Saturday night's dragnet was the most aggressive yet in the sniper investigation, with the first police cars taking only several minutes to seal off interstate ramps in a highway shutdown that soon stretched 34 miles along I-95, authorities said.

But despite the speed and scope of the response, police are now 0 for 4 with dragnets, pointing to the problems inherent in such tactics even when hundreds of law enforcement officers are available.

The shooter could have taken a prearranged route on secondary roads, then listened to radio reports to learn when it was safe to return to major highways, law enforcement experts said. Or he could have ditched his weapon, talked his way past police checkpoints or simply walked away from the scene, they said.

"I think they're doing it out of desperation as much as anything else," said J.T. McCann, a former D.C. homicide detective who investigated the shotgun stalker who terrorized neighborhoods in Northwest Washington for two months during 1993.

"It's better than nothing," McCann said of the dragnets. "You would be remiss if you had the resources and you didn't do it."

After Saturday's shooting, police pulled over white vans as far north of the Ashland, Va., shooting site as Montgomery County, but they said they lacked a credible description of the shooter or his vehicle from that night, rendering a dragnet far less effective.

Authorities don't know how the shooter escaped, but some other motorists found a way out, much as they did in the earlier dragnets.

Kenneth Carlton, 23, said he left his family's store on the same main strip in Ashland soon after the shooting. He encountered roadblocks on the major routes but said he soon slipped past using a secondary road just off Route 1.

"They said they had everything blocked off, but they didn't have that" cordoned off, said Carlton. "The sniper could easily have taken it."

Law enforcement experts said it's nearly impossible to suddenly seal off an area, though police have been using dragnets since establishing a rudimentary one in Prince William County on Oct. 9, after a Gaithersburg man became the seventh fatality linked to thesniper. They deployed more elaborate ones after a shooting in Spotsylvania County on Oct. 11 and one in Seven Corners on Oct. 14.

Such tactics are not often successful in catching suspects, experts say, but routine traffic stops have tripped up some serious criminals. The shotgun stalker, who attacked 14 times, was arrested after he ran a red light.

Even with the elaborate plans that police prepared for another snipershooting, they could not know where he would strike.

And for dragnets to be successful, police generally need a clear description of what to search for. The only consistent element in all the sniper shootings has been the choice of ammunition: a .223-caliber bullet.

Saturday night's incident has not been conclusively linked to thesniper, but authorities have treated it as one while evidence is being collected.

When the shot struck a traveler leaving the Ponderosa restaurant in Ashland, police said they moved quickly to seal off the area.

The call came at 7:59 p.m. Police stationed at the Interstate 95 entrance ramp closed it almost as quickly as the first officer reached the victim, at 8:01 p.m., said Hanover County police Lt. Doug Goodman.

Other roadblocks unfurled on major highways in all directions of the shooting site within 10 minutes of the call, officials said. I-95 was closed between I-295, about eight miles south of Ashland, and Route 606 at Thornburg, 26 miles north.

Police quickly set up checkpoints on bridges. Motorists were stopped more than 90 miles away, on the D.C.-Maryland border. Police posted throughout the Washington suburbs looked for the white vans described in previous shootings.

Police kept the checkpoints on highways and bridges in place for several hours, snarling traffic and delaying thousands of travelers.

"This is much broader, much wider, much more thorough than any [dragnet] I've seen," said Northeastern University criminal justice professor James Alan Fox. "I've seen some big ones, but not this big."

Yet even with the massive effort, numerous smaller roadways, such as the one taken by Carlton, remained an option for motorists who are familiar with the area -- or map out an escape route on roads that would not likely be the top priority of police searching for a suspect.

"We feel like we did as well as we could," said Col. W. Gerald Massengill, the Virginia State Police superintendent. "As with any strategy, we go back and look at everything."

Beyond Ashland, the response was a patchwork. The FBI stopped vehicles at Chevy Chase Circle and inspected them and their trunks. In Northern Virginia, which is closer to Ashland, roadways were not blocked off though police pulled over and searched many white vans.

Radio and television reports, meanwhile, gave motorists -- and possibly the sniper -- warning about the checkpoints, allowing them to take detours.

Yaw Day, 59, of Rockville said he waited about an hour to travel on Connecticut Avenue through Chevy Chase. "What can we do?" Day said. "I really don't mind. It's for the benefit of the whole community."

But even though complaints have been muted, the tactics of the dragnets are beginning to generate concern among civil libertarians.

Police are generally free to pull over cars but usually must get permission from drivers to conduct searches.

There are exceptions when there is plain evidence of criminal activity, such as an inebriated driver, or if a major crime scene is nearby, said Kent Willis, executive director of the Virginia chapter of the American Civil Liberties Union.

He said police enter a legal gray area when they conduct searches far from a crime scene without getting motorists' permission.

"Now that the dragnet has become so large, it's an important time for constitutional experts to advise the police on what their limits are," Willis said.

Staff writers R.H. Melton, Arthur Santana and April Witt contributed to this report.