A quick stop for a bathroom break on the shoulder of the Capital Beltway left a couple dead and their teenage daughter seriously injured after their sedan was struck by a car that had veered off the highway.

Maryland State Police said the victims were returning to New York from a Thanksgiving holiday in Richmond when they pulled off the busy highway just south of Route 210 in Prince George's County to allow their 10-year-old son a chance to relieve himself in some woods nearby.

Shortly after the child got out, the family's 1995 Mazda was struck from behind by a 1994 Pontiac Grand Prix, killing the boy's parents and seriously injuring his 16-year-old sister, police said.

The two deaths brought to 30 the number of people killed in crashes on roads in Maryland, Virginia and the District during the four-day Thanksgiving break, police said. The holiday is traditionally among the busiest for travel, according to the American Automobile Association, as the greatest number of people take to the roads in the shortest stretch of time, most waiting until yesterday to scramble home.

Authorities identified the driver of the Mazda as John Michael Hall, 47, of Northport, Long Island. They said he and his daughter were sitting in the front seat, and his wife, Carole Hall, 47, in the back seat when the accident occurred at 8:24 a.m. Relatives said the family had spent Thanksgiving with John Michael Hall's brother, Stephen.

"They usually spend Thanksgiving in New York, but this weekend they decided to drive to Virginia," said Gail Hall, of Dallas, a sister-in-law of John Michael Hall.

Trooper Rob Moroney, spokesman for the Maryland State Police, said authorities had yet to determine why the Grand Prix left the highway.

"At this time, we're unaware why the Pontiac veered off the road and struck the Mazda from behind," Moroney said.

The Mazda did not have its emergency blinkers on. The car was stopped on a wide shoulder next to a wooded area, and was clearly visible to other cars, said Sgt. R.E. Morris, of the Maryland State Police, who was at the scene.

"You can see them from a distance," Morris said.

The impact killed the parents. They were pronounced dead at the scene, police said. Their daughter was taken to Prince George's Hospital Center, where she was listed in serious condition. She was identified by relatives as Sarah. None of the family members was believed to be wearing a seat belt at the time of the crash.

The boy, whom relatives identified as Michael, was released to New Jersey relatives who drove down yesterday afternoon.

The driver of the Pontiac, Jason Jones, 19, of Fort Washington, was reported in stable condition at Prince George's Hospital Center. State police would consult with the Prince George's County state's attorney to determine whether charges will be filed in connection with the accident, Moroney said.

In Maryland, 13 people died in traffic accidents during the four-day Thanksgiving break, raising the death toll this year on state roads to 516, compared with 498 for the period in 1998, said Maryland State Police Sgt. R. Lee. Of the 13 holiday crashes, speed was a factor in seven, alcohol in two. The remainder were attributed to various driver mistakes or were undetermined in cause, Lee said.

In Virginia, 16 people died in crashes on state roads during the extended weekend, compared with 13 who were killed in crashes during the 1998 holiday break, said Virginia State Police Sgt. Robert Evans. The crashes brought the total killed on Virginia roads to 771 for the year, down from the 818 died through Nov. 28 last year.

Evans said alcohol was a factor in at least two of the holiday break crashes, including one early yesterday on Interstate 64, south of Ivy, in which a driver lost control of his car, was ejected and struck by another vehicle.

U.S. Park Police have investigated two fatal wrecks in the Washington area since Wednesday, including a head-on crash Friday on the Rock Creek Parkway in the District that killed a 14-year-old boy. Park police also investigated an accident Saturday in Maryland on the Baltimore-Washington Parkway that killed a 44-year-old Greenbelt man, a crash counted by Maryland State Police as among the state's 13 holiday fatalities.

The deaths of John Michael and Carole Hall are hitting the family hard, Gail Hall said.

John Michael Hall worked for a computer company in Suffolk County, Long Island, and his wife Carole worked part time helping disabled students get on and off school buses, Gail Hall said.

"Carole was my best friend," Gail Hall said. "She was a wonderful mother, fun-loving, with a great sense of humor."

John Michael, she added, "was a very awesome family man, very involved with the life of his children."

News Researcher Nancy Shiner contributed to this report.