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A revealing comparison in how we've come to use two very different modes of transportation.

If you've ever wrestled with the moral implications of flying from Washington to Chicago when you could perfectly well drive there, you may be comforted to know that airplane travel actually demands less energy per person, per mile. This hasn't always been the case. But, according to some national data recently compiled by the University of Michigan's Michael Sivak, air travel in the U.S. became less energy-intensive than driving around the year 2000, and it's remained that way ever since.

This chart, compiled from Sivak's data, illustrates the efficiency gains by the two modes of transportation over the last 40 years, measured in BTU per person mile. This takes into account the typical passenger load on domestic flights for all certified air carriers, as well as the number of occupants carried on average by light-duty vehicles. Think of it as the total amount of energy it takes to move one person one mile, by either plane or car (including the energy needed to cool our cars along the way, or to pressurize airplane cabins mid-flight):

These historic trends are calculated from national, aggregated data. And the point isn't that you'd be better off flying to your office five miles away than driving there. But, over long-distance trips where you might realistically pick between the two modes, flying is on average the more energy-efficient option.

What's more interesting, though, is why plane travel has made such efficiency gains in 40 years while car travel has not. Part of the story is that airplanes, as machines, have better fuel efficiency now than they did in 1970. This is true for cars, too (although the improvement is less impressive). But the other part of the story is that we simply use planes much more efficiently: We cram more passengers onto them now, and planes tend to fly with fewer empty seats. In short, we're less wasteful with them.

The exact opposite has happened over the last 40 years with cars. Their engines may be more efficient. But we're more likely now than in 1970 to drive in them alone, and that drives up the amount of fuel needed per person per mile. Put more people into a car, and that number goes down. The average vehicle load of light-duty cars in America in 1970 was almost two people per car (1.9 to be exact). By 2010, it was 1.38 people.

If we wanted to bring cars back in line with planes in energy intensity, we'd have to do one of two things: We could dramatically improve their fuel economy (Sivak calculates that this would mean increasing the entire fleet from 21.5 miles per gallon today to at least 33.8). Or we'd have to start doing a lot more carpooling (increasing the average occupant load from 1.38 people to somewhere around 2.3).

This is primarily a thought experiment, looking at how we use these transportation modes in broad, national patterns. You could of course pile your five closest friends into your Prius to drive to Chicago, and your personal math would look quite different. Sivak's calculations also don't take into account the energy you'd need to drive to the airport, or the fact that a plane trip from Washington to Chicago might inexplicably layover in Dallas.

But the historic comparison between flying and driving raises an interesting question: If airlines have been intent on using airplanes as efficiently as possible, why don't we make the same calculation with our cars?

Top image: MountainHardcore /Shutterstock.com (left); SP-Photo /Shutterstock.com (right)

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