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Dan Rutherford and Irene Kwan
Is flying really greener than driving? In previous work, we concluded that it’s likely not – at least for the average flight and road trip on comparable distances (300 to 500 mile trips). In a new analysis, “Energy Intensities of Flying and Driving,” Professor Michael Sivak came to the opposite conclusion, finding that “the energy intensity [the amount of energy needed to transport a person in the U.S. a given distance] of driving was 57% greater than the energy intensity of flying” in 2010, with the gap increasing in 2012. Let’s dig deeper.
Looking more closely, these two analyses aim to answer different questions. Professor Sivak’s targets this: Overall, what is the most efficient transport mode in the U.S.? He uses fuel efficiency data for each mode on average trips in the U.S. Thus, efficiency for buses is estimated from transit buses operating in urban areas, trains from Amtrak, planes from a typical domestic flight, and cars from a mix of predominately shorter commuting, shopping, and leisure trips. Professor Sivak notes that the average U.S. domestic flight was 914 miles in 2013 compared to 9 miles for driving trips, and recognizes that “flying is a viable alternative to driving only for a subset of driving trips that involve relatively long distances.”
The danger of this approach is that, though internally consistent, it does not provide an apples-to-apples comparison of transport efficiency. In practical terms, one does not fly a plane to the store to buy a gallon of milk, nor does one ride a city bus cross-country. Instead, a common question that’s sometimes raised here in the Bay Area is: What’s the greenest way to get to my relative’s house in Southern California for Thanksgiving? In this case, car, plane, Amtrak, and interurban buses are all viable options.
Our analysis addresses this second question: Given a choice between modes, what is the most efficient means of traveling on a given trip? We assessed the efficiency of each mode across comparable 300 to 500 mile trips (e.g. Washington DC to Boston, Los Angeles to San Francisco). Thus, we compared Greyhound buses, Amtrak trains, planes on shorter flights, and cars on the highway for a typical road trip. Calculated this way, planes become somewhat less efficient as the aircraft flown are generally smaller, burn relatively more fuel in landing and takeoff, and have somewhat lower load factors. Meanwhile, cars look better because they operate more efficiently on the highway than in the city and have a higher average occupancy on longer road trips.* Likewise, buses also get much better, while Amtrak is consistent across the analysis. We also corrected for the differing energy content of fuels.
Our conclusion was that on comparable trips in the U.S., planes fly on average 43 passenger miles per gallon (pmpg) and are less efficient than trains (51 pmpg), cars (53 pmpg), and interurban buses (152 pmpg), all on a gasoline equivalent basis. In addition, the relative efficiency of cars will vary strongly based upon two factors: the underlying fuel economy of the vehicle used, and the number of occupants. So we ran a quick sensitivity analysis of these factors, summarized below.
Our starting point was the U.S. Environmental Protection Agency’s miles per gallon (mpg) estimates used for labeling new vehicles, which approximate the real-world efficiency of cars, trucks, and vans over average city and highway driving. Next, we used the highway fuel economy label values for this comparison, which reflect the fact that cars and trucks typically operate about 20% more efficiently on the highway than the overall label indicates, with the exception of hybrids, which perform similarly in city and highway driving. Finally, to convert from highway mpg to passenger mpg, we corrected for differences in the weight of a vehicle when carrying more or fewer passengers than assumed under EPA testing. Here, we assumed an additional 200 pounds per passenger (belongings included), and a 0.4% decrease in fuel efficiency for a 1% increase in weight relative to EPA’s test value.
Our results are summarized in the table below. It shows the estimated passenger miles per gallon, on a gasoline equivalent basis, for highway driving for typical cars, trucks, and large vans as a function of fuel economy and number of occupants (driver alone equals one). Combinations of overall label fuel economy (far left column) and vehicle occupancy (rightmost columns) with overall efficiency worse than the average 300 to 500 mile U.S. flight are highlighted in red, while combinations that are better than a plane but worse than a Greyhound bus are highlighted in blue. Particularly efficient combinations of fuel economy and car occupancy that are better than the average interurban bus are highlighted in green. (Note that we’ve left out trains, which are rated at 51 pmpg, to keep the table clean.)
Though a quick estimate, the table highlights that the answer to the question of whether flying is better than driving is: “it depends.” If you are driving alone in a vehicle that gets 40 mpg or less – somewhat close to Professor Sivak’s average car trip in the U.S. including commuting, shopping, etc. – flying may be more efficient. If you have one or more additional passengers,* driving is typically more efficient unless you are in a large vehicle. Notably, even a 12 mpg full-sized cargo van just edges out a plane if you have three people on board. And if you are driving a hybrid with three or four passengers, in addition to yourself, congratulations! You are beating Greyhound and are four to five times as efficient as a plane over a similar distance.
There’s more to be said about this table – sharp-eyed readers will note some interesting discontinuities once labeled fuel economy exceeds 35 miles per gallon and hybrids take over – but it’s worth noting that this analysis is consistent with our previous conclusion that, given a choice between modes, planes are on average less fuel-efficient than other modes of transport. If your vehicle gets 20 mpg or more, and like me, you enjoy company on a longer road trip, driving is a greener choice than flying. And this will become even truer as new vehicle fuel economy improves and the fuel efficiency of U.S. airlines stalls.
And yes, when in doubt, just take the bus.
* Clarification: Data suggest that the motor vehicle occupancy for longer trips could vary from 2.2 (overall social/recreational trips; NHTS 2009) up to 3.1 (NCHRP REPORT 735 to TRB, 2012). This compares to an overall average occupancy of 1.4 to 1.7 for all trip purposes including commuting, shopping, etc. (RITA 2012; NHTS 2009).