Cars on American roads have improved their fuel economy. The people who drive them? Not so much.

Our driving habits have collectively diminished some of the fuel-economy gains provided by advances in technology over the past four decades, says a new University of Michigan study. Automakers have increased the fuel efficiency of the nation's fleet by 40 percent since 1970. But there's been a net gain in collective fuel efficiency of only 17 percent over the same timeframe.

Here's why: More than ever, Americans like driving alone. In 1970, there were an average of 1.9 people per vehicle. In 2010, the average was 1.38.

"We increasingly value the flexibility of going where we want and when we want, despite the incremental cost of gasoline, says Michael Sivak, the study's author. "Clearly, we would save a lot of gasoline if we would car pool more, but that comes at a cost of flexibility."

Fewer people per car means more people driving their own cars, each using their own gasoline and thus decreasing the country's net efficiency. The shift has come at a time when the cost of gasoline has comparatively skyrocketed from an average of 36 cents per gallon in 1970 to $2.79 in 2010, according to the study.

Sivak, the director of the Sustainable Worldwide Transportation consortium within the university's Transportation Research Institute, culled his data from research statistics kept by the U.S. Department of Transportation.

He says another reason why the country's efficiency increases haven't matched that of automakers is that even as drivers are more conscious of their fuel economy, they tend to drive more as they increase their fuel savings. So in the end, they don't save as much on gas costs as they think.

"If you look at a large group of people who have bought new cars, they tend to drive more," he said.

Some more numbers from Sivak's study, which you can find in its entirety here:

- The distance U.S. vehicles have traveled increased 155 percent between 1970 and 2010.

- Because of the drop in people per vehicle, the corresponding increase in distance occupants have traveled is only 84 percent.

- In the same timeframe, the country consumed 53 percent more fuel, going from 303,333 million liters in 1970 to 463,233 million in 2010.

Pete Bigelow is an associate editor at AOL Autos. He can be reached at or followed @PeterCBigelow.

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