A Southwest Airlines Co. Boeing 737-7H4 plane takes off from Love Field Airport in Dallas, on Wednesday, April 20, 2011. (Matt Nager/ Bloomberg)

If you want to live a green and energy conscious lifestyle, then the travel or transportation choices you make are crucial. That’s because travel uses a great deal more energy than, say, spending time at home, or cleaning, or eating.

So what should you do when Thanksgiving is coming, and you’ve got to decide whether to pack up the car or just spring for that really pricey ticket to Detroit from Washington, D.C.?

new analysis, just out from Michael Sivak of the University of Michigan Transportation Research Institute, offers a surprising answer. Namely, Sivak finds that driving today is actually considerably more “energy intensive” than flying, where energy intensity is defined as “the amount of energy needed to transport one person a given distance.”

One principal reason? While airlines and cars have both gotten more energy efficient over time, one key factor in determining the energy intensity of a particular form of travel is how many people are being transported per trip. And on this score, jam-packed modern passenger planes have cars totally beat.

“Flying domestically in the U.S. used to be much more energy intensive than driving, but that is no longer the case,” said Sivak by e-mail. “One of the main reasons is that the proportion of occupied seats on airplanes has increased substantially, while the number of occupants in cars and other light-duty vehicles has decreased.”

In his analysis — based on data from the Department of Transportation — Sivak found that in 2012, the average energy intensity of driving a light duty vehicle, such as a car or SUV, in the United States was 4,211 BTUs (British thermal units) per person mile, while the energy intensity of flying domestically was 2,033 BTUs per person mile. (BTUs are a relatively small amount of energy, defined as “the amount of heat necessary to raise one pound of water by 1 degree Farenheit.”) Thus, driving is more than twice as energy intense (on average).

The analysis follows on a report of Sivak’s from last year, which had found that driving is 57 percent more energy intensive than flying. Now, Sivak has gone back to update the analysis through 2012 and add some corrections to the data on the energy intensity of flying. And it turns out that as a result, “the advantage of flying has increased even further.”

It’s important, though, to clarify what this analysis does – and does not – mean. We all drive different cars different distances, with different numbers of passengers in them; and our flying habits are also divergent. So these averages can hide quite a lot of diversity in transportation practices – and how much energy they use.

For instance, when it comes to driving, having a more fuel efficient vehicle obviously means you’ll use less energy. And to reduce driving energy intensity still further, having multiple people in the same vehicle also makes a huge difference. Hence the virtue of carpooling.

“As vehicle load increases, the amount of fuel consumed per person mile decreases (even after taking into account the increased weight to be carried),” the report noted.

When it comes to flying, meanwhile, longer distance flights are also less energy intensive, principally because “airplanes use a disproportionate amount of fuel during takeoffs.”

Thus, if you carpool with a large group of people over a moderate distance – say, driving from D.C. to Detroit for Thanksgiving — you may still beat flying on an energy intensity basis.

Moreover, if you fly a lot, your total energy use from flying may still outdistance your energy use from driving. That’s because a single flight tends to be a lot longer (and thus more energy consuming) than a single drive. The average drive, noted Sivak, is 9 miles, whereas the average domestic flight is 895 miles.

Sivak’s research also notes that this energy intensiveness comparison has changed over time. Back in 1970, based on these same metrics, flying was actually much more energy intensive than driving. Both forms of transportation have since improved their efficiency, but flying has improved it a lot more.

So if you’re worried about the energy intensity of your transit choices, and you still want to use your car, the answer is clear: Try to carpool more. And for longer distances, remember that you have more options than just these two. According to the Department of Transportation, buses and taking Amtrak both have a lower energy intensity than either driving or flying.

Update: Dan Rutherford and Irene Kwan of the International Council on Clean Transportation have put up an analysis that challenges some aspects of the report by Michael Sivak discussed above. Suggesting that the analysis “does not provide an apples-to-apples comparison of transport efficiency” when it comes to planes vs. cars, the authors sought to compare instances when there’s a real choice to be made between taking one or taking another. Thus, they “assessed the efficiency of each mode [of transportation] across comparable 300 to 500 mile trips (e.g. Washington DC to Boston, Los Angeles to San Francisco).”

The resultant analysis found some instances where planes are less energy intensive, but an even larger number where cars were more efficient than planes. “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,” they concluded. “If you have one or more additional passengers, driving is typically more efficient unless you are in a large vehicle.”

Sivak responded by email that his conclusions, and those of ICCT, “are not that far apart.” But he did note two points in relation to the comment above. “On the average you ARE driving alone,” wrote Sivak, “and the average fuel economy IS less than 40 mpg (by a lot, even on limited-access highways).” Sivak also commented that the ICCT analysis “does not take into account the fact that a non-trivial part of the energy consumed by airplanes is due to transporting revenue freight and mail; my analysis did take that into account.”