http://jama.jamanetwork.com/article....icleid=1362037
Helmet Use Among Motorcyclists Who Died in Crashes and Economic Cost Savings Associated With State Motorcycle Helmet Laws—United States, 2008–2010
"The findings indicated that, on average, 12% of fatally injured motorcyclists were not wearing helmets in states with universal helmet laws, compared with 64% in partial helmet law states (laws that only required specific groups, usually young riders, to wear helmets) and 79% in states without a helmet law"