Aaron Edlin first noticed this imbalance more than 15 years ago. “I was a graduate student at Stanford,” he says, “and I drove maybe 2,000 miles a year. But I paid roughly the same $1,000 as if I’d driven 10 times as much, which was a huge portion of my budget.” A few years later, Edlin was serving on the President’s Council of Economic Advisers when he floated an idea that economists had long found attractive: pay-as-you-drive (PAYD) insurance. It seemed like an obvious solution. Since no one expects to pay the same price for, say, a 60-minute massage as they pay for a 15-minute massage, why should people pay the same for insurance no matter how many miles they drove?

“The objection within the White House,” Edlin recalls, “was there wasn’t good academic research on the subject.”

Edlin and a few others, including Jason Bordoff and Pascal Noel at the Brookings Institution, have since done such research. It makes a compelling case that PAYD insurance would work well, reducing the carbon emissions, congestion and accident risk created by too much driving while leading drivers to pay the true cost of their mileage. Bordoff and Noel put the total social benefit at $52 billion a year.

The better news is that PAYD insurance is no longer just an academic exercise. G.M.A.C. has begun using OnStar technology to offer mileage discounts, and next month Progressive will roll out a comprehensive PAYD plan called MyRate. Progressive, the huge Ohio-based insurer that has long prided itself as an innovator, will first offer the plan in six states, having run a similar pilot in three other states. Drivers who sign up for MyRate will install a small wireless device in their cars that transmits to Progressive not just how many miles they drive but also when those miles are driven and, to some extent, how they are driven: the device measures the car’s speed every second, from which Progressive can derive acceleration and braking behavior. Which means that Progressive will not only be able to charge drivers for the actual miles they consume but will also better assess the true risk of each driver.

If PAYD is such a great idea, why has it taken so long? There are at least three reasons: the tracking technology has only recently become affordable; insurers were anxious about drivers’ privacy concerns; and there was a substantial risk for whichever company was first to offer PAYD on a large scale.

Participation in the MyRate program is voluntary, and that’s where the economics get interesting. As with most incentive changes, there will be winners and losers. The clearest winners are people like Zelda, who can drive the same distance they used to drive and pay less. What’s less obvious is whether Progressive will be a winner; there are, in fact, a couple of situations in which Progressive could lose out. If all MyRate accomplishes is to give Progressive’s low-mileage customers the rate cut they deserve, then Progressive is doing little more than lowering its own revenues. It could, of course, try to compensate by raising rates on all its high-mileage Arthurs, but then there’s nothing to stop Arthur from buying his insurance elsewhere. (Of course, losing its riskiest customers to other companies might also prove profitable for Progressive.)

If, however, Progressive can corner the Zelda market by stealing millions of Zeldas from other insurers, then it could make a killing by being the first to sell accurately priced insurance for low-mileage drivers. The bigger goal for society  and the wild card in this or any incentive shift  is to create real behavior change. And that is always easier said than done. But if Progressive’s PAYD insurance can induce some of its high-mileage customers to drive less and especially to drive more safely, resulting in smaller claims payouts for Progressive and fewer negative externalities for everyone, then it could truly be a win-win-win situation.

Except, perhaps, for Progressive’s rivals.