The 2016 Acura RLX Sport Hybrid. (PRNewsFoto/Acura)

Carmakers have crash-prevention technology that could radically reduce rear-end collisions, which account for about half of America's two-car accidents and kill 1,700 people a year.

But to enjoy the life-saving benefits, drivers have to pay up first: The systems — which alert drivers or automatically slam on the brakes if they sense an upcoming accident — are optional and often expensive, leading many buyers to go without.

Federal regulators this month strongly recommended car companies make the systems a standard feature in all new cars, much like airbags and anti-lock brakes.

“You don't pay extra for your seat belt,” National Transportation Safety Board Chairman Christopher A. Hart said earlier this month. “And you shouldn't have to pay extra for technology that can help prevent a collision altogether.”

But that demand, analysts said, has sparked a new tension for automakers, who offer the technology as an option in fewer than half of new cars — and typically earn a few thousand dollars from each installation.

"Consumers want to be in the driver's seat when it comes to deciding on how they spend their safety dollars, and automakers agree," said Wade Newton, a spokesperson with the Alliance of Automobile Manufacturers, an industry group. "Consumers should be deciding what vehicles they drive and what technology is in those vehicles."

A staggering 87 percent of rear-end crashes were blamed on driver inattention, a 2007 federal study found, leading regulators to estimate that 80 percent of rear-end collision deaths and injuries could have been avoided if crash-prevention systems were more widespread.

The systems come in two parts: Driver warnings that beep and flash if car sensors suspect an upcoming hazard and automatic brakes that trigger in case of a potential accident, even at high speed. Because they never get drunk, drowsy or distracted, the systems are increasingly seen as a crucial safety upgrade for drivers: The Insurance Institute for Highway Safety has made crash-prevention systems the sole criterion for its ratings of the country's safest cars.

The tech is spreading quickly: About 40 percent of new 2014 models offer optional crash-avoidance systems, up from about 10 percent in 2010, industry data show.

But only four of the 684 new models last year, regulators said, came with the systems as a standard feature: Subaru's Forester, Legacy and Outback, and Mercedes-Benz's $114,000 G-Class. And in the Subaru's case, that barely counts: The technology only comes standard in cars with Subaru EyeSight, which costs, at its cheapest, about $1,300.

When the features are offered, they are often tucked deep into pricey upgrade bundles and overshadowed by glitzier upgrades. The 2015 Hyundai Genesis, which starts at $38,000, offers automatic emergency brakes as the seventh item in its $3,500 "tech package," after ultra-premium leather seats and a powered driver's seat cushion extender.

Most carmakers are enthusiastic about spreading liability-limiting, active-safety features, said Michelle Krebs, a senior analyst with AutoTrader.com: "Their philosophy is [it's] best to avoid an accident than protect when one occurs."

But they have not rushed to spread the technology for free when they can still reap the rewards of extra upgrade revenue, analysts said.

"It's a race to see who can produce those features first, offer them first and use them as a selling point," said Karl Brauer, a senior director of automotive industry insights for Kelley Blue Book, "before it's not a big deal to have them anymore."

U.S. drivers have not always clamored to pay for the upgrades, either. A third of the drivers polled by AutoTrader late last year said they wouldn't pay extra for in-car technology such as safety upgrades, but would wait until the features became standard instead. And in the car showroom, analysts said, crash-warning indicators are far harder to get people excited about than, say, back-up cameras or in-car WiFi.

"Drivers are more willing to pay for comfort and convenience standards than they are safety," said Jeremy Carlson, a senior analyst with IHS Automotive. The reasoning: Drivers can "tangibly feel the value" of comfort upgrades, and they believe the cars are safe enough already.

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Crash-warning technology, while advanced, remains far from perfect, and drivers could face a rude awakening if they fail to realize why their car is suddenly hitting the brakes. Crash-warning systems and automatic brakes have been known to trigger at phantom hazards such as plastic bags or the shadows of trees. Acura this month recalled nearly 50,000 vehicles because the systems determined that fences and guardrails were potential crash risks, and automatically hit the brakes.

Even when the systems work as expected, they can face criticism for not doing more. After bloggers in the Dominican Republic posted what they called a "self-parking car accident," in which a Volvo XC60 scarily lurches at a crowd, the automaker said that it looked as if someone was testing the car's automatic brakes.

But the car, which comes standard with "City Safety" upgrades that help it drive in heavy traffic or hit the brakes in case of a crash, appeared not to have an extra "pedestrian detection" upgrade installed, which would have prevented it from lurching at the crowd. "Keeping the car safe is included as a standard feature," Fusion writer Kashmir Hill wrote, "but keeping pedestrians safe isn’t."