Today, seat belt use in the U.S. stands at about 80 percent, and primary law states (where drivers can get pulled over simply for not wearing a seat belt), tend to have somewhat higher rates of seat belt use. So, case closed, seat belt laws save lives. Not so fast.
It's interesting to compare, for example, the state with the highest seat belt use (Washington), to the state with the lowest seat belt use (New Hampshire). Washington is a primary law state, while New Hampshire is the only state that has no seat belt law at all (most states have secondary laws). While both states have motor vehicle fatality rates below the national average, New Hampshire's rate (1.01 deaths per 100 million vehicle miles traveled) is actually much lower than Washington's (1.20).
Now, of course, this compares only two states, but it does show the importance of keeping your eye on the ball. We don't want higher seat belt use as an end in itself, fewer deaths is what we're after (and I prefer to slice it finer still, placing the deaths of innocents atop the priorities list).