Driving slowly in the fast lane is more than just annoying, it's also illegal in many states. Wanna know if your 45-mph, Buick-borne grandma is breaking the law on the interstate? Just consult our handy map.
The most popular law follows the Uniform Vehicle Code, which says a car driving below the "normal speed of traffic" should be driven in the right-hand lane. Because it indicates "normal speed" instead of saying "speed limit" a driver going above the speed limit but slower than most traffic is still in the wrong.
The states indicated in green dictate that the left lane should be used exclusively for passing or turning left, though most of these limit enforcement to multi-lane highways. The simplest and best laws simply state drivers in the left lane must always yield to faster traffic regardless of the passing car's speed or other factors.
Unfortunately, most of these laws are only enforced as a way of pulling over suspicious vehicles or passengers. This is why Georgia is in the process of passing a tougher "Slow-Poke Law" with real penalties for drivers.
(Hat tip to JohnDecaf!)