If you really want to reduce emissions or conserve energy, get rid of your car.
For the first time in several decades, the American Association of State Highway and Transportation Officials' National Report on Commuting Patterns and Trends found an increase in the share of American households that don't own cars.
Starting in 1960, the number of American households without a car began dropping, reaching a low of 8.7 percent in 2007.
However, by 2011--the latest year for which data is available--that number had risen to 9.3 percent.
While the economic fallout from the Great Recession may have had some impact, the study's authors believe a proliferation of alternative types of transportation is a major factor in the decline of car ownership.
People are more likely to bike, walk, or take mass transit, they said, and many people are now working from home thanks to advances in communications technology.
Car sharing is another new option. Zipcar or local city car-sharing services can substitute for an individual vehicle owned by some city dwellers.
Finally, there's the persistent narrative that Millennials don't care much about owning cars, because of a combination of lack of funds, environmentalism, and a preference for digital technology.
Ditching a car would certainly be a thorough way to reduce your personal emissions profile; a car can't pollute if it doesn't exist. Even an all-electric Nissan Leaf can't beat that logic.
Still, given the suburban infrastructure and society we've built since World War 2--which requires a vehicle, if not two or three--cars have little chance of disappearing, at least in our lifetimes.
What would it take for your household to go without a personal vehicle?