A new study by the University of Michigan's Dr. Michael Sivak found that flying was actually now more energy efficient per passenger-mile than driving, on average. That didn't use to be the case a few decades ago, but as both cars and planes got more energy efficient, planes improved faster than cars. Here's what the study found:
[The study] measured BTU per person mile from 1970 to 2012.
Sivak found that the energy intensity of driving is 2.07 times that of flying. In 2012, BTU per person mile was 4,211 for driving compared to 2,033 for flying. Consequently, the entire fleet of light-duty vehicles would have to improve from the current on-road fuel economy of 21.6 mpg to 44.7 mpg for driving to be as energy intensive as flying.
But there's a risk that this information will be misinterpreted. I can already see the headlines ("flying greener than driving, so fly as much as possible!") and imagine how many people will grab unto them to justify a lot of unnecessary flying. Let's take a closer look at what this truly means.
First, this is a per-mile figure. But people tend to fly a lot farther than they would drive, so the total amount of energy (and thus fossil fuels burned) from flying might be higher than for driving. People rarely drive across the U.S., but many frequently fly from coast to coast. And driving to Europe or Asia isn't exactly easy...
Second, for the types of flights that are more competitive with driving, over short distances, planes don't fare as well. This is a bit counter-intuitive, but the energy-efficiency per passenger-mile is higher on longer flights than shorter ones. That's because taking off uses a disproportionate amount of energy, so if you're not going very far, your average energy used per mile will be higher.
Third, the numbers from the study are averages. If you drive a car that is more efficient than average, obviously you'll do better. Hybrids like the Prius are about twice as efficient as the average car, and plug-in hybrids are even more efficient. And all-electric vehicles, especially if charged from clean sources of power, will always be better than flying. Also note that some airplane models are more efficient than others, so there's variability there too.
Finally, we must remember that we're in a transition period in the transportation world. Over time, more and more vehicles will be plug-ins, either hybrids or all-electric, and that will help reduce the impact of driving (or course it's even better not to drive and to walk, bike, or take transit, but if you have to drive, look into the cleanest model that meets your needs).
This probably means that more cars will overtake planes in energy-per-passenger mile, and readers of this site who drive will hopefully pick those. And the amount of energy isn't the only thing; plug-ins can get their energy from clean sources, while planes get it from fossil fuels right now.
So don't start flying more because of this!
The only way that planes can compete over time is if they become much more efficient (that might require new body designs, such as "flying wings"), and are powered by advanced biofuels that are at least carbon neutral (ideally carbon negative). This type of progress could make flying a lot better for the planet, and I hope it happens soon.