But if gasoline prices stay at current levels — around $2.80 per gallon — or drop further, then Americans are expected to continue to buy S.U.V.s and other gas guzzlers, as they have been doing in increasing numbers the past few years.
Second, states could try to enact other fresh policies to try to cut emissions from the transportation sector and blunt the impact from Trump’s rollback. California and New York, for instance, have been offering tax breaks for people to buy electric vehicles, and they have been investing hundreds of millions of dollars in new charging infrastructure.
Other Northeastern states have been participating in discussions on how to reduce vehicle emissions, through steps like expanding mass transit, buying electric buses or reconfiguring cities to make them denser and more walkable.
But some of these state policies can be politically difficult and take time to enact. In the absence of stricter federal fuel economy standards, states like Connecticut and Maryland that have set legislative targets for reducing economywide emissions might struggle to meet their goals.
“Transportation is extremely complicated and it really takes all levels of government working together,” said Vicki Arroyo, the executive director of the Georgetown Climate Center, who has been working with states on plans to cut emissions from transportation. If the federal government pulls back, she said, “it’s a tremendous setback.”
The automakers themselves are another wild card. While many manufacturers have been developing new electric car models in response to the ever-rising fuel economy standards, it’s not clear how many would completely pull back if the standards were frozen. China and Europe are continuing to push hard on fuel efficiency and battery-powered vehicles, and automakers have those international markets to consider.
And the biggest wild card of all? What the next president might do. “If a new administration came in, they’d have a blank slate for rethinking the standards entirely, and there are a lot of ideas out there for standards that would be even more effective” than the Obama-era rules, said Mr. Houser.
If a future president ultimately managed to put even stricter vehicle rules in place, he said, “that would certainly reduce the magnitude of the emissions impact that we’re projecting.”