We have examined the trend in Maryland, The District of Columbia and nationwide of an overall reduction in car and truck crash deaths. http://www.maryland-law.com/library/analysis-of-maryland-car-accident-truck-crash-motorcycle-fatalities.cfm
Yet motorcycle injuries and fatalities continue to increase. The C.O.D.E.S. program instituted by the National Highway Transportation Safety Administration and discussed at length in a recent article here (see http://www.maryland-law.com/library/reducing-deadly-vehicle-crashes-in-maryland-is-codes-the-answer.cfm has identified motorcycle helmet use as a critical factor in reducing injuries and wrongful deaths on our roadways.
CODES found that helmet use was 67% more effective in reducing brain injury in motorcycle crashes than non-use. One of the mechanisms employed in arriving at this conclusion was examining brain injury data in states where mandatory helmet laws were repealed.
The evidence was overwhelming that repeal substantially increased brain injury both overall and on a per collision basis.
What seems clear is that helmet use, whether mandatory or not is essential to reduce the advent of brain injury.