Many industries in the U.S. have cut their carbon footprints in recent years, but transportation remains one sector where emissions are still rising. And in an effort to put the brakes on this trend, the Obama administration announced a set of new fuel-efficiency standards for heavy-duty trucks, vans and buses.

“The main reason emissions are up in the transportation sector is due to increased emissions from medium and heavy-duty trucking,” says Brandon Schoettle, the project manager for Sustainable Worldwide Transportation at the University of Michigan Transportation Research Institute. “That, of course, we think is mostly due to increases in the actual use of medium and heavy trucks—things like construction, transporting goods and cargo around the United States.”

LISTEN: Making Our Truck Fleet More Fuel-Efficient

Schoettle says there has been a 30 percent increase in the number of light-duty vehicles on the road since 1990. And emissions from most other parts of the transportation sector—like cars—have stayed relatively flat during that time.

“Vehicles have just been getting more efficient during that time,” he says. “Recently, we’ve hit some records in terms of new vehicle fuel economy—especially over the past 10 or 15 years. So those gains—even though we’ve been also gaining quite a few vehicles—have led to a sort of stable performance in terms of emissions.”

U.S. greenhouse gas emissions by economic sector as percentages of total emissions, 1990-2014. Graphic: Michael Sivak and Brandon Schoettle / University of Michigan

U.S. greenhouse gas emissions by economic sector as percentages of total emissions, 1990-2014. Graphic: Michael Sivak and Brandon Schoettle / University of Michigan

But Schoettle says reducing the collective carbon footprint of the country’s truck fleet could be tricky.

“For cargo, it’s a little different. They get what we would generally think of as pretty poor fuel economy—five, six, seven miles per gallon. But, of course, they’re hauling tens of thousands of pounds of cargo around. So they’ve been increasing their cargo efficiency significantly during the same period we looked at. But the increase in just cargo operations moving around the country has led to the increase.”

Schoettle says his team is looking at ways to improve the fuel efficiency of heavy trucks. Their new study is due out later this summer.

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This story comes from our partners at Michigan Radio's Environment Report, a program exploring the relationship between the natural world and the everyday lives of people in Michigan.