A tractor-trailer driver was fatally injured and Capital Beltway traffic snarled for several hours yesterday afternoon after the truck jackknifed on the Cabin John Bridge and its cab plunged more than 50 feet into the Potomac River.

The driver, Arthur B. Wright, 56, of Baltimore, was pulled from the wreckage by passing motorists who dove into the near-freezing waters from the shore shortly after the 3:40 p.m. incident. Despite resucitation efforts at the scene, he was pronounced dead on arrival at Suburban Hospital. e

Rush-hour traffic on the Beltway in both directions slowed to a crawl for several hours following the accident.

According to police, the tractor-trailer was traveling along the inner loop of the Beltway headed from Virginia into Maryland when it suddenly swerved and jackknifed. The cab went through a bridge guardrail and fell into the river, partially submerging. The rig's trailer continued down the highway, traveling more than 100 yards north of the bridge before it stopped. No other vehicles were involved in the accident, police said.

Investigators said the cause of the accident still was unknown last night, but they said the rig may have been forced out of control by winds on the bridge. Winds of 28 miles an hour gusting to 46 miles an hour were reported yesterday.

"I got there about five minutes after it happened," said Peter Stankiewicz, 25, who was on his way at the time to his Air Force job in Virginia from his home in Gaithersburg. "I was about the 10th car in line, stopped on the bridge. I got out, asked what was happening, whether anybody had gone down there to get the guy out. They said no -- so I just went down.

"That really got me mad -- I mean, they [the other motorists] were just standing there. One guy was taking pictures," Stankiewicz said.

Stankiewicz said he was the first would-be rescuer into the approximately 35-degree water. He said three unidentified motorists scrambled down the gravel-covered embankment behind him and the four worked the driver free of the wreckage in a few minutes. The driver was submerged when they arrived, according to Stankiewicz, who was treated at Suburban for exposure.

Stankiewicz and the others continued mouth-to-mouth and cardiopulmonary resuscitation attempts until rescue workers from McLean and Montgomery County arrived. The three others "disappeared," said Stankiewicz, who learned the resuscitation as a water safety instructor "a long time ago," and he went to sit in the warm cab of a fire truck.

"I don't know what the guy's chances were anyway -- it was a long way down -- but if someone had gone down there sooner, I think it might have made a difference," said Stankiewicz.

An autopsy will be performed to determine the cause of the driver's death.