Hundreds of Northern Virginia commuters are complaining that their longtime practice of forming impromptu car pools in the Pentagon's parking lot each afternoon is being disrupted by Defense Department police who chase away motorists stopping to pick up passengers. But a Pentagon spokesman said the police have not changed their policy in dealing with "slug lines," where commuters stand to hitch rides with motorists. Car pools of at least three people are eligible to travel in Shirley Highway's express lanes. The stepped-up activity by the Defense Protective Service is aimed simply at improving traffic flow and enhancing safety around the military headquarters, the spokesman said. The complaints come six months after D.C. police sparked a furor by preventing motorists from picking up riders from similar lines along 14th Street NW, saying the practice was delaying downtown traffic. D.C. Police Chief Charles H. Ramsey, who initially suggested moving the pickup site, later dropped that idea after drawing a barrage of criticism from Northern Virginia officials who view the informal car-pool system as a unique, inexpensive way to move hundreds of people into and out of the city each workday. The suburban officials say that car pools in the Shirley Highway corridor will be particularly important during the next several years, as they seek to get cars off the road during construction of a massive Springfield interchange. At the Pentagon, hundreds of commuters -- uniformed and not -- line up each weekday afternoon in four queues along a sidewalk in an island on a parking lot access road. Motorists usually stop on either side of the island to pick up passengers bound for destinations in Prince William and Stafford counties. In recent months, however, police have begun chasing away some motorists who stop on the main, six-lane access road, forcing them to circle around and pick up riders along a single-lane roadway on the opposite side of the sidewalk. That can cause cars to back up for more than 75 yards, causing the slug lines to more than double in length and threatening the car-pooling system, frequent riders say. "When the police try to stop them, everything grinds to a halt," said Jamie C. Reed, who commutes between his Woodbridge home and his job as a salesman at Pentagon City Mall. "It creates a huge traffic jam. I'm sure many drivers just give up and get on the freeway." "It really slows down the slug line," said Carolyn Broussard, who rides a Metro train from her D.C. bank job to the Pentagon, where she joins a southbound car pool. "Just leave them alone. They work." Of more than a dozen slug-line locations in the District and Northern Virginia, none attracts more people than the Pentagon, according to a slug-line site on the World Wide Web (www.slug-line.com) that was established two months ago by a car-pooler. Even without police interference, the crush of cars stopping to pick up riders sometimes is so great that some drivers prefer to avoid either roadway and stop in nearby parking lots, calling out their destinations to prospective passengers. The lines can grow to dozens of people, but the typical wait for a ride rarely exceeds 15 minutes. Although days can pass without any intervention by the Pentagon police, riders said pickups were disrupted three afternoons last week. Glenn Flood, a Defense Department spokesman, said the heightened effort to keep traffic flowing in the main access road is necessary because of congestion resulting from construction in the parking lot at the Pentagon, which is undergoing a years-long renovation. He called it a temporary measure but said it would continue as long as police are concerned about cars stopping in a lane often used by buses. "It's not unusual for an officer on the scene to say, Let's move it on,' " he said. "The officer on the scene makes the determination what is safe procedure." Flood said the chief of the Defense Protective Service had stressed that there has been no change in the Pentagon's "slug philosophy" of encouraging car-pooling. Though regular riders said they were aware of Pentagon concerns about bus traffic, they urged the police to keep their hands off a ride-sharing system that transportation analysts have said is a national model -- and one that has contributed to making Washington the country's car-pooling capital. Recent surveys indicate that about 15 percent of area commuters car-pool to and from work. "This is the best thing for mass transit," said Dave LeBlanc, who commutes between Lake Ridge and his job in Crystal City. LeBlanc suggested the police show more flexibility. "Because of the total number of people moved by slug lines, you'd think they'd try to make an accommodation," he said. "But they haven't done that."