WASHINGTON — Can we improve fuel economy without sacrificing vehicle safety? That’s one contentious issue at the center of the debate over federal fuel economy standards for cars and light trucks.
The Obama administration concluded the answer was yes and issued strict rules in 2012 that would have required automakers to roughly double the fuel economy of new cars, S.U.V.s and pickup trucks by 2025.
But the Trump administration, in its big new proposal to roll back those rules, is now arguing the opposite: Forcing automakers to build cleaner cars will lead to more highway accidents and deaths.
The administration contends that scrapping the Obama-era standards after 2021 would prevent more than 12,700 deaths from road accidents over the following decade, compared with keeping the standards in place. To support this claim, the proposal makes three main arguments.
First, people who buy fuel-efficient vehicles will end up driving more, increasing the odds that they will get into a crash. Second, the fuel-efficient vehicles will themselves be more expensive, slowing the rate at which people buy newer vehicles with advanced safety features. Third, automakers will have to make their cars lighter in response to rising standards, slightly hurting safety.
These arguments have been greeted with skepticism by outside experts who reviewed an earlier draft of the proposal. “I don’t know how they are going to defend this analysis,” said Antonio M. Bento, a professor of public policy and economics at the University of Southern California whose research is cited throughout the document. “I just don’t think it’s correct.”
Here’s a closer look at the three main arguments on auto safety that the Trump administration is putting forward in its proposal.
The ‘driving more’ argument
Of the more than 12,700 extra road deaths that the Trump administration estimates would result from keeping the Obama-era standards in place, compared with halting them after 2021, about half are attributed to a phenomenon known as the “rebound effect.”
Economists have been studying this dynamic for years: As cars and trucks become more fuel-efficient, the cost of driving goes down and people are likely to drive more. But there’s a lot of debate over how big this effect actually is.
The Obama administration, in its analysis of the original rules, concluded that the rebound effect was fairly modest: For every 1 percent increase in vehicle fuel economy, people would drive about 0.1 percent more.
The Trump administration, in its new proposal, reworked that analysis and concluded that the rebound effect was essentially twice as large. People with more efficient cars would drive more miles than previously assumed, and hence were likely to get into many more accidents.
Some economists are questioning the Trump administration’s newer, higher estimate of the rebound effect. Kenneth Gillingham, a Yale economist, points out that the Trump rollback proposal cited one of his papers, which inferred a larger rebound effect from changes in oil prices, but ignored some more recent studies, including one that he led, that found a much smaller effect.
There’s also some evidence, Dr. Gillingham said, that the rebound effect shrinks as Americans get richer, which suggests that this should be less of a problem in the future — an argument that the Trump proposal rejected.
“I think it’s fair to say that their number is at the high end,” Dr. Gillingham said. “And there are several arguments they dismissed that could bring it down.”
The ‘pricier vehicles’ argument
The Trump administration also argues against the Obama-era fuel economy standards by estimating that they would add about $1,900 to the average cost of a new car. That, in turn, will deter people from buying newer vehicles with advanced safety features like automatic emergency braking systems and keep them in older, less-safe vehicles for longer.
The Transportation Department laid some groundwork for this argument in April, when it published a report showing that more vehicle fatalities occurred in older cars than in newer cars. And the agency developed a new model to quantify this effect.
This line of reasoning is likely to receive pushback from a variety of fronts. Some economists called the analysis too simplistic, arguing that it did not properly consider key factors like age, gender and other driver characteristics that affect crashes.
“I think they are substantially overestimating the impact here,” Dr. Bento said.
Other critics note that this particular safety argument is inconsistently deployed. For instance, President Trump is also considering tariffs on imported cars and car parts that, according to estimates by the Alliance of Automobile Manufacturers, could increase prices of American-produced vehicles by $2,000 and imported vehicles by up to $5,800.
“It’s strange that the administration only uses this safety argument on fuel-economy standards and not for trade,” said Daniel F. Becker, director of the Safe Climate Campaign. “Even though the trade effects could be much bigger.”
The ‘lighter vehicle’ argument
The Trump administration argues that strict fuel economy rules could also hurt vehicle safety by forcing automakers to produce lighter vehicles that are less capable of withstanding crashes.
There’s little question that automakers have been improving fuel economy by reducing the weight of their vehicles over time. Ford, for instance, shed 700 pounds from its popular F-150 pickup by switching from steel to high-strength aluminum.
This can potentially affect road safety in two big ways: On average, smaller, lighter vehicles can be worse at protecting their occupants in an accident. But lighter vehicles also cause less damage to other cars on the road. The big question is which effect dominates.
Experts who have looked at this question have developed a rule of thumb: If automakers are mainly reducing the weight of their largest vehicles, like S.U.V.s and pickup trucks, then that makes the roads safer overall. But if manufacturers were to focus more on reducing the weight of their smallest passenger cars, that could be worse for overall auto safety, since those cars would be more vulnerable in crashes with bigger vehicles.
“The most important question is whether cars on the road are getting more similar in weight, or more dissimilar,” said Mark Jacobsen, an economist at the University of California, San Diego. “If you’re bringing down the weight of the heaviest vehicles but not the lightest vehicles, then in the average accident, the cars will be better matched.”
When the Obama administration looked at the data, it found that automakers were indeed mostly cutting weight from their larger vehicles in response to fuel-economy rules — one reason they found no harm to overall vehicle safety. But the Trump proposal argues that this might not necessarily be the case in the future, and suggests that smaller cars could be negatively affected.
It’s worth noting, however, that this particular argument accounts for only about 1 percent of the Trump administration’s fatality estimates.
That’s because, in recent years, experts have found that the size of a vehicle is more important to safety than its weight. And as the National Research Council concluded in 2015, the Obama-era standards were designed to encourage automakers to make their vehicles lighter without making them smaller, a change that had alleviated their earlier safety concerns.
Brad Plumer is a reporter covering climate change, energy policy and other environmental issues for The Times’s climate team. @bradplumer