A line of semi trucks stranded in the eastbound lane of U.S. 150 on Jan. 6, 2014. (John Dixon/News-Gazette via Associated Press)

THE OBAMA administration announced this week groundbreaking rules on heavy trucks, promising that by 2027 the vehicles that move massive amounts of freight across the country will be as much as 25 percent cleaner. This is good news. But it also highlights the fact that the executive branch has been given only command-and-control tools to combat climate change. If Congress would act, the country could do much better.

Big trucks are about 5 percent of the nation’s vehicle fleet but account for a fifth of transportation-sector emissions. Most of that comes from big rigs that carry up to 50,000 pounds of freight at a time and often travel more than 150,000 miles a year. The Environmental Defense Fund reckons that one of these big trucks equates to about 50 passenger cars in fuel consumption.

The new rules would tie specific fuel efficiency and carbon dioxide emissions requirements to each vehicle size and type, making them steadily more stringent over the next decade. Past standards were based on vehicle technologies already in use. The new ones will require innovation and deployment of new fuel-saving technologies, such as aerodynamic alterations that reduce wind resistance. Trucks will be more expensive to buy but cheaper to operate.

The rules would allow companies that overshoot the targets to sell credits to those that have a harder time complying, making for a more flexible system. And the regulations would apply evenly across the country: The Environmental Protection Agency, the Transportation Department and the state of California, which has special dispensation in federal law to set vehicle emissions standards, collaborated to send a unified signal to industry.

Given the importance of fighting climate change and Congress’s resistance to act, actions such as these, which are fully legitimate under existing executive authorities, are valuable. But they also represent a second-best approach. Last month brought some bad news for a different set of the administration’s vehicle rules — those on cars and light trucks. The EPA announced that the nation’s car fleet will not meet the 54.5 miles per gallon fuel efficiency goal the administration previously set for 2025. This is not as bad as some of the headlines made it sound. New cars will still have to meet government standards, which vary according to their size, and fuel efficiency will still rise. But Americans are buying more light trucks than they were a few years ago, pushing down the miles per gallon that the light-duty fleet is, on average, projected to meet.

This does not show that rules such as these are ineffective. The fleet will still meet an average fuel efficiency of 50.8 mpg. But regulators cannot perfectly predict the future, and government mandates can never be fine-tuned enough to account for unanticipated changes.

What the country needs is a simple carbon dioxide policy that applies equally across all sectors and empowers consumers and businesses to determine the details. Placing a simple, significant and rising tax on carbon dioxide would make it unnecessary for the auto industry to wait for the EPA’s latest decree on emissions regulations. It would nudge consumers to economize and businesses to get greener on their own, making emissions-cutting relatively cheap, maximally efficient and minimally coercive.