|Energy intensities of flying (blue) and driving (green), 1970-2010. Data: Sivak, UMTRI-2014-2. Click to enlarge.|
Currently, the energy intensity (BTU per person mile) of driving is 57% greater than that of flying, according to a new analysis by Dr. Michael Sivak, Director, Sustainable Worldwide Transportation at the University of Michigan Transportation Research Institute (UMTRI). To make driving less energy intensive than flying, the fuel economy of the entire US fleet of light-duty vehicles would have to improve from the current 21.5 mpg (10.9 l/100 km) to at least 33.8 mpg (7.0 l/100 km) at the current vehicle load, or vehicle load would have to increase from the current 1.38 persons to at least 2.3 persons.
In the report, Sivak considered domestic operations of all certified air carriers were considered and all light-duty vehicles (cars, SUVs, pickups, and vans) over the past 40 years. During that period, the energy intensities of both driving and flying decreased. However, the improvement for driving (17%) was substantially less than for flying (74%).
In 1970, the energy intensity of driving was about half that of flying, but decreased with each five-year increment examined. The situation reversed in 2000, and the advantage of flying increased from then on. For the latest year analyzed (2010), the energy intensity of driving was 57% greater than that of flying.
In 2010, driving was the least efficient of five major transportation modes (4,218 BTU/person mile), with rail (Amtrak) being the most at 1,668 BTU/person mile. Motorcycles were the second most efficient (2,675 BTU/peson mile), followed closely by flying (2,691 BTU/person mile). Transit buses came in fourth (3,347 BTU/person mile).
Sivak then used the ratio of energy intensities of driving and flying (1.57 (4,218/2,691 = 1.57) to calculate, at the current vehicle load, the requisite fuel economy to bring driving to parity.
Without any improvement in vehicle fuel economy, an average vehicle load of more than 2.3 persons would result in the energy intensity of driving to be less than that of flying.
It would not be easy to achieve either of the two changes … Although the fuel economy of new vehicles is continuously improving, and these improvements are likely to accelerate given the new corporate average fuel economy standards, changes in fuel economy of new vehicles take a long time to substantially influence the fuel economy of the entire fleet. This is the case because it takes a long time to turn over the fleet. For example, the 14.5 million light-duty vehicles sold in 2012, accounted for only about 6% of the entire fleet of light-duty vehicles.—Sivak, UMTRI-2014-2
Simply looking at the history provides perspective on “the daunting task”, Sivak notes. Although a minimum 57% improvement in vehicle fuel economy is required for parity between the two modes, during the 40 years that were examined in the study, vehicle fuel economy improved by only 65%.
Reducing vehicle load by at least 67% might be even more difficult to achieve, Sivak concludes; vehicle load has recently been continuously dropping, from 1.90 in 1970 to 1.38 in 2010.
However, Sivak concludes, although flying is less energy intensive than driving, flying is a viable alternative to driving only for a subset of driving trips that involve relatively long distances.
Michael Sivak (2014) “Making Driving less Energy Intensive than Flying” (UMTRI-2014-2)