An out-of-control tractor-trailer cut across four lanes of traffic on the Beltway in Fairfax County yesterday, sideswiping several cars before smashing through a guardrail and killing the driver, police and witnesses said.

The wreck, which temporarily closed the inner loop between Georgetown Pike and George Washington Parkway and snarled traffic for hours afterward, occurred around 10 a.m. a few hundred yards north of the Georgetown Pike exit, less than a mile from the Cabin John Bridge.

The driver of the truck was identified as Keney Watkins, 40, of Martinsville, Va.

Three other people were taken to Fairfax Hospital after the crash, which involved six vehicles in addition to the truck. They were not seriously hurt, authorities said.

"It's safe to say . . . there were defective brakes" on the truck, Virginia State Police Trooper W.S. McKinney said. He said state police were investigating the crash. The truck was northbound, and carrying plums.

Yesterday's collision, the third major accident involving a truck on the Beltway in just over a month, added new fuel to the controversy over whether trucking should be restricted on the heavily traveled road.

Matt Whalen, of Bethesda, said at the scene of the accident yesterday that his was the first of six vehicles to be struck by the truck.

Whalen said he was driving two lanes in from the median when he saw the truck overtaking him in the lane to his left. He said the truck crossed into his lane and hit his vehicle, smashing the trunk of his sedan all the way into the back seat.

Whalen walked away from the collision uninjured. "I feel very good about it," said Whalen, a project manager for a local construction firm. "I'm lucky just to survive this."

After striking Whalen's car, witnesses said, the truck struck a red Datsun carrying three passengers, flipping the car into the air to clip another car while airborne before skidding further down the highway.

" The truck slingshotted the car right on its roof," said Ricky Shevitz of Manassas, whose vehicle was also struck by the speeding truck. "The car went speeding right past me on its roof."

The three passengers in the car, who were taken to the hospital, were not identified.

"It's just fascinating to me that something like this could happen and that no one was seriously hurt except for the truck driver," said Jim Webb of Manassas, whose car was hit by the airborne car as it was coming down from its flip.

Police closed the Beltway from the Georgetown Pike exit to the George Washington Parkway exit for about an hour after the crash, Plaugher said.

Although some lanes were later reopened, it was not until after 4 p.m. that the wreckage was fully cleared, and traffic remained backed up as far as two miles for much of the afternoon, a Fairfax County police spokesman said.

Trucks are involved in 60 percent of the accidents on the Beltway, according to a spokesman for the Metropolitan Washington Council of Governments, which later this year will cosponsor a conference on Beltway safety.

Fairfax County Board of Supervisors Chairman John F. Herrity said he believes curbs on trucks such as a 40-mile-an-hour speed limit and a no-passing rule may be necessary to make the Beltway safer.