Americans could save about 10,000 lives a year by a simple, painless act that costs nothing and takes virtually no time. They could buckle their seat belts.

They could, but they don't. Despite reams of grim statistical proof and a generation of "Buckle Up" campaigns, between 75 and 90 percent of American motorists don't use a seat belt.

One of every 140 Americans dies in a traffic accident, and one in three suffers an injury that is disabling for at least a day. Isn't that incentive enough to buckle up?

Apparently not. Trouble is, most people look at the risk in terms of short hops to the grocery store instead of a lifetime of driving.

They know the risk of being injured or killed in any one car trip is minuscule. About one trip in 4 million ends in a fatal accident, and one in 100,000 causes a disabling injury.

An individual motorist may look at the tiny per-trip risk, forgetting that the average American takes about 50,000 car trips in a lifetime. And each safe trip may reinforce the idea that buckling up is unnecessary.

"If you tell people how much the seat belt will help them on their next trip," said Lester Lave, president of the Society for Risk Analysis, "there are so many zeroes after the decimal point they get bored with it."

Psychologist Paul Slovic and his colleagues at Decision Research, a A motorist may look at the tiny per-trip risk, forgetting that the average American takes about 50,000 car trips. Eugene, Ore., firm specializing in risk assessment, tried a different approach. They won a federal contract several years ago to design a campaign emphasizing the cumulative lifetime risk of driving.

After creating a series of dramatic television commercials on the theme of lifetime risk, they showed them to large groups and monitored the groups' seat belt use afterwards.

The campaign was yet another flop. Its failure led a frustrated Slovic to conclude that no educational campaign can persuade more than a small percentage of American motorists to use seat belts voluntarily. He now endorses legislation to require seat belt use.

Such laws have been enacted in 25 states and the District.

Seventy-six percent of Americans don't use seat belts, last week's Journal of the American Medical Association reported, based on a survey of 28 states and the District. Among those least likely to buckle up are blacks, people with no more than a high school education, 18- to 24-year-olds and drunken drivers.

Seat belt use -- or nonuse -- shows that numbers alone can't persuade the public to protect itself against a proven risk.

"On the technical side, we have more tools to manage risk today," Slovic said, "but somehow, on the social side, things are getting messier."