The Freakonomics guys have just given Pay As You Drive (or PAYD) insurance some much-needed publicity. Also called 'distance-based insurance', this turns motor insurance payments, which are usually a fixed cost, into a variable cost. This makes it possible to save money by driving less. They write in their April 20 column in the New York Times Magazine . Imagine that Arthur and Zelda live in the same city and occupy the same insurance risk pool but that Arthur drives 30,000 miles a year while Zelda drives just 3,000. Under the current system, Zelda probably pays the same amount for insurance as Arthur. While some insurance companies do offer a small discount for driving less — usually based on self-reporting, which has an obvious shortcoming — U.S. auto insurance is generally an all-you-can-eat affair. Which means that the 27,000 more miles than Zelda that Arthur drives don’t cost him a penny, even as each mile produces externalities for everyone. It also means that low-mileage...