About 9,000 lives a year would be saved if all states passed laws requiring car drivers and front-seat passengers to use seat belts, the National Safety Council estimates. Twice that many -- 18,000 lives -- could be saved if all drivers and passengers used seat belts at all times.

American motorists have been ignoring such numbers for years. Despite all the grim evidence and multimillion dollar safety campaigns with slogans like "Buckle up for safety" and "Lock it to me," seat belt use nationally has hovered below 15 percent.

"Risk is not a rational topic," says Chuck Hurley, the National Safety Council's executive director for federal affairs.

"Most people who work in emergency rooms wear their seat belts religiously," says Dr. Prudence Kline, an emergency room physician at George Washington University Medical Center. "If people could see what we see, they would wear seat belts."

But they don't. For public health experts whose logic is stymied, for insurance companies who pay the bills, for emergency room doctors who treat the trauma, it is a source of frustration verging on rage.

Seat belts are a readily available, easy-to-use, cheap and effective life-saving technology. And they're spurned by six out of seven Americans.

They think, if they think about it at all, that accidents only happen to other people (despite the fact that the typical American is involved in at least one traffic accident every decade). They think they need a seat belt only on long trips (even though 75 percent of accidents occur within 25 miles of home) or only on roads like the Capital Beltway (even though 80 percent of accidents happen at less than 40 mph). They think a seat belt will trap them in a fiery crash (even though a passenger's chances of being killed are 25 times greater when he or she is thrown from the vehicle).

"It's not even carelessness," says Dr. Joy Drass, medical director of the MedSTAR shock trauma unit at the Washington Hospital Center. "It's thinking, 'It won't happen to me.' "

But it happens to a lot of somebodys. About 44,600 Americans died in motor vehicle accidents in 1983 -- the equivalent of a jumbo jet crashing and killing all aboard every three days -- and 3.4 million were injured. Traffic accidents are the fifth-leading cause of death -- and the leading cause among people aged 5 to 34.

When New York Gov. Mario Cuomo signed the nation's first buckle-up law last July, seat belt use among New Yorkers was about 14 percent. Last month, after traffic officers began slapping unbelted New York drivers with $50 fines, use jumped to 70 percent, according to surveys by the state Department of Motor Vehicles.

New York's law requires drivers and front-seat passengers to use seat belts or face a $50 fine. Similar laws take effect in New Jersey on March 8 and Illinois on July 1. Seat belt legislation is expected to be considered by legislatures in 42 other states this year, though passage is considered likely in fewer than 15.

In Virginia, a seat belt bill with a $25 fine was passed by the House this month and is pending in the Senate. In Maryland, a similar bill cleared the Senate last week, but had already been defeated in a House committee. In the District, a bill has been introduced in the City Council, with no action yet.

Seven of Canada's 10 provinces and some 30 nations, including Australia, Britain and most other European countries, already require use of seat belts.

Given the time-honored unwillingness of Americans to buckle up, many safety experts have long recommended use of automatic crash-protection devices or "passive" restraints called air bags. These are cushions that inflate automatically on impact at more than 12 mph to keep a driver or front-seat passenger from crashing into the steering column, windshield or dashboard.

A federal safety standard requiring front-seat air bags in cars was first proposed in 1969. But automakers have fought the air bag requirement in court, on Capitol Hill and before the Department of Transportation because of its cost. DOT estimates that front-seat air bags would cost $320 per car.

One of the auto industry's arguments throughout the protracted regulatory fight over air bags has been that seat belts could save nearly as many lives as air bags -- if they were used. But proponents of air bags argue that it's irresponsible to base public policy on a "voluntary" strategy that six out of seven Americans reject.

The Reagan administration tried to rescind the federal safety standard requiring passive restraints -- either air bags or belts that fasten automatically. But consumer and insurance groups sued and won a Supreme Court ruling forcing DOT to reinstate the rule.

Transportation Secretary Elizabeth Dole reissued the rule last July, requiring automakers to provide automatic crash protection in all 1990-model cars. But she said DOT would suspend the rule if states with at least two-thirds of the U.S. population enact mandatory seat belt laws by April 1989.

That ruling, plus passage of the New York law, gave sudden impetus to buckle-up laws on the state level. The auto industry, hoping to head off the air bag requirement, formed a group called Traffic Safety Now to lead a $15 million lobbying effort for belt laws.

Auto safety experts agree that the best crash protection is a combination of air bags and seat belts. Air bags are most effective in frontal and front-corner collisions, the most common type. Seat belts add protection in rollovers, side impacts and low-speed crashes.

Some experts worry that passage of seat belt laws covering two-thirds of the nation could allow DOT to overturn the air bag requirement, leaving motorists in the other third unprotected.

Both strategies have disadvantages. Since the average car lasts 10 years, even if air bags are required in all new 1990-model cars, they won't catch up to the entire fleet of cars until after the turn of the century. Seat belts are available now, but even strictly enforced laws with stiff fines won't bring use close to 100 percent.

"You can get about 70 percent usage with seat belt laws when they're enforced," says Hurley, of the National Safety Council, which supports both seat belt laws and air bags. "The problem is, the 30 percent you don't get is disproportionately at risk: the kids, the drunks, the speeders."

Opponents of seat belt laws often raise the so-called Big Brother issue, claiming, as a Maryland legislator put it last week, that "people have a right to be stupid." But even libertarians cannot repeal the laws of physics, and proponents note that nonwearers' "stupidity" raises insurance rates and medical and social costs for everyone.

Hurley says there has been a "fairly fundamental political shift" recently on the degree of political intervention the public will tolerate on auto safety issues. He points to passage of state laws requiring use of child safety seats and growing efforts to curb drunken driving, led by pressure groups such as Mothers Against Drunk Driving.

The impetus, he says, has come not from national statistics or safety slogans but from organized groups of doctors and family members expressing outrage and grief about specific tragedies.

"The real constituency of auto safety are the survivors," Hurley says. "They're the ones who know the real costs."