Blog: Sustaining the Great American Road Trip

For a couple years in the late 2010s, there was a concerted movement to get people to give up flying. The term that was adopted for this comes from Swedish, flygskam, and was associated with the then-teenage climate activist Greta Thunberg, though she didn’t originate the concept. Of course, the pandemic grounded world airline fleets shortly after the idea was gaining ground. Today, however, the number of passengers on domestic and international flights is barely off the early 2020 peak and will likely hit a new monthly record before the year is out.

The story is the same for automobile trips in the U.S. According to the U.S. Federal Highway Administration, U.S. vehicles traveled 287 billion miles in May 2023, the second-highest level for a May. If summer highway traffic jams are anything to go by, the United States will be start setting new records in vehicle miles traveled very soon.

The impulse behind flygskam may have been noble if a bit misguided. As Michael E. Webber wrote in 2019, “Travel is also necessary to actually fight climate change. Renewable energy developers need to travel globally, and field researchers often must get to remote corners of the world where trains don’t reach. Travel can be the glue that holds together international research collaborations, such as the multinational partnership that facilitated the lithium ion battery’s development.”

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The case against automobile travel is perhaps stronger. According to a December 2022 study by the Congressional Budget Office, the average carbon dioxide emissions air travel in the United States was 0.34 pounds per passenger-mile. If that’s shameful, consider car travel. “The most popular size car in 2020 emitted 0.60 pounds of CO₂ per vehicle mile,” the report stated, “whereas one category of large pickup trucks built in that year had average emissions that were nearly twice that amount—1.18 pounds of CO₂ per vehicle mile.”

Other modes of transportation listed by the CBO include bus travel (0.39 pounds per passenger-mile), passenger railroad (at 0.30 pounds), and rail transit (at 0.17). Transit is especially good since in most cases subway and light-rail trains are electric-powered; the downside is that most people can’t hop the subway to a vacation spot.

Battery-electric automobiles will eventually bring down emissions from cars, as even if the electricity comes from a mix of coal and gas power plants, they are generally more energy efficient than conventional automobiles. But there is a way to cut any residual “driving shame” without getting a plug-in car.

The emissions figures listed for passenger vehicles was per “vehicle miles” while those for other modes were for “passenger mile.” For planes, trains, and buses it makes sense to divide the emissions of a large vehicle by the number of passengers to assign a personal share of emissions. And generally, since those vehicles are large, a few empty seats won’t affect the carbon-emissions impact.
But with a car or light truck, filling the empty seats can make a big difference. Take that “0.60 pounds of CO₂ per vehicle mile” for a popular-sized car: If that car is carrying only the driver, then it has an impact nearly twice as great as a typical airline flight, on a passenger-mile basis. An 800-mile road trip would emit 480 pounds of CO₂ per the one passenger, compared to 272 pounds for a flight that distance. But fill the car with four people—say two parents and two kids on a family vacation—and the emissions per passenger mile get quartered. The 0.60 pounds of CO₂ per vehicle mile becomes 0.15 pounds per passenger, which is better than virtually any other available mode of transportation.

My family doesn’t own a car (in Brooklyn, it’s more trouble than it’s worth) but this summer we rented a car for a road trip to Maine and Quebec. While the trip was not without its scares (no one had prepared us for the steep drop at Dixville Notch, N.H.), the four of us didn’t have to fret about the climate impact of our adventure. I’m not sure what’s the opposite of flygskam—“driving pride” doesn’t quite capture it—but it was nice to know we had picked the greenest way to go.

Jeffrey Winters is editor in chief of Mechanical Engineering magazine.