Few transport methods are truly green, requiring the input of fuel at some point in the process to generate the energy required to move.

Flying is often derided as one of the worst, given the spectacular amounts of fuel aircraft can burn through. But are they any worse than cars? Indeed, could they be better for those cross-country trips?

The University of Michigan's Transport Research Institute has drawn up a report looking into exactly this scenario, and has come up with a surprising conclusion--for the most part, it's greener to fly than drive.

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In fact, for cars to be a more efficient means of moving people about, average fuel economy of light-duty vehicles would have to increase by around 50 percent, or average passenger numbers would need to double.

In the report, Making Driving Less Energy Intensive Than Flying, author Michael Sivak analyzes the amount of energy needed to transport a person in the U.S. a given distance, either in a light-duty vehicle or on a scheduled airline flight.

While automobiles have generally become more efficient over the decades, the airline industry has leapfrogged road transport as the greenest way of moving people about.

Popular Mechanics cites a table in the report showing the energy intensities of flying between 1970 and 2010.

Back in 1970, airline travel used twice the energy per passenger to move people about. But by 2000, energy use per passenger had decreased below that of driving, and today it's 57 percent less energy-intensive.

To restore the balance, according to the report, the current fuel economy of the entire fleet of light-duty vehicles would have to increase from 21.5 mpg to 33.8 mpg, or average passenger numbers would have to increase from 1.38 persons to at least 2.3.

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In reality the jump wouldn't be quite that steep--the very latest data shows the average fleet-wide fuel economy of brand-new vehicles in the U.S. is 24.8 mpg, so cars are getting more efficient all the time.

For the purpose of the report they're still being held back by many of the used vehicles on the road--but removing these from the road and replacing them all with new vehicles is a completely different can of worms...

On the face of it though, the numbers are confusing. How can cars be getting more efficient, yet driving itself become more energy intensive?

The main problem is that second, average passenger number figure. Back in 1970, the average car had 1.9 people in it--compared to the 1.38 of today. Driving alone has really harmed per-person efficiency. And airlines cram ever more people onto flights--less comfortable, but certainly a more efficient use of fuel.

Efficiency does vary depending on the trip and your passengers, though--so as Popular Mechanics points out, for that trip from L.A. to Vegas it could well be more efficient piling yourself and a few friends into a car, rather than taking a plane.


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