Your tailpipe emissions represent, by most lights, an engineering problem. Build a cleaner, more efficient car, and the problem starts to get smaller.

Fuel efficiency standards are the most obvious way to reform a transportation sector that accounts for about 29 percent of the country’s greenhouse gas emissions. The Obama Administration was certainly thinking this when last May it mandated all new cars in America by model year 2016 must get at least 35.5 miles to the gallon.

But what if instead of changing the cars — or, better yet, in addition to that — we changed the way we use them? What if we treated the problem not as an engineering challenge, but a behavioral one?

This controversial idea drew a packed ballroom at the annual meeting of the National Academy of Sciences’ Transportation Research Board. Thousands of international transportation geeks huddled this week for hundreds of sessions in a series of Washington hotels largely to mull how we ought to get around in a low-carbon economy.

How do we pay for infrastructure improvements if better fuel economy leads to fewer gas taxes? How do we rebuild roads with greener materials or move drivers off those roads onto even greener public transit? And what do we do when the American Dream that took so many people into the suburbs now bogs them there in congestion?

One phrase that rouses the professionals who think about these problems is “vehicle miles traveled.” Some officials have suggested we could improve emissions not just by increasing fuel efficiency, but also by decreasing the miles people drive (and, say, having them live more densely). While we’re at it setting targets, we could make VMT reduction one as well. But with its sheen of coercion — telling people how much they can drive, where they can go, and which trips are frivolous — VMT reduction has a lot of detractors.

Researchers also aren’t sure the strategy will yield significant emissions cuts.

Todd Alexander Litman, a VMT proponent with the Victoria Transport Policy Institute, concedes this last point. But, he says, if we look beyond emissions reduction targets, getting drivers to decrease their VMT would have a host of additional benefits making the exercise worthwhile: We’d also improve safety, public health and land-use options, and reduce traffic congestion, fatalities and costs.

Litman looks at our mobility the way an economist considers goods. We get diminishing marginal benefits from driving; some trips we take are essential, while others are not (where the dividing line exists is hard to say). But while the benefits of driving tail off, the cost we pay for fuel doesn’t. That means most of us aren’t being efficient drivers in an economic sense.

He suggests that in a market that optimizes social welfare, we’d each be driving about 3,000-5,000 miles a year.

Instead, he says the current market is full of distortions — we don’t actually have to pay anything for the congestion we create, or the wear on roads we use, or even often the parking we take for granted (consider these costs to be externalities in the same way a coal plant doesn’t have to pay for the pollution it causes). Litman suggests we correct these distortions by, for example, making your state vehicle registration fee dependent on the number of miles you drive, or by charging you more to use certain roads during rush hour.

Critics counter that when people drive less, it hurts the economy, a calculation Litman disputes.

“There’s kind of a European model here that cars only belong to the elites, and that somehow we just don’t know our place … that we’re supposed to take some kind of a vow of abstinence here,” said Alan Pisarski, a transportation consultant who played Litman’s foil at the conference.

He also questioned the impacts of a VMT reduction policy on low-income workers, who researchers tell us often have to travel longer distances than higher-income people to get to work.

“How come when black people got to where they could own cars,” Pisarski asked, “it suddenly became a bad thing to own cars?”

Or to live on a big lot in the suburbs?

Litman insists behavior change doesn’t mean forcing people to forgo something valuable. Many people today don’t share the old aspiration of a full four-car garage in the suburbs because gas prices and congestion have made that life unlivable. Pricing the auto market until VMT drops (and more of us live in walkable, denser mixed-use neighborhoods) might, on the other hand, give many people just want they want: less time in the car.

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