Police dropped a massive dragnet over Montgomery County and threw up dozens of shifting checkpoints across the Washington region yesterday morning, trapping thousands of people in the most extensive and disruptive hunt yet for the sniper.

It was the fifth time in two weeks that police had blocked major thoroughfares and secondary roads, but it was the first attempt during the morning rush on a weekday when hundreds of thousands of suburbanites were headed to jobs, classrooms and appointments.

Trips that would usually take 20 minutes stretched to three hours. People bound for doctors missed medical appointments, cars idling on the Capital Beltway ran out of gas, children spent hours squirming in sputtering school buses.

Motorists tried to be patient, but some started to question the value of a dragnet that yields nothing but full bladders and frayed nerves.

"I got stuck in Northern Virginia because of a shooting . . . 25 miles away," said Glenn B. Manishin, a lawyer from McLean who missed a crucial business flight because he couldn't reach Reagan National Airport. "I don't see there's any other way to stop this guy. But I don't think it makes a lot of sense to close the Beltway on the other side of the river in Northern Virginia."

Others said they were surprised at how easy it was to evade the roadblocks, with nothing more than a good road map or sense of direction.

Rajesh Sharma, 34, of Rockville said he simply followed the compass on his car along back roads after getting off the jammed Beltway. "I didn't see a single police car on the way in," said Sharma, who was heading to his consulting job in College Park. "If you're trying to stop someone, you'd have to have a tighter net, and that simply wasn't there. I was a novice trying to make my way through, and it was fairly easy."

Several law enforcement experts questioned the effectiveness of dragnets but said police may have few alternatives. Massive roadblocks are labor-intensive and costly to the public, they said.

Jim Kouri, vice president of the National Association of Chiefs of Police, said roadblocks usually don't work because suspects can slip onto side streets that police cannot cover.

Although some suspects are caught in dragnets, "in this case you're talking about someone who is very, very clever and probably plans his escape routes ahead of time," said Kouri, former police chief for the New York City Housing Authority.

However, police may feel compelled to try, Kouri said. "They'll do anything to capture this guy," he said.

Patrick V. Murphy, a former New York City police commissioner and former director of public safety in the District, said yesterday's dragnet was the most disruptive he had ever seen.

"To me, roadblocks don't have the greatest payoff," said Murphy, who lives in Bethesda. "But if you don't have anything else you can do, roadblocks do show some [police] activity to the public."

Maryland State Police and Montgomery police refused to comment on the extent of the dragnet.

Arteries squeezed yesterday included some of the busiest Maryland commuter routes: Connecticut Avenue, Georgia Avenue, 16th Street, River Road, Wisconsin Avenue as far south as Western Avenue at Friendship Heights, Interstate 270 at Route 118, and roads in Howard and Anne Arundel counties.

Police closed the American Legion Bridge in both directions about 6:10 a.m., backing up traffic on the Beltway in Maryland and Virginia. By 7:30 a.m., police stretched the dragnet to Northwest and Northeast Washington.

Police moved checkpoints from place to place, making travel more challenging for motorists, and asked traffic reporters not to broadcast the locations -- a request with which most complied.

"Traffic is standing still, and we can't tell you where it's standing still," Lisa Baden, a traffic reporter, told radio audiences.

Jim Russ of Metro Networks said his company, which provides traffic reports to local television and radio stations, agreed to withhold locations after police said doing otherwise would hinder their investigation. More than 170 listeners called in, with the "overwhelming majority" supporting the information blackout, Russ said.

Police wearing bulletproof vests searched vehicles at gunpoint, and not just white vans.

Officers boarded several D.C. public school buses, which were transporting special education students to private schools in Maryland, said Alfred Winder, head of the school system's bus service.

Motorists trying to avoid the American Legion Bridge headed for the tiny White's Ferry, which had a 90-minute wait.

The traffic backup was felt as far away as Loudoun County, where motorists looking for an alternate way over the Potomac River from Maryland drove across the Point of Rocks bridge in Frederick, clogging Route 15 south toward Leesburg.

Gordon Russell, 55, who runs a farmer's market just off Route 15 in Loudoun, said southbound traffic from Maryland became bumper to bumper at times. It meant more sales at the vegetable stand, Russell said.

John Harding and Wes Amick, both 43, steamed in their Strohman's Bread truck. They had struggled to make deliveries to supermarkets and stores near several of the shooting sites. Yesterday, they were three hours late with their bread delivery to the Shoppers Food Warehouse at Randolph and Veirs Mill roads in Wheaton. "I'm just fed up with it," Harding said. "I wish I could catch him. He's putting people out of business every day."

Motorists stuck in traffic at Colesville Road and the Capital Beltway commiserate during the lengthy delays that started about 7:30 a.m.A Montgomery County police officer checks out the bed of a pickup truck at a roadblock at Colesville Road and the Capital Beltway.Stranded motorists talk on Colesville Road. Police roadblocks forced drivers across the region to contend with empty gas tanks, full bladders, bored children and missed appointments.