The death of a Vienna man who was struck by a truck after he parked his disabled car on the side of Interstate 66 was the ironic consequence of what Virginia officials said yesterday was their effort to improve highway safety by converting the shoulder to a deceleration lane.

John J. Pyne, 71, was killed Tuesday when his Acura was hit by a tractor-trailer as he sat on a deceleration lane that the Virginia Department of Transportation had extended along a shoulder in 1993 to prevent cars from backing up onto the main lanes of I-66 as they exited to the Capital Beltway.

"The advantage of [converting] the shoulder was for safety," said Tom Farley, VDOT's district administrator for Northern Virginia. "This is a fairly common and standard practice to extend these decel lanes."

Richard Retting, a transportation engineer with the Insurance Institute for Highway Safety, agreed that highway departments typically decide to forgo shoulders when designing interchanges, judging that it is better to provide generous exit and entrance lanes than to leave space for disabled vehicles. There is often not enough room to do both, he said.

"This is a trade-off, and trade-offs are made every day in traffic design," Retting said. "Clearly there's a need to provide adequate room for deceleration. When traffic backs up onto a high-speed road, that's a safety concern."

The seven-year-old decision to extend the exit lane came at the same time that VDOT chose to use the shoulders of I-66 between the Beltway and Route 50 as an additional travel lane during the morning and evening rush hours.

In the latter instance, transportation officials deemed that the hazard posed by eliminating the shoulder during rush hours was outweighed by the need to continue providing three ordinary travel lanes in each direction for heavy interstate traffic. At the time, one of the three existing travel lanes was being converted to a car-pool lane.

Though Retting did not criticize VDOT's decision, he said the merits of this second trade-off are less clear. "It does come at the expense of a relatively safe stopping area," he said.

Some state and county officials echoed those worries yesterday. "We're very concerned about it," said Col. M. Wayne Huggins, the Virginia State Police superintendent. "In a perfect world, where we had all the resources we needed, I would like to see the shoulders about three times wider than they are. But unfortunately we don't live in a perfect world and we have to work with shoulders that are available to us."

Maryland highway officials said yesterday they have not converted any shoulders to ordinary travel lanes except as a temporary measure during construction.

Farley said VDOT has monitored whether the use of the I-66 shoulders during rush hour has contributed to more crashes. "We haven't had an unusual set of conditions occurring that makes us want to revoke the use of shoulders," he said.

Questions about the wisdom of eliminating the shoulders on I-66 previously surfaced after a traffic fatality in March. In that accident, a motorist who got out of his car during rush hour to check the damage from a fender bender was killed when traffic started moving again and a tractor-trailer ran into him. VDOT officials said they doubted that the lack of a shoulder played a role in the crash because the driver had stopped his car in a center travel lane.

State police examined Pyne's car yesterday to determine what might have caused the vehicle to stall. They also were checking its emergency hazard lights. "We believe the button was pushed to have them up, but it's unclear whether they were on or not," said Lucy Caldwell, a state police spokeswoman.

According to a family member, Pyne told his wife that the hazard lights were working when he called her on his cell phone moments before being struck by the truck about 3:30 a.m.

"The flasher button was depressed, and the truck driver apparently told police that the lights were on, but he wasn't sure if it was flashing," said Pyne's son, Tom Pyne, who said he got that information from a police officer. The son said the black 1991 Acura Integra had been serviced just last week.

The driver of the truck was not charged. An official with Browning-Ferris Industries Inc., the owner of the truck, said the driver was "very upset and distraught" over the incident and would be unavailable for an interview.