There's been no shortage of media reporting on the ongoing decline in driving in the United States. Since 2004, Americans have been decreasing their per-capita miles driven every year.

What's less clear is why it's happening. A new report shows just how little we understand the trend.

In some circles, skeptics have balked at the dip, arguing it's merely an effect of the sluggish economy. Owning a car and paying for gas isn't cheap, so it's logical to assume when times are tough, people would cut back on automobile trips.

"Among the potential causes, certainly the most important is the economic situation, with steeply declining household incomes and the worst economic situation since the 1930s," Wendell Cox wrote in a blog reflecting that line of the thinking.

Researchers at U.S. PIRG, trying to figure out if that was true, came up with something of an experiment: If the sagging economy was the reason Americans are driving less, then theoretically, the states hit the hardest by the recession would see the biggest declines in ridership.

That didn't happen.

"There's no real relationship at all," says Phineas Baxandall, senior tax and budget policy analyst at U.S. PIRG. "When people dismiss it, they say 'it's the economy.' But if that was the case, you'd expect states hit the hardest by the economy to have reduced driving the most. That's not at all the case." (Baxandall also notes that the decline in driving pre-dates the recession by several years.)

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Alaska, for example, weathered the recession relatively well -- its unemployment rate from 2005 to 2011 rose less than that of almost any other state. Yet it saw a huge decrease in per capita miles driven.

Nevada, meanwhile, was devastated by the recession -- its unemployment rate tripled from 2005 to 2011 -- yet in the same time period it was one of the few states where vehicle miles traveled actually increased.

The implications of the study could be big. If the decline in driving isn't largely the result of the recession, then transportation planners might want to think twice before assuming driving miles will rebound if the economy picks up.

"While certainly a contributing factor and an economic rebound could be expected to have some upward lift on driving, the recession does not appear to be the prime cause of the fall-off in driving over the past eight years," Baxandall writes.

So researchers looked at some of the other common explanations for the decline in driving. Much has been written about the trend of Americans returning from the suburbs to the cities, where walking, biking and mass transit makes it easy to get around without a car. Others have speculated the rise of telecommuting can explain the change.

But the rate of telecommuting in each state also doesn't seem to correspond to the changes in the amount of driving. And states with the fastest rates of urbanization did not consistently drive less.

The reasons for the end of the "boom" period of driving -- from the 1950s to the middle of the last decade -- are well-known. Gas was cheap for most of that time period, residential development moved to the suburbs, and women were increasingly entering the workforce, introducing a huge number of new motorists commuting between home and work.

What's less clear is why the trend is actually reversing. "All in all, there appears to be no single factor behind the average amount of driving for residents in any given state," according to Baxandall.

States that saw the biggest drop in annual per-person vehicle miles traveled from 2005 to 2011 include Alaska (16.23 percent), Delaware (11.71 percent), and Georgia (11.68 percent). Washington, D.C. also saw a 14.4 percent decline. Based on federal estimates, Wisconsin experienced a 12.14 percent drop, but the researchers noted a discrepancy between federal and state data showing that may not be the case.

It's unclear why the trend was so pronounced in Alaska. Billy Connor, director of the Alaska University Transportation Center, speculated it might be due to a growing number of tourists who are visiting Alaska by cruise ship or choosing to fly there and tour the state by bus, rather than using a rental cars or motor homes. He also noted large portions of the population travel by snowmobile and all-terrain vehicle instead of cars.

On the other hand, Alabama, Louisiana, Nevada and North Dakota are the only states where per capita miles driven in 2011 eclipsed their 2004 or 2005 peaks.

Baxandall says he was surprised to discover that the decline in driving was so widespread across so many different parts of the country.

Overall, per capita vehicle miles traveled peaked in 2004 and have fallen each of the last eight years, amounting to a 7.4 percent dip in that time.

Other factors that may explain the decline include increasing gas prices, the rise of e-commerce replacing shopping trips, and the simultaneous aging of baby boomers and emergence of millennials who are delaying the age at which they get a driver's license.

Changes in Miles Driven by State

Figures shown below refer to vehicle miles traveled (VMT).

Source: U.S. PIRG compilation of Federal Highway Administration Statistics