According to the report, the energy intensity of driving is 2.07 times that of flying, based on 2012 data. That breaks down to 4,211 BTUs of energy per person mile for driving, and only 2,033 for flying. The American car fleet would have to reach an average of 44.7 mph for driving and flying to have approximately the same energy profile. In 2012, cars and light trucks together averaged 21.6 mpg, though the numbers have steadily improved since then.
Michael Sivak of Michigan’s Transportation Research Institute tells me, “Flying domestically in the U.S. used to be much more energy intensive than driving, but now it is the other way around.” Two factors make that true: The increased fuel efficiency of our airplane fleet and “the increased passenger load in planes.” In 1970, for instance, only about half of airplane seats were occupied on average, compared to 83 percent in 2012. This older EarthTalk analysis gives the edge to driving, but the big switcheroo has been in progress for a while.
Of course, cars are also getting much more fuel efficient, but as Sivak points out “changes in new vehicle fuel economy take a long time to substantially influence the fuel economy of the whole fleet.” In 2014, 16.5 million new cars and trucks were sold, but they amounted to only seven percent of all such vehicles actually on the road. We keep our cars an average of more than 11 years, after all.
An interesting footnote to all this is that federal Corporate Average Fuel Economy (CAFE) standards are aimed at getting American new car fleets to average an impressive 54.5 mpg by 2025. So, considering it takes a while for the newer, cleaner cars to influence the entire fleet, at what theoretical point will driving be back as the cleaner and greener choice?
Yet another factor is the cost of fuel, which has been very high for airlines (and 30 to 50 percent of the bottom-line cost of sending out flights). So that was an incentive both to build more fuel-efficient planes like the new Boeing 787, which is 15 to 30 percent better on jet fuel than most of the aircraft it replaces, and to fly with more passengers aboard. But fuel prices have plunged now, so maybe the industry will get lax again—just as the automakers ratchet up their game with cleaner cars.
One more factor involves that "We're flying at an altitude of 30,000 feet" thing. According to the Center for Biological Diversity:
Got it. Expect the flying vs. driving thing to remain fluid.
Aircraft account for 11 percent of carbon dioxide (CO2) emissions from U.S. transportation sources....In addition to CO2, aircraft emit nitrogen oxides, known as NOx, which contribute to the formation of ozone, another greenhouse gas. Emissions of NOx at high altitudes result in greater concentrations of ozone than ground-level emissions. Aircraft also emit water vapor at high altitudes, creating condensation trails or “contrails,” visible cloud lines that form in cold, humid atmospheres and contribute to the warming impacts of aircraft emissions. The persistent formation of contrails is associated with increased cirrus cloud cover, which also warms the Earth’s surface. All told, aircraft’s high-altitude emissions have a greater global warming impact than they would if the emissions were released at ground level.