Is Flying More Energy Efficient Than Driving?

By Aaron Crowe
 
Depending on where you live, deciding between flying and driving to your vacation destination can be a complicated weighing of time vs. cost.
The 400-mile trip from the Bay Area to Los Angeles, for example, can take about six hours by car with less than two tanks of gas for the average car, and can be an inexpensive way to travel. Flying, however, can be a lot faster but costs much more.
Another factor to consider is how much energy is used to make that trip, and using the method of travel that’s most efficient. It’s called energy intensity — the amount of energy needed to transport one person a given distance.
In an April report by the University of Michigan Transportation Research Institute, author Michael Sivak expands on a 2014 report he did comparing energy intensities of flying and driving from 1970 through 2010. The main finding of the 2014 report was that flying domestically in the United States used to be much more energy intensive than driving.
Driving is double the energy intensity of flying
Sivak’s most recent analysis found the reverse. The energy intensity of driving is now greater than flying — twice as much.
If you want to use less of the world’s energy, flying makes more sense over driving — unless your car is more than twice as fuel efficient as the average car.
Among the findings by Sivak, who is director of Sustainable Worldwide Transportation at the University of Michigan, are that the amount of British thermal unit, or BTU, in 2012 for driving was 4,211 per person mile and 2,033 for flying domestically. The energy intensity of driving was double that of flying.
44 mpg need to equal flying
The energy intensity of driving, along with other means of transportation, Sivak writes, depends on two primary variables: vehicle fuel economy and vehicle load (the number of people aboard). In 2012, light-duty vehicles in the U.S. had an average vehicle load of 1.38 people and 21.6 miles per gallon, or mpg.
To match the energy intensity of driving to flying would require improving vehicle fuel economy by the current ratio of the energy intensities of driving and flying, which is 2.07, according to the study. Fuel economy would have to improve to 44.7 mpg for driving energy intensity to equal that of flying.
But just as vehicle fuel economy improves over time, so does the energy intensity of flying continue improving, Sivak notes. That could push up higher mpg needed to make driving lower than or equal to the energy intensity of flying.
Driving vs flying trips
Flying is less energy intense than driving, but that doesn’t make it a feasible alternative. The average driving trip is nine miles, but the average domestic flying trip is 100 times longer at 895 miles, according to the study.
Each serves a different purpose, with driving used mostly for trips that are too short for flying. Long-distance driving, however, is an area where flying may be a viable alternative.
“As the trip length increases, so does the average fuel economy of driving,” according to the study. That’s because long-distance driving is frequently done on highways where vehicle fuel economy is better than average.
The average fuel economy of flying also increases as the trip length increases, partly because airplanes us a disproportionate amount of fuel during takeoffs. On short flights, takeoffs can use as much as 25 percent of the total fuel consumed.
Other factors for drivers
Energy intensity and cost aren’t the only factors used to determine if driving 600 miles is a better choice than driving. Driving long distances can also take a physical toll on the driver.
“When driving long distances, the driver must be constantly alert and focused on the road,” says Jared Heathman, a family psychiatrist in Cypress, Texas. “If children are traveling as well, this can put added stress on the driver.”
“Instead of focusing on the road, the driver may end up consoling or breaking up fights in the back seat,” Heathman says. “Multitasking drivers will expend quite a bit of energy. Solo fliers in comparison have the option of reading books and magazines. Parents can focus on their children and not worry about constantly monitoring the road.”
The exception, he says, is people with a severe anxiety to flying in the enclosed space of a plane and the inability to escape until the plane lands.
Another factor is the type of fuel used. While some airlines are adding biodiesel and other alternative fuels to their fleets, most airplanes don’t have all the options that cars do.
“Cars can now be propelled by electricity, hydrogen and even natural gas, which makes them ultimately more potentially efficient,” says Bill Seavey, who writes about green cars.
The University of Michigan study didn’t include plug-in hybrid electric and fully electric vehicles, though it pointed out that the energy intensities of driving may be slightly underestimated by the use of electric vehicles. In 2012, less than 1 percent of all vehicles on the road were electric or plug-in hybrids.
Lastly, there’s the headache of getting through an airport that can make the lower energy intensity of a flight not worthwhile to some people.
“While airplanes get you there faster, the hassles of airports and security are big downers,” Seavey says.
That may be reason enough to take a long road trip in your car.
Aaron Crowe is a journalist who covers the auto industry for CheapCarInsurance.net.