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The Trump administration is using a decades-old safety argument to help justify rolling back Obama-era fuel-economy standards. But auto industry watchers and safety experts say advances in technology have made that claim outdated.

As expected, the administration announced Thursday that it intends to freeze fuel-economy targets at an average 37 mpg when the auto industry reaches that goal by 2020. The original Obama plan was for the industry to continue improving fuel efficiency—and curb tailpipe emissions—so that autos averaged more than 50 mpg by 2025.

Part of the Trump administration’s argument is that the higher standards would force automakers to build lighter cars, which they say would be less safe in the event of a crash. But critics say recent gains in safety technology and fuel economy show that lighter cars aren’t necessarily less safe.

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“The administration has trotted out the old safety canard that the government’s own data disproves,” says Daniel Becker, director of the Safe Climate Campaign and a longtime environmental lobbyist in Washington. “Better engineering, stronger materials, and improved technology, including airbags and antilock brakes, deliver safer cars. Weight does not.”

The safety debate is likely to be one of the key issues in the impending court battle over fuel-economy standards. Nineteen states led by California plan to sue the federal government to prevent any rollback in the rules, arguing that the stricter standards are needed to make cars more efficient and to curb pollution.

Impact on Consumers

The outcome of the legal battle, which could end up in the Supreme Court, will have a profound impact on the types of cars and trucks that consumers buy—and how much they will cost—in the years ahead.

“Blocking all progress after 2020 and lowering pollution targets will undermine consumer savings, weaken our economy, and put the brakes on automaker innovation,” says David Friedman, vice president of advocacy at Consumer Reports.

But until the conflict is resolved, automakers will have a hard time planning future models, which have to be designed years in advance.

For that reason, the Alliance of Automobile Manufacturers says the industry supports continued improvements in efficiency as well as incentives for specific new fuel-saving technologies while balancing priorities like affordability, safety, jobs, and the environment.

“With today’s release of the Administration’s proposals, it’s time for substantive negotiations to begin,” the trade group said in a statement. “We urge California and the federal government to find a common sense solution that sets continued increases in vehicle efficiency standards while also meeting the needs of America’s drivers.”

In addition to the effect on cars themselves, consumers will feel the impact on the environment. Freezing the fuel-economy standards could cause an additional 2.2 billion tons of car emissions to be spewed into the Earth’s atmosphere by 2040, according to an estimate from the Union of Concerned Scientists.

Another key element of the administration’s plan for fuel economy is to revoke California’s ability to set its own standards. Because of its historic pollution problems, the Clean Air Act gave the state the power to have rules that are more stringent than federal requirements. More than a dozen states follow California’s rules rather than federal.

So do lighter cars really pose a safety hazard? Here’s a look at the debate.

Lighter vs. Safer

At its core, the thought is simple: Lighter cars are more dangerous than heavier ones, all things being equal. The physics behind car crashes show that an occupant in a smaller, lighter car is at a disadvantage in a head-on crash with a bigger, heavier car. There’s less mass in the surrounding vehicle to absorb the forces of the crash.

So weight is a factor, but it’s by no means the only factor in how well a vehicle protects its occupant. Government crash tests show that vehicles with similar weights have much different scores. Lighter vehicles can be engineered to still provide substantial crash protection.

In recent years, the safety vs. fuel economy fight has been dormant. Today’s fuel-economy standards have been intentionally structured so that there’s no incentive for manufacturers to meet them by building a lot of small, lightweight cars—the kind that are most dangerous in highway crashes.

“The best available evidence shows that if you make the fleet somewhat lighter across the board, you’ll save fatalities and injuries,” says David Greene, a University of Tennessee engineering professor who has spent decades studying the effect of fuel economy on safety.

The current fuel-economy rules were written in a way that encourages automakers to use ways of meeting the targets other than downsizing vehicles, says David Zuby, senior vice president of vehicle research at the Insurance Institute for Highway Safety.

For instance, there are new complex formulas for each automaker so that companies like GM and Ford, which sell a lot of pickups and SUVs, aren’t penalized if consumers choose larger vehicles. The system seems to be working, he says.

Automakers also have used some of their engine improvements to deliver acceleration and horsepower rather than better mileage, Zuby says. That allows them to deliver better efficiency without reducing weight, he says.

“There’s flexibility in the current technology to meet more stringent fuel-economy standards,” Zuby says.

In an effort to evaluate the link between weight and crashworthiness, the Consumers Federation of America looked at federal crash-test results for 19 vehicles that had been redesigned for the 2018 model year.

It compared how these cars fared in new crash tests with the results of the previous versions. The CFA found that 14 of the 19 dropped weight and were more efficient. But when it came to crash-test performance, eight of the 14 had the same rating as before, and six received higher ratings. None saw their crash-test performance drop.

“Today’s new cars, which are meeting commonsense efficiency standards, are the safest, cleanest, most fuel-efficient vehicles American consumers have ever known,” says Jack Gillis, executive director of the CFA.

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