Traffic death rates dropped sharply in several states that have strengthened their seat belt laws to the toughest on the books, a leading highway safety group said on Thursday.

An analysis of federal crash data by the Insurance Institute for Highway Safety concluded that fatality rates in nine states and the District of Columbia fell by an estimated 7 percent when they adopted primary seat belt laws. They made the change at different times between 1989 and 2003.

“In states with primary laws, safety belt use rates are higher. The result is that crash deaths are reduced,” said Susan Ferguson, senior vice president of the safety organization funded by the insurance industry.

“Where primary laws are in effect, drivers are more likely to buckle up because the perception is that they’re going to be pulled over if they don’t,” Ferguson said.

Auto safety advocates have made increasing the number of states with primary belt laws a priority in 2005. So far, 21 states and the District of Columbia have them. The rest have secondary laws, which permit authorities to ticket beltless drivers but only after pulling them over for another reason.

New Hampshire is the only U.S. state without a belt-use law.

Federal highway data show that belt use rates averaged 84 percent in states with primary laws compared with 73 percent in states with a less stringent measure.

Ferguson said about 700 lives would be saved annually if the 28 states without primary belt laws were to adopt the tougher standard. More than 42,000 people were killed on U.S. roads in 2003, according to the last available figures.