More than a half century of safety advances, public-relations campaigns, legislation, and advertising pitches by the Department of Transportation and the world’s automakers have persuaded 88.5 percent of Americans to fasten their seat belts when they get into their cars.

But that also means 11.5 percent of vehicle drivers and passengers still don’t buckle up. That translates to almost 25 million people who ignore the public-service ads, the reminder chimes in their vehicles, the 49 state laws that make seat belt use mandatory, and the nagging from loved ones who do click into their seat belts.

At this point in the history of the seat belt, the research behind its effectiveness is so conclusive and well-socialized that few abstainers can claim ignorance, forgetfulness, or confusion. To regulators and safety advocates, the remainder of stubborn people who refuse to buckle up represents an incredibly high number of unnecessary vehicle fatalities.

The societal consequences extend beyond the tragedies themselves. The added risk from unbelted motorists raises car-insurance rates and healthcare costs for everyone. And automakers have made numerous design compromises to their vehicles to account for unbelted occupants, resulting in cars that are less safe for the rest of us.

Seat belts have been standard equipment in passenger cars since 1968. Usage was low at first, but in the 1980s states began to set mandatory seat belt laws. The DOT then pushed out a large-scale public-education campaign, which turned crash-test dummies into cultural icons. (Scroll down to see a history of seat belts.)

The results were significant. After New York State passed the first seat belt law in 1984, observed belt use rose from 14 percent to 37 percent within two years. By the end of the 1990s, it was above 70 percent.

“The ‘Vince and Larry’ ads and high visibility enforcement campaign ‘Click It or Ticket’ have proven tremendously helpful in increasing seat belt use,” says Mark Rosekind, chief of the National Highway Traffic Safety Administration (NHTSA).

Rosekind’s agency is America’s official record-keeper when it comes to traffic accidents. According to its data, seat belts represent the single greatest public-safety innovation in automotive history. In the past 50 years, 14 safety innovations have saved 613,501 lives. They include frontal airbags, child safety seats, and side impact protection and curtain airbags. But not one has come close to seat belts, which are responsible for saving 329,715 lives—more than half the total.

Consumer Reports recently posted an online request to hear from seat belt abstainers. A majority of respondents complained of discomfort, especially shorter drivers and women with large breasts.

Others expressed a fierce libertarian streak, seeing seat belt laws as onerous and unjustified. Some were baby boomers who grew up not wearing belts and never got into the habit of using them. Many said they buckled up on highway trips but not when driving around town. We even heard from someone—who said he’s in law enforcement—who refuses to wear a seat belt and doesn’t think anyone else should have to, either.

More than a few who responded said they had been in accidents while unbelted but continued not to wear a seat belt. Some even theorized that being ejected from the car actually saved their life.

There are always stories of people defying the odds and surviving a car crash unbelted, but the likelihood of being injured or killed as a result of being ejected is actually very high, says Jennifer Stockburger, director of operations for Consumer Reports’ Auto Test Center.

“The odds are not in your favor,” Stockburger says. “You’ll only hear survival stories from survivors, but unfortunately not from the many whose lives could have been saved by buckling up.”

Statistically speaking, you’re twice as likely to die in a crash if you’re not wearing a lap/shoulder seat belt. Your chances are even worse if you’re in a light truck or SUV.

According to the Centers for Disease Control and Prevention, women are more likely to buckle up than men, and drivers in urban and suburban areas are more likely to wear their seat belts than those in rural areas.

Find out what it will take to get to 100 percent seat belt use, and learn about the 5 dangerous seat belt lies people tell themselves.

Backseat Bad News

Data on rear-seat passengers are the most distressing. Adult seat belt use in 2014 was only 73 percent in rear seats, down from 78 percent the previous year. Worse still, rear seat belt use among younger passengers (ages 16 to 24) fell significantly, to 68 percent from 78 percent, a possible reason motor-vehicle crashes are the leading cause of death among teens. NHTSA data show that more than half of teenagers who died in crashes weren’t wearing a seat belt.

Rear-seat passengers are three times more likely to die in a crash if they’re unbelted, according to a recent study done for the Governors Highway Safety Association. It noted that 883 unrestrained rear-seat passengers ages 8 and older were killed in crashes, but that buckling up might have saved 436 of them.

The study also revealed a macabre fact: A driver wearing a seat belt is more than twice as likely to be killed in a frontal crash when an unbelted person in the backseat is hurled forward.

“By being completely unrestrained in a vehicle, the child or the adult is free to fly around inside the vehicle and strike basically anything,” says Dennis Durbin, M.D., director of the Office of Clinical and Translational Research at The Children’s Hospital of Philadelphia. “Anything” can include hard surfaces such as the side pillars, glass—or other occupants.

An unrestrained passenger in the backseat might not only become a projectile that hits the driver in the back of the head; he or she can also be propelled forward with enough force to compress the driver against the steering wheel or windshield, Durbin says.

“I suspect that when people choose not to wear their seat belt, particularly in the backseat,” he says, “they don’t realize that they are putting others in the vehicle at risk.”

Society Picks Up the Tab

In 2010 NHTSA studied the social and economic costs of motor-vehicle crashes. The conclusion? A direct cost to society of $242 billion that year. When pain and suffering were included, it came to $836 billion.

Seat belts saved 12,500 lives and prevented 308,000 serious injuries that year. As a result, seat belts saved $50 billion in medical care costs and the lost productivity and other costs they would entail. But crashes involving unbelted passengers cost society more than $10 billion.

Those of us not directly involved in crashes pay for more than 75 percent of all crash-related costs, direct and indirect. Those costs are primarily related to higher insurance premiums, taxes, travel delays, and excess fuel consumption because of traffic, according to NHTSA’s study.

Also, about one-third of what insurers pay out in auto-insurance claims is for medical bills. Therefore, about one-third of what consumers pay for liability coverage is affected by medical costs, says Robert Passmore, assistant vice president of the Property Casualty Insurers Association of America.

“If you are using your seat belt, you are much less likely to sustain an injury in the first place, and the severity of that injury is going to be less,” Passmore says. “Generally speaking, the less severe the injuries, the lower medical costs will be. But among the worst things that can happen to you is to go through the windshield or be thrown outside the car. That’s where you hear of the most catastrophic injuries occurring.”

The more severe an injury, the higher the related costs. According to NHTSA’s 2010 report, the cost of an accident in which a passenger is critically injured is estimated at more than $3.3 million in terms of medical bills, property damage, and lost wages. For a fatality, that number climbs to more than $5.3 million.

An infographic that shows how people use seat belts in different U.S. states

How Seat Belts Affect Car Design

Auto-industry experts consulted by Consumer Reports say that modern cars are designed to protect unbelted occupants in a crash, but that those protections result in compromises that can actually reduce safety and comfort. One result is that automotive engineers essentially have to supersize airbags.

“The belted occupant is subject to a much larger, more powerful airbag than necessary,” says Chris Caruso, a former GM crash-safety engineer and now a consultant with Automotive Safety Consulting in Las Vegas. He spent more than 20 years designing three generations of airbag safety systems, leading to the airbags that are in vehicles today.

Modern cars take into account how passengers are seated, their weight, and whether or not they’re belted, then adjust the force at which airbags deploy accordingly. But there are design and engineering limitations for how much the airbag can be depowered. Also, airbags could be made smaller if federal standards didn’t require them to protect unbelted occupants, Caruso says.

“The larger size and volume of the driver airbag to compensate for unbelted occupants results in an airbag that is larger than the steering wheel rim diameter,” Caruso explains. That can result in injuries to the driver’s hands, wrists, and forearms in a crash.

In Europe, where a higher percentage of people use seat belts, regulators don’t require automakers to build airbags to protect unbelted passengers, so airbags are smaller.

If the U.S. adopted similar regulations, “belted occupants actually could be much safer, although that would mean unbelted occupants would be at a much greater risk,” Caruso says.

Sam Campbell, the head of U.S. safety engineering at BMW, says that the requirement to accommodate unbelted passengers makes a difference in cabin design as well.

BMW’s vehicles, especially its smaller cars, could be roomier and lighter, reducing emissions and fuel consumption, Campbell says. But to protect the unbelted, the instrument panel has to be closer to occupants to keep them from being hurtled as far and fast toward the airbags. “If they were belted, you would have a little bit more design freedom to make the instrument panel a little bit slimmer.”

And without that requirement, BMW could do away with part of the instrument panel and knee airbags, Campbell says.

But those airbags are designed to protect the lower extremities of unbelted occupants, who could otherwise be thrown into the footwell. Smaller, less expensive airbags could also mean a reduction in the cost of a vehicle or the addition of features, Campbell adds.

Rolling Out New Tech

Technology exists that could get more people to buckle up and keep them safer when they do. The new Chevrolet Malibu, for instance, can mute the vehicle’s stereo when people in the front aren’t wearing their seat belts.

Adding seat belt reminder chimes for rear seats like those for up front could also get more people to buckle up. That’s why Congress, at the urging of Consumer Reports and other safety advocates, asked the Department of Transportation to propose a rule for rear seat belt reminder systems by October 2014. The agency still hasn’t done so.

Other technologies are also available to further improve seat belt performance. For years, front seat belts have had pre-crash pretensioners and load limiters to help reduce chest injuries for belted occupants in a collision­, yet those systems are rare to nonexistent for rear seat belts. And some Fords and Mercedes-Benzes offer seat belts with a small airbag in the shoulder portion for rear seat occupants.

Getting drivers—and their passengers— to buckle up “is one of the great success stories in our society, about how we can change behavior,” says Rosekind of NHTSA. But more work needs to be done.

“We are at 88.5 percent,” he says. “We need to get all the way to 100.”

A History of Seat Belts

Timeline of seat belts dating back to 1885.

Editor's Note: This article also appeared in the August 2016 issue of Consumer Reports magazine.