Moments after he called his wife on a cellular phone to say his car had broken down, a Vienna man was killed on Interstate 66 early yesterday when he was struck by a truck hauling medical waste in what was once a shoulder but is now an exit to the Capital Beltway.

Virginia State Police said John J. Pyne, 71, a longtime Washington trial lawyer, had called for a tow truck and was sitting in his black 1991 Acura about 3:30 a.m. waiting for his wife to pick him up when he was struck from behind by the tractor-trailer.

Pyne's wife, Betty, was one of the first people to arrive at the crash scene, Pyne's son, Tom, said in an interview yesterday. The couple's town house is only five minutes away.

"She hasn't even been able to tell us what she saw," Tom Pyne said. "She's still in shock."

The incident is the most recent in a rash of fatalities involving drivers who have pulled over on the region's busy highways. The crashes--three in the past two months--have prompted some officials to question whether the age-old safety advice of staying in your car in an emergency isn't inviting disaster.

"Now with several incidents where people have stayed in their cars and died, I'm telling motorists to get out of the car and get as far back as they can," said Lon Anderson, spokesman for the Potomac Chapter of the American Automobile Association, adding that behind a guardrail or barricade might be the safest spot.

State police are somewhat more cautious. Stranded motorists should first call for help from police by dialing #77 on a cell phone. Then, if traffic is heavy, they should move away from the highway if possible, authorities advise.

In November, a New York husband and wife were killed and their teenage daughter seriously injured after the family stopped on the shoulder of the Beltway in Prince George's County to let their 10-year-old son take a bathroom break.

On New Year's Day, a 35-year-old Gaithersburg man and his 7-year-old daughter were killed when their car, disabled on the shoulder of Interstate 95 south of Baltimore, was struck by a car driven by a man police said was drunk.

Yesterday's incident also points up the potential danger of transforming shoulders--where drivers can pull over in emergencies--into travel lanes to relieve the region's traffic congestion. The former shoulder where Pyne had pulled over became an exit to the Beltway several years ago to accommodate the growing stream of motorists shifting from eastbound I-66 to the outer loop of the Beltway, according to Virginia Department of Transportation spokeswoman Joan Morris.

Morris said she did not know exactly when the shift was made but said it may have been in the early 1990s.

State officials said the driver of the truck will not be charged. Speed was not a factor, according to state police spokeswoman Lucy Caldwell. Investigators also inspected the truck and conducted a lengthy interview with the driver and determined there was no infraction or mechanical defect that would have caused the crash, she said.

The tractor-trailer, owned by Browning-Ferris Industries Inc., had just picked up medical waste from American Medical Labs in Chantilly and was heading to an incinerator in southern Virginia, state officials said.

"It was a tragic accident," Caldwell said, adding that there was little Pyne could have done to protect himself. His car was stranded where only 18 inches separate traffic from a five-foot-tall concrete barrier.

Even if Pyne had gotten out of his car, he probably would not have been able to scale the barrier because of his age, his son said.

"There was no place to go. There was no escape," said Tom Pyne, who examined the accident site shortly before the vehicles were towed away.

Caldwell said Pyne was only a hundred feet from a break in the barrier and a grassy area to pull his car out of the traffic. Pyne, who was preparing for a major trial starting yesterday, was driving to his law office in Chevy Chase, his son said.

CAPTION: John J. Pyne was driving to his law office when his car broke down. A truck hit the stopped car.