A Monroeville woman who was killed Tuesday night in a crash while driving the wrong way on the Pennsylvania Turnpike was awaiting trial in two cases in which she was charged with driving under the influence of prescription drugs.

Andrea Baker, 36, and Aidan, her son, were trapped after her car crashed head-on with two tractor-trailer trucks less than two miles from the Monroeville interchange at 9:18 p.m., police said.

State police said investigators had not determined how Ms. Baker got onto the turnpike on the wrong side of the highway.

Ms. Baker was pronounced dead at 10:48 p.m. Tuesday at Forbes Regional Hospital. The cause of death was blunt force trauma to the leg and abdomen, the Allegheny County medical examiner said. Toxicology results were not yet available.

Her son, who was strapped in a safety seat, suffered only minor injuries. He was alert and speaking with police at the scene during the rescue before he and his mother were taken to Forbes Regional.

The boy was transferred by helicopter to Children's Hospital and later discharged, said a hospital spokeswoman.

The eastbound lanes of the Pennsylvania Turnpike were closed Tuesday night near the Monroeville exit until about 5 a.m. yesterday, police said.

According to county court records, Ms. Baker was arrested twice this year -- in May and June -- on charges of driving under the influence of psychotropic prescription drugs.

She had been free on her own recognizance, pending trial.

Records also show she was arrested in April 2007 in Springdale Township, and in November 2005 in York, Pa., on similar counts.

Charges were dismissed in both cases by magisterial district judges, according to court records.

On Tuesday night, Ms. Baker drove head-on into the path of an eastbound tractor-trailer truck driven in the left-hand lane by Patrick H. Miller, 63, of South Euclid, Ohio.

Mr. Miller, who was transporting furniture to Maryland, said a pickup truck that was driving ahead of him swerved suddenly to the right lane, revealing Ms. Baker's car. He said he was unable to move out of the car's way because an 18-wheeler was to his right and the concrete divider was to his left.

"There was nothing I can do," he said.

Ms. Baker's car careened into the path of a second truck, driven in the right-hand lane by James A. Balmer, 60, of Port Orange, Fla.

Neither truck driver was injured, police said.

"It's very difficult to figure out what this woman did to get on the wrong side," said Bill Capone, spokesman for the Pennsylvania Turnpike Commission.

"There's no physical barrier to stop people from crossing over at the interchange," he said.

The area remains free of barriers because motorists and police frequently use the interchange area as a turn-around.

Mr. Capone said signs are posted in a number of places at the interchanges to guide motorists to safe, proper routes.