From an article in the Willamette Week (link via Jim Romenesko):
In 1865, this geezer named William Jevons noticed that every time a new innovation came along to get more energy out of the same amount of coal, coal use actually went up. Jevons realized that increasing efficiency reduces cost, reduced cost leads to increased demand, and increased demand leads to increased consumption....If the Jevons paradox applied to hybrid cars, we'd expect to see folks driving more, or faster, or less efficiently or all three. Sure enough, a 2009 study by an insurance-industry group found that hybrid owners were driving as much as 25 percent more miles than their all-gas counterparts, and getting more traffic tickets.