Rome Hopkins, who chairs Montgomery’s volunteer Walter Reed BRAC Integration Committee and lives nearby. “It’s sequential, cumulative and unpredictable.”

Debbie Michaels, a liaison to the committee from the Glenbrook Village Homeowners Association, said she and her neighbors must head north on Rockville Pike by 2:30 p.m. — a half-hour earlier than before the expansion — or it can take 30 minutes to drive the mile or so between Walter Reed, NIH and the Beltway.

“It’s pretty grueling,” Michaels said. “Most people I know, if they don’t have to do it, they don’t.”

Local residents said they realize much of the congestion stems from sources other than the base. Several emphasized that they appreciate the importance of Walter Reed’s mission and the base officials’ significant work in persuading more staffers to carpool and take transit.

Still, some say, it’s too early to declare any traffic victories. Another analysis that Walter Reed did, known as a “gate count,” found that the number of vehicles passing through its gates daily increased by a total of 1,561 — a 23 percent jump — during the morning and evening rushes in October, compared with 2006. Traffic leaving the base in the afternoon was up by 35 percent. Moreover, nearby residents say, none of the base’s peak period counts reflects the thousands of additional patients driving to Walter Reed clinics throughout the day.

Phil Alperson, Montgomery’s BRAC coordinator, said he’s concerned that the declining vehicle counts, which he believes are inaccurate, are part of the military’s environmental review of further expansion plans, including building 900 new parking spaces.

“If you build more parking,” Alperson said, “the vehicles will come.”

Local officials also are keeping an eye on NIH’s long-term growth plans, which are under environmental review. They could result in an additional 3,000 workers.

Matt Snare, a travel forecaster for the Maryland State Highway Administration, said there is hope on the horizon. Over the next four years, new turn lanes will begin moving more traffic through the intersections. And a new pedestrian tunnel and high-speed elevators will allow Walter Reed employees to reach bus stops and the Metrorail station without having to cross busy Rockville Pike.

Yes, Snare said, major road construction brings more delays. But the highway agency hopes to at least raise the intersections’ “F” grades to “E’s,” allowing motorists to get through them on the first green signal.

As for determining exactly who’s making traffic worse and by how much, Snare said with a chuckle, “It’d be an interesting PhD dissertation.”