The $10 I cost the economy was based on Parry’s algorithm, which calculates that drivers should pay a tax of at least $1.25 a gallon. Forty percent of that price, he says, is the cost that each vehicle adds to congestion. Another 40 cents or so offsets the price of accidents if we divided the full cost — more than $400 billion annually — by each gallon of gas consumed. (Only about 32 cents would be needed to offset the impact on the environment.) According to Parry’s logic, if we paid a tax of $1.25 per gallon instead of the current average of 50 cents, the price of gas would increase by about 25 percent to around $4 a gallon, which is still well below what much of Europe pays. But it would still encourage us to drive less, pollute less, crash less, lower the country’s dependence on foreign oil and make cities more livable. Not surprisingly, several studies have found that people — especially in Europe, where the gas tax is around $3 a gallon — drive a lot less when they have to pay a lot more for gas.

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The idea of raising taxes to help society might sound like the ravings of a left-wing radical, or an idea that would destroy American industry. Yet the nation’s leading proponent of a Pigovian gas tax is N. Gregory Mankiw, chairman of President George W. Bush’s Council of Economic Advisers and a consultant to Mitt Romney’s 2012 campaign. Mankiw keeps track of others who support Pigovian taxes, and his unofficial Pigou Club is surely the only group that counts Ralph Nader and Al Gore along with leading conservatives like Charles Krauthammer, Alan Greenspan and Gary Becker as members.

Republican economists, like Mankiw, normally oppose tax increases, but many support Pigovian taxes because, in some sense, we are already paying them. We pay the tax in the form of the overcrowded roads, higher insurance premiums, smog and global warming. Adding an extra fee at the pump simply makes the cost explicit. Pigou’s approach, Mankiw argues, also converts a burden into a benefit. Imposing taxes on income and capital gains, he notes, punishes the work and investment that improve society; taxing negative externalities allows the government to make money while discouraging activity that hurts the overall economy.

There are some obvious problems with their approach. Nobody actually knows the precise cost of any negative externality. (Estimates for the collective impact of a ton of carbon range from $1 to $1,500, for instance, which could lead to all kinds of price disagreements on a Pigovian gas tax.) So Mankiw prefers to focus on simpler factors to deduce externalities. It’s not terribly difficult to figure out how many people drive on a certain road per hour and how much time they lose by being stuck in traffic, he told me. Still, he said, “you’ve got to take your best guess.”

Another major drawback is that it’s hard to know where to stop. All of us are constantly affecting those around us in positive and negative ways, which in turn affect the economy, however indirectly. In my Brooklyn neighborhood, I notice that some neighbors have well-tended gardens that make my walk to the subway more enjoyable. Others, less so. (Taken as a whole, they also play a subtle but significant role in determining property values.) Can’t we tax the sloppy and subsidize the beautifiers? Every time someone drops out of high school, he increases the likelihood of crime and reliance on public assistance and decreases the overall rise in G.D.P. Should we tax them? Or their teachers? What about taxing obese people who increase the costs of our health care system? Or should we tax fast-food companies instead? (This fall, voters in California defeated ballot measures to impose a tax on sugary drinks.)