Are you making plans for a spring break trip? One of the first decisions you may make is how you're going to travel. Flying is certainly the fastest option, but it can get expensive. If your choice is determined in part by which mode of transportation will make your travels greener, the results of a new study may surprise you.

Driving has long been thought of as the more efficient mode of transportation, but according to data compiled by a researcher at the University of Michigan, air travel became less energy-intensive per person per mile around the year 2000. Think about it as the amount of energy required to move one person one mile in each vehicle while accounting for the number of individuals in each vehicle. When measured in British thermal units (BTU), the average energy intensity of driving in the U.S. is 57 percent greater than flying, a stark contrast from 40 years ago when the intensity of driving was 50 percent less than flying.

How did this happen?

Dr. Michael Sivak's data, which spans more than 40 years, indicates flying surpassed driving in terms of energy demand more than a decade ago. When you dig into the numbers, the reason flying has surpassed driving is mainly because automobile efficiency has improved far less than that of airplanes, coming in at 17 and 74 percent respectively.

In the last 40 years, the energy intensity of driving has remained relatively flat. While fuel economy has improved, today's average car carries 1.38 people, compared to 1.9 in 1970, raising the amount of energy consumed to carry each person one mile. In order for cars to compete with planes at today's energy efficiency levels, the average gas mileage of a passenger car would have to increase from 21.5 miles to about 33.8, or the average car would need to carry around 2.3 people.

The increase in airplane efficiency is twofold. In the last 40 years engines, materials and aerodynamics have improved, making planes more fuel efficient. However, part of the improvement in plane efficiency is from the way planes are used. Airlines now tend to fly with fewer empty seats and your minimal leg room means more passengers can fit on each flight.

In 2010, according to Sivak's data, driving ranked last among the five major transportation modes, with train services such as Amtrak ranking first, followed by motorcycles, planes and buses. But don't buy your plane tickets just yet. A vehicle with more than two people reduces driving's energy intensity below that of flying to a level comparable to motorcycles.

What's the most energy-efficient way to travel?

When deciding whether to travel by plane or car, fuel efficiency isn't the only factor to consider. Flying is not as simple as takeoff and landing, as security checkpoints and travel to and from the airport can make a short flight an all-day event. Additionally, driving is usually much cheaper than flying, especially if you are able to split transportation costs between several people and are driving efficiently.

Flying certainly makes more sense for your New York to Los Angeles business trips, however, for shorter trips you can reduce your environmental impact by seeking out alternatives to flying and limiting the amount you travel alone.

In the end, modern driving and flying habits have had more of an impact on overall energy intensity than any engine modifications. More people board each plane and fewer ride in each car. However, most newer small to medium-size cars get about 25 miles per gallon, and if cars utilized every available seat the same way planes do driving would re-emerge as an efficient and environmentally friendlier option than flying. So for your next trip, pack into your car and hit the road!

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