From Public Citizen:

Seat belts are the single most effective traffic safety device for preventing death and injury, according to NHTSA. Wearing a seat belt can reduce the risk of crash injuries by 50 percent. Seat belts saved more than 75,000 lives from 2004 to 2008. Forty-two percent of passenger vehicle occupants killed in 2007 were unbelted. A 2009 NHTSA study estimates that more than 1,600 lives could be saved and 22,000 injuries prevented if seat belt use was 90 percent in every state.

It amazes me that there have been a few people I ridden with who don't use a seat belt. I tell them I won't move my car until they put on their belt, and they always have, sometimes unhappy about it. I should just tell those people that it is an anti-terrorist device that will save 1,600 lives every year from Middle Eastern terrorists. Then they'd have federal checkpoints to make sure everyone is belted in.