Driving is more energy-intense than flying, a University of Michigan study concluded last month, revealing surprising trends about energy consumption in the United States.
Lead researcher Michael Sivak of the University’s Transportation Research Institute found transporting one person one mile by aircraft used 2,465 British thermal units (Btus) in 2012 compared with driving, which used 4,211 Btus. Btus are a standard measure used to compare the energy intensities of various petroleum products, like kerosene and gasoline that fuel aircraft and light-duty vehicles (LDVs).
Sivak’s analyses correct earlier inconsistencies in publicly-available information about flying, like the effects of carrier groups, and cargo, on energy intensity. The relative advantage of flying increased between 2010 and 2012, and now accounts for less than half the energy intensity of driving, the pair of researchers found. Flying consumed 10,185 Btus per person/mile in 1970 compared with driving, which consumed 5,067 Btus.
The crossover point for driving and flying occurred recently—at the millennium.
A 2009 International Council on Clean Transportation (ICCT) study found airplanes have doubled their fuel efficiency today over five decades ago. The first major advancements came just before 1970—the advent of the modern piston engine—when fuel economy improved 25 percent over 1960 levels, ICCT says.
Airplanes have doubled their fuel efficiency today since five decades ago.
Between 1970 and 2000, fuel economy improvements in the airline industry slowed pace—growing 25 percent over a thirty year period. Aerodynamic advancements, like those in wingtip design and engine cowling, as well as the recent integration of composite materials like carbon fibers and lightweight plastics propelled major breakthroughs in aircraft flight efficiency. At the current level of investment, tomorrow’s airplanes could be 20 to 40 percent lighter than they are today.
The fuel efficiency of passenger cars also improved, although less so by comparison.
Following the oil price shocks of the 1970s, Congress passed historic CAFE standards that have so far raised nationwide fuel economy to 25.3 miles per gallon (mpg). At the same time, cars became heavier and more powerful. Americans drove more frequently, owned more vehicles, and journeyed longer distances.
Today, LDV fuel economy has improved nearly 80 percent above 1960 levels, while LDV vehicle miles traveled (VMT) contemporaneously grew 135 percent, from 587,000 to 1.37 million miles in 2013.
The cumulative effects of these habits increased gasoline demand—now 45 percent of total transportation sector consumption—despite important improvements in technology across the board.
Airlines, meanwhile, increased capacity, allowing airplanes to carry more passengers further. “As vehicle load increases, the amount of fuel consumed per person mile decreases,” the University of Michigan report says.
The total number of U.S. passengers occupying domestic and international airline seats grew 355 percent since 1960.
As airplanes connected every major city in the country—and many smaller ones—more people were able to access air travel, causing growth in overall ridership. According to the World Bank, the total number of U.S. passengers occupying domestic and international airline seats grew 355 percent since 1960, with 743 million seats occupied by passengers in 2013. In the intervening time, seat miles per gallon improved from 27 mpg for domestic travel to 67.1 mpg for new aircraft this year, the Energy Information Administration says.
“It is important to recognize that the energy intensity of flying will … continue to improve,” Sivak told the University of Michigan press office. “Consequently, because the future energy intensity of flying will be better than it currently is, the calculated improvements underestimate the [those] that need to be achieved in order for driving to be as energy intensive as flying.”
One question is how economic and population growth will affect the variables Sivak and his colleagues studied, particularly as demands for air travel grow across population and income demographics. In the coming decades, heavier cargo and new passengers will place additional burdens on freight carriers and commercial airlines.
The airline industry may look to displace variably-priced petroleum products, like jet fuel—one-quarter of which is used during take-off, by some estimates. Under these circumstances, technological developments in energy efficiency may only go so far.
According to the EIA’s 2015 Annual Energy Outlook, airlines will be able to increase efficiency in the short term by utilizing technology that trims consumption, like lighter metals that allow airplanes to fly more smoothly through the skies. In the long term, however, EIA says the rise of private air travel will grow energy consumption, possibly offsetting the coming gains.
As our energy intensity continues to increase, one outstanding question will be the level of investment required for efficiency to match demand growth in aviation.