One recent Friday night, a 16-year-old girl and some friends were driving around shopping centers in Fairfax County, when they spotted the girl's father leaving a store parking lot. He was followed in another car by a woman whom the girl recognized as her father's lover.

The girl urged her friends to follow the car. Driving two car lengths behind, they followed it into a medical building parking lot where the teenagers watched the couple make love.

Several weeks later the girl left home. She said she was tired of her father cheating on her mother, her father beating her mother, and her father pushing her around, especially after he knew she found out about his affair.

The girl is from a middle-class McLean family. Her disruptive family life is one example of an emerging factor causing teenagers to run away from home, according to several psychologists and youth counselors. Grim relationships between parents and parent and child have now outpaced the flower children desire for adventure and freedom in the 1960s as a reason for running away, youth experts say.

Statistics show a general decline in the number of reported runaway children since a peak was reached in the late 1960s. While the disbruntled children of the 1960s and early 1970s hitchhiked hundreds of miles away from home to find their identities, emotionally upset teen-agers today may take a bus or walk less than a mile to stay with friends or at a run-aways' shelter to excape their family problems.

Some youth specialists balem this phenomenon on what they see as a breakdown of the traditional family. Others exonerate the parents and blame a deterioration of a society that often controls children more their parents do. Still others say teen-agers today are more interested in finding jobs than running off to parts unknown.

"The '60s and early '70s saw the end of the longterm runaway," said Peggy Kyle, assistant director of Alternative House, a privately and publicly funded home for runaways in McLean. "Most are short-term runaways in the neighborhood.

"There's been a breakdown of the nuclear family," Kyle said. "There are more and more cases of moms marrying three or four times, kids living with an unaffectionate step-parent, a single parent and more physical abuse," Kyle said.

"Of the cases I deal with, a good 60 per cent are from broken homes," said Lolly Zip of the Office of Youth Services in Alexandria. Teen-agers "are running away from it. The kids bear the brunt of their parent's problems."

Because all runaways are not reported, it is difficult to determine accurately their number.

But from 1970 to 1975, the number of runaways reported in one FBI survey of several large cities decreased from 149,978 to 141,895.

Alexandria officials said the number of reported runaways "decreased significantly" there within the last seven years. Fairfax County police said reported runaways in the county decreased from 2,028 in 1973 to 1,796 last year.In Montgomery County, the number of reported runaways for the fiscal year 1975-76 was 777. It decreased to 282 last year, according to the state Juvenile Services Department. The number of runaways in Prince George's County dropped from 365 to 326 in the same period. Such statistics for the District were not readily available.

But the number of local teen-agers who left home and resided at Alternative House has jumped from 152 in 1973 to 245 last year, Kyle said.

As it stands now, if a child runs away in Virginia there is virtually nothing a parent can do about it. According to the state juvenile code revisions that became effective last July 1, police can no longer pick up a runaway and take him home or to a detention center unless the child is a habitual runaway. Habitual is not defined in the law, but Fairfax has decided a habitual runaway is a child who has run away three times or for more than 10 days once, according to Vincent Picciano, director of Fairfax's Juvenile and Domestic Relations Court.

But even if a juvenile is deemed a habitual runaway and has been picked up by police on a detention warrant, the Virginia courts by law must release the juvenile the next day. The child cannot be forced to go home with his parents, to a foster home or an institution. But it is one opportunity for a parent to talk to a child before he or she decides to take off again.

"There was a feeling in the state legislature that too many kids were being picked up and detained for many minor, nonserious kinds of behavior and the court should not be involved in those matters, Picciano said.

One McLean youth, who has run away several times from his father, a well-to-do corporation executive, said his father beats his younger sister and has chased him and his sister with axes, although not actually hit them.

"Since I was 5 I've seen my father beat my mother to a pulp," the youth said. The youth vows never to return home and says that his parents hope he keeps that vow.

Kyle recalled a teen-ager who told her the story of her life that made her "feel like I'm watching soap operas."

She said the girl's mother married, had a child, got divorced, brought a boyfriend into the home, got rid of him and started sleeping with someone else. The first boyfriend returned one day in a large and shot the second boyfriend and the mother in front of the girl. The girl went to live with her father, but he had remarried, had more children of his own and did not want her.

"This isn't in the middle of the ghetto," Kyle said. "This is middle-class."

James Smith, head of Georgetown University's counseling center, said that not only are youngsters running away from home because of their parents' problems but because of an increasing incidence of incest.

"There seems to be such a high degree of incest," Smith said. "It's not new. It's just that society is much more mobile and (teens) look at running away as an alternative to putting up with it."

A recent report by the Carnegie Council on Children in New York says that the blame should be shifted from the parent's to society. Factors outside the home have more impact on the child's development than the few hours spent with their families, the report states.

"The Dick and Jane family is no longer true, if it ever was," said Christopher Cory, a council spokesman.

"There are new strains" on families. Cory said, such as mothers working and few families working together in business activities. "Families are changing shape drastically."