Not all vehicle occupants are protected equally when it comes to car crashes. Until 2017, cars weren't even routinely crash-tested on the passenger side, just the driver's. There's still other low-hanging fruit, too; thousands of rear-seat passengers die in cars each year in the United States because they're not wearing seat belts, despite decades of evidence on the effectiveness of buckling up. The National Highway Traffic Safety Administration has now had enough and has proposed a new rule that would mean new cars, trucks, and even some buses would need to have seat belt warning alerts for all occupants, not just the driver.

Seat belts have been mandatory equipment for all seats in cars and trucks (but not buses) since 1968, thanks to the US Department of Transport. But the US has lagged behind much of the world when it comes to requiring their use; this is determined at the state level, and it wasn't until 1984 that New York became the first US state to require seat belt use.

Since then, 48 other states, along with the District of Columbia, now require front seat occupants to wear belts—New Hampshire remains unconvinced—but a total of 10 states don't require rear passengers to wear seat belts by law.

As a federal agency, NHTSA doesn't have the power to change those state laws. But if this rule is adopted, it hopes to increase belt wearing, which in 2021 was around 90 percent for front-seat occupants and about 78 percent for rear-seat occupants.

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"Wearing a seat belt is one of the most effective ways to prevent injury and death in a crash," said acting NHTSA administrator Ann Carlson. "In 2021, almost 43,000 people lost their lives on America’s roads, and half of those in vehicles were unbelted. This proposed rule can help reduce that number by getting more to buckle up."

The current required seat belt alert is mandated in federal motor vehicle safety standard 208. To be satisfied, the rule requires "a short-duration audio-visual seat belt warning for the driver’s seat belt on passenger cars, most trucks and MPVs with a GVWR [gross vehicle weight rating] of 4,536 kg (10,000 lb) or less, and buses with a GVWR of 3,855 kg (8,500 lb) or less." The audio alert must last at least four seconds, and the visual alert at least 60 seconds.

NHTSA notes that while the regulation does not require the same alert for the front passenger seat, almost every model year 2022 vehicle includes this safety feature. But only 47 percent also include seat belt alerts for the rear seats. And the agency notes that most vehicles provide much longer warnings than the minimum requirements.

What is NHTSA proposing?

The new rule would apply to "passenger cars and trucks, buses (except school buses), and multipurpose passenger vehicles with a GVWR of 4,536 kg (10,000 lb) or less."

One component of the new rule is a requirement for seat belt warning alerts for all rear seats. This would include a visual warning informing the driver that lasts at least 60 seconds once the car is turned on and a 30-second audio-visual warning if a rear belt is unfastened while the car is on and in gear.

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NHTSA says the system should be able to detect when adults, teenagers, and children in booster seats are occupying a rear seat. However, children in rear- and forward-facing child seats are not part of the target population "because these children are restrained by the CRS [child restraint system] harness, not the seat belt. The intent of the reminder is not to warn of CRS misuse, but to warn of occupants not restrained by a belt alone."

Consequently, the alert would be triggered if the seat is occupied by a person who weighs at least 46.5 lbs (21 kg) and is at least 45 inches (114 cm) tall, which NHTSA says is a proxy for a 6-year-old, the age at which a child would move to a booster seat.

NHTSA also wants to refine the regulation regarding the front passenger seat, which would be required to have an audio-visual seat belt warning alert like the driver's seat. Both front seat alerts would be required to remain active until all occupied front seats are strapped in, not just for 60 seconds. Like the rear seats, there would be another audio-visual alert whenever a front seat is unbuckled during a trip unless a front door is open. This alert would remain on until the belt is refastened.

"Extending these protections to all seating positions is urgently needed," said Peter Kurdock, general counsel for Advocates for Highway and Auto Safety. "The rulemaking was required by Congress over a decade ago in the Moving Ahead for Progress in the 21st Century (MAP-21) Act (Pub. L. 112-141). Advocates has repeatedly pushed for the final rule to be issued and, in 2019, filed comments in response to the Advance Notice of Proposed Rulemaking (ANPRM). We commend NHTSA for moving forward with the NPRM and urge the agency to complete this rulemaking without further delay."

The new rules should prevent more than 100 deaths and 300 non-fatal injuries a year, NHTSA estimates. The agency will be collecting public comments on the rule for the next 60 days.