This new article in Journal of Safety Research reveals that despite the fact that bicycle helmets are highly effective at reducing the risk for head injuries, including severe brain injuries and death, less than half of children and adults always wore bicycle helmets while riding.
Jewett, A., Beck, L. F., Taylor, C., & Baldwin, G. (2016). Bicycle helmet use among persons 5 years and older in the United States, 2012. Journal of Safety Research, 59, 1-7. doi:http://dx.doi.org/10.1016/j.jsr.2016.09.001
Vargo J, Bicyclist Deaths Associated with Motor Vehicle Traffic — United States, 1975–2012. MMWR 2015; 64(31);837-841.