As the first cruisers raced onto Interstate 95, they seemed headed toward an accident, maybe caused by rain. But in seconds, they were everywhere, blocking entrance ramps and exits. Officers were out of their cars, standing in the middle of asphalt lanes, assault rifles slung over shoulders. They peered down from overpasses, radios in hand.

More than 100 armed officers from local, state and federal agencies descended on I-95 between Fredericksburg and Alexandria for three hours yesterday, hoping to turn an escape route into a dragnet for a killer. They slowed every car, truck and van heading north, pinching the region's main north-south artery and trapping thousands of vehicles and motorists -- but not the one they sought.

It was a plan that police said they had prepared in advance -- and finalized during a conference call Thursday -- to try to capture thesniper whose varied targets recently seemed connected by one common element: proximity to major highways.

"We immediately contacted the locals to get all the ramps shut down within minutes," Virginia State Police spokeswoman Lucy Caldwell said. "Every single police agency [along the route] was out there. They came together quickly."

Police said they saturated the northbound, rather than the southbound, side of the interstate because witnesses reported seeing a white van flee the area around the shooting in the direction of the ramp onto northbound I-95. But troopers and local officers also stopped vans on southbound I-95, Route 1 and secondary streets.

When asked whether state police, in hindsight, should have had more coverage on the southbound lanes, Caldwell said: "Police in this case shut down every avenue and every roadway they thought needed to be shut at the time. They did everything they could."

Col. W. Gerald Massengill, the Virginia State Police superintendent, said authorities had been planning "for several days" to shut down all bridges leading from Virginia to the District and Maryland if a shooting took place near an interstate.

Maryland State Police took up positions at the American Legion Bridge but did not interfere with traffic flow. Maryland authorities would not comment on whether they have a similar plan to stop traffic on highways, but State Police Cpl. Rob Moroney said motorists could expect more possible traffic tie-ups and roadblocks if thesniper strikes again in Maryland. "We will do whatever we have to do in an attempt to catch this guy," he said.

The area around the shooting was sealed off from the site northward for nearly three hours. But the dragnet yielded little more than jittery motorists, and police abandoned their effort shortly after noon.

"When he walked up with the M-16 assault rifle, I was a little worried," said Doug Cokenour, 26, one of several plumbers in white trucks on their way to routine jobs when they were pulled over.

Police did not technically shut down I-95; cruisers were positioned at every exit and entrance ramp but did not prevent any vehicles from leaving or entering the highway. At the Springfield interchange, all northbound traffic was funneled into one lane that inched through a gantlet of officers.

Asked how the shooter could have escaped if roadblocks had been erected swiftly, Massengill interjected, "Obviously, not quick enough." But he defended the strategy. "It made sense," he said.

The shooting occurred at approximately 9:30 a.m. Within minutes, authorities called the Virginia Department of Transportation and requested that the agency dispatch highway vehicles equipped with arrow signs to help corral traffic to checkpoints along I-95, VDOT spokesman Ryan Hall said. On a typical day, 430,000 vehicles travel I-95 through the Springfield interchange; it carries local traffic and also is a main north-south route for anyone traveling along the East Coast.

It was the tail end of the morning rush, and hundreds of officers swarmed the interstate, pulling over any vehicle that fit a description of a white van seen leaving the area around the Four Mile Exxon in Massaponax after yesterday's shooting. In Alexandria alone, patrol officers stopped 176 white vans. "We were getting multiple calls on the same vehicle that had already been checked out," said Alexandria police spokeswoman Amy Bertsch, noting that motorists on cellular telephones were calling in descriptions of fellow drivers.

Police also were stopping cars on Route 1, the smaller, older roadway that parallels the interstate. Shortly after 10 a.m., police were out in force on Route 1 as far north as Alexandria, pulling over white vans. Lt. Col. John Scott of the Virginia State Police said officers were also sent south of Fredericksburg to monitor ramps on I-95 ramps, and onto nearby roads as well.

"We had people south, and people east and west, on all the primary highways and the secondary roads," he said.

Fairfax County police set up a roadblock on northbound Route 1 in front of the Gables at Mount Vernon Townhouses, stopping all types of white vehicles: plumbers' vans, box trucks and even a florist delivery vehicle decorated with blossoms.

George Boan, 32, of Fairfax was among the plumbers pulled over. Police asked for his driver's license and registration and permission to search the van.

"I heard them rumbling around" in back, he said. He said that he understood the rationale for the roadblock but said it had kept him from making a turn and he would have to explain to his customer that he was late because he had been stopped and searched. "I haven't had a chance to use that one before," he said.

The sniper has scattered his targets throughout the Washington region, but in his most recent attacks, he has tended to choose locations close to major highways -- Route 50, I-95, I-66 -- that afford quick escape.

"Most seem to be happening near interstates -- we were trying to shut down the interstate as fast as we could," said Julie Hersey, spokeswoman for the Fairfax County police.

Police in Virginia and Maryland said the clampdown on I-95 caused traffic backups on state roads that reached as far as five miles into Charles County on Route 301.