Cheap gas changes a lot of things — the kind of vehicles we buy, the places where we want to live, even the political odds on unpopular ideas like raising the gas tax. The price of gas also potentially influences how much of it people buy, which, in turn, would change how much we all drive.

What happens next on this front will be particularly interesting in light of recent history. Since the mid-2000s, American car travel has been on the decline, an anomaly relative to, well, the entire history of personal automobile travel:

The picture looks more striking when we look at vehicle miles traveled per capita (taking into account population growth), in this zoomed-in view:

There's a lot of debate about what's happened over the last decade: Have we witnessed Peak Car in America, or just a byproduct of a bum economy (and, recently, pricey gas)? The shape of these two lines — or, more precisely, the shape of where we think they're heading — heavily influences how we invest in and plan for transportation for years to come.

If you believe America's love affair with the automobile is finally waning, then it might be time to stop building so many new roads and redirect resources to other forms of transportation. If you think we're in a temporary driving lull, then agencies will continue planning for decades of driving growth in the future.

This question is still up in the air, which means that any surge in driving thanks to what feels like dirt-cheap gas may obscure the long-term answer.

"All else equal, cheaper gas makes for more driving," says Phineas Baxandall, a senior analyst with the U.S. Public Interest Research Group, who has followed the decline in driving. "It’s possible people will take some kind of temporary uptick as a way to deny a long-term trend — especially if they don’t want to believe in that long-term trend. But I think, if ever there was a time to have an uptick in driving, it would be when the economy is accelerating and the price of gas is plummeting."

Baxandall argues that other systemic factors potentially depressing car travel — in demographics, housing, technology, labor — will still override the lure of cheap gas. And, of course, this cheap gas won't last forever. But the federal government has a long history of wildly overestimating America's appetite for future car travel, as this chart of recent dead-wrong protections shows:

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U.S. vehicle miles traveled (in trillions). The black line shows "actual" trends as tracked by the Federal Highway Administration. The colored lines are projections from the U.S. Department of Transportation's Conditions and Performance reports. State Smart Transportation Initiative.

And if it looks like we're finally coming out of our long driving rut, it may be easier to go back to transportation policymaking as usual.

"Small as these differences may be, they mean hundreds of billions of miles a year difference once they're projected out decades," Baxandall says. "And perhaps more importantly, the small gradations in how much driving is expected to happen often mean the difference between whether or not some very expensive highway expansion is judged to be worth the money."

The Federal Highway Administration, Baxandall points out, has only recently begun to scale back its projections after decades of assuming that car travel will outpace population growth. What we'll see in the coming months could blur those assumptions again.