Alan Spearman / The Commercial Appeal file Driver Jimmy Vinson waits for the scene of a motorcycle accident to be cleared on March 26, 2012, in Memphis, Tenn. No one died in this crash, but more than 5,000 riders were killed nationwide last year, according to preliminary accident data.

U.S. motorcyclist deaths surged 9 percent last year to more than 5,000 -- near a record high -- in part because warm weather lured more riders out on the road last spring, but also because of a long-term decline in helmet laws, according to a report issued Wednesday by the Governors Highway Safety Association.

"All of the trends with motorcycle deaths are really going in the wrong direction," said GHSA Chairman Kendell Poole, highway safety director for the state of Tennessee. "We are talking about 5,000 tragedies a year with no sign of progress."

The projected rise, based on preliminary accident data, outpaced the national increase in overall traffic fatalities, which rose from 5 percent to 7 percent in 2012, and marked the 14th time in the past 15 years that motorcycle deaths have increased. Over the same period, traffic deaths involving all varieties of vehicles have fallen by 23 percent, according to the GHSA.

"The most notable thing was the warm weather," said Dr. James Hedlund, the author of the report and a former official with the National Highway Traffic Safety Administration. "Motorcyclists in a number of Northern states were out on the road a lot earlier." Thirty-four states east of the Rocky Mountains notched record average temperatures from March to May 2012.

Read the GHSA report: Motorcyclist Traffic Fatalities by State

The report also cited two economic factors that may have contributed to an increase in the number of miles traveled via motorcycle in 2012 and a higher probability of accidents: a better economy and high gas prices.

"An improving economy and produces more discretionary income with which to buy and ride motorcycles," said the study. Several states reported an increase in the number of registrations in 2012, likely because of improved economic conditions. Hedlund has found that over the past three decades, the number of registrations tracks closely with the number of deaths.

The improving economy may also have contributed to the increase in overall motorist fatalities in 2012, since more disposable income means more miles driven in all types of vehicles. With gas prices still high, however, the report suggested that motorcycle riders may have decided "to use their fuel-efficient motorcycles rather than automobiles for commuting and other everday travel," thus increasing rider deaths.

The report recommended that states address a number of issues in order to reduce motorcycle deaths, especially encouraging an increase in helmet use. "The most effective strategy by far," asserted the report, "is to enact a universal helmet law in the [states] that lack them." Only 19 states now require riders to wear helmets, down from 26 in 1997.

The last state to enact a helmet requirement was Louisiana in 2004. In 2012, Michigan dropped its requirement and made helmets voluntary.

Michigan's new law went into effect last April. Preliminary totals from the Michigan Office of Highway Safety Planning projected 129 motorcyclist deaths in the state for 2012, an 18 percent increase over 2011.

The Michigan chapter of ABATE, a motorcyclist group opposed to mandatory helmet laws, said in a statement to NBC News that the fatalities "fall within the normal range of variation" and can't be attributed to the relaxed law. ABATE also referenced the warm weather and an increase in registration as possible factors. "Our primary concern about the accuracy of the data from (the state Office of Highway Safety Planning) is that two single points of data are being compared, when a multiyear comparison will give the clearest and most accurate depiction," said Vince Piacenti, statistician for ABATE Michigan.

Hedlund said that he had not analyzed the Michigan data, but said the variation was not small. "Numbers can vary, particularly in fairly small states," he said, "but Michigan's numbers went up substantially."

ABATE also noted that more than half of Michigan bikers who died in 2012 were "not endorsed to operate a motorcycle." GHSA's recommenddations for cutting motorcyclist deaths include reducing alcohol impairment, reducing speeding and ensuring that motorcyclists are licensed. According to GHSA, NHTSA data shows that in 2010, nearly a quarter of riders involved in fatal accidents did not have valid licenses.

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