Earlier this week Michigan's governor Jennifer Granholm called for a lower speed limit, though she neglected to offer a new target, and yesterday Virginia Senator John Warner renewed the call for 55mph speed limits.

It wouldn't be the first time lower speed limits were imposed in an attempt to save fuel during an energy crunch, but it's still not clear that the lower speed limits actually achieve their stated goals.

In 1974 the U.S. set a national speed limit of 60mph, a policy which lasted until 1995. The price of oil had a large role in the institution and eventual repeal of the policy, but some advocates also played up the safety angle of the move. The problem with reducing speed limits is that reality and theory do not align well.

Warner cites studies that show as much as 2% of U.S. fuel consumption per day could be saved with a 55mph speed limit. It's true that fuel consumption for most cars falls of significantly above 60mph, and it's also true that lower speeds usually result in less severe crashes. But lowering the speed limit presupposes people will actually drive slower, and historically, this has not been the case.

According to a U.S. NHTSA study from 1992, during the height of the last 60mph national limit period, neither raising nor lowering the speed limit had any significant effect on the speed of traffic. In fact the only significant change noted by the study was the increase in speeding tickets issued when the speed limit fell too low.