Two and a half years ago, Pamela Kostmayer was at her home in Bucks County, Pa., when she heard a crash. She rushed outside to the end of her driveway and saw a car "wrapped around a phone pole. Two children [including the teen-age driver] were dead instantly," she recalls, "and at first I thought the third one was dead, too. Then he moved."

Passing motorists called for help while she waited with the surviving teen-ager who was in shock. It took the rescue squad an hour and a half to cut him out of the wreckage. None of the occupants of the car had been wearing seat belts. The children died of internal or head injuries -- the very kind that seat belts often can prevent or lessen.

In the weeks and months that followed, Kostmayer, who has worked in public relations and is the wife of Rep. Pete Kostmayer (D-Pa.), read everything she could about auto safety. She founded Mothers United to Save Children's Lives, a Pennsylvania group that promotes the use of car restraints for children and rents car seats for 50 cents a month, regardless of income. The organization also buys and sells car seats at cost and shows parents how to use them.

Kostmayer, who has two daughters ages 10 and 11, is on the national board of Mothers Against Drunk Driving (MADD) and on the school bus safety task force of the National Child Passenger Safety Association. She has become an active proponent of laws requiring seat belts on school buses, an issue that came up in the course of a car safety campaign she did in conjunction with the Chrysler Corp. She toured 17 cities doing interviews and talk shows and, she says, "the number one question was, why aren't seat belts on school buses?"

The answer, she found, came from school bus manufacturers who oppose it as unnecessarily costly and impractical and who point to the extraordinarily good safety record of school buses, which accounted for only 0.2 percent of all highway accidents in 1983, according to the National Safety Council. School buses also have been built to more strict safety standards since 1977. Kostmayer was surprised to discover that seat belts on school buses, which she believes will help reinforce children's habits of using seat belts in additon to ensuring greater safety in case of accidents, is a controversial issue. It pits a small but growing number of activist parents against a business lobby. Bus drivers have opposed seat belts, saying they would be unable to enforce usage and they could be used as weapons.

Kostmayer's answer to that is to have manufacturers install retractable belts. And, she says, the several jurisdictions that have passed laws requiring seat belts on buses -- including two in New York state -- have found the system works.

According to Safe Ride News, published by the American Academy of Pediatrics, seat belts were required last fall on buses in Skokie, Ill. Last spring, school officials took portable bus seats with seat belts into the schools so young children could practice buckling up, and they showed films to convince children of all school ages to use seat belts. Estimates are that it would cost between $1,000 and $1,500 to install seat belts on a bus. "I have PTAs that say we'll raise the money," says Kostmayer. "Just give us the anchors." Her husband has introduced legislation that provides $10 million over three years to help states pay for seat belts. He, too, believes that encouraging children to use restraints in cars and not on buses confuses them, and dilutes long-term efforts to get them to buckle up when they are older.

Kostmayer points out that car crashes are the number one killer of adults under age 36 and of children under the age of 5. Each year, more than 40,000 people die in crashes. "I cannot find a policeman or fireman who has unbuckled a dead man," she says.

Every state except Wyoming has now passed legislation requiring restraints for children in cars. Several states, the latest being Illinois, have passed mandatory seatbelt laws for adults as well. Gradually, activists such as Kostmayer have helped the nation become more aware that properly used restraints save lives.

But the battle is far from over, which is one of the best arguments for putting seat belts on buses and instilling a good habit in children. Despite all the seatbelt campaigns aimed at adults, highway safety officials say their use has not increased. And last year, despite all the recent efforts to curb drunk driving and improve highway safety, the rate of deaths per miles traveled remained almost the same -- and the actual number of deaths increased by 3 percent.

The tragic fact at year's end was that, by federal count, 43,862 people had perished on the nation's roads.