Although head injuries from biking accidents can be severe, most adults don't wear helmets. Only 22 percent of adults seriously injured while biking were using a helmet at the time of their accident, according to information from the National Trauma Data Bank on 76,032 bicyclists who sustained head or neck injuries from 2002 to 2012.

Men were less likely than women to wear helmets (21 percent vs. 28 percent). They also spent more time in the hospital than did women, and their injuries were more severe. The stats for helmet use among young people were worse: Just 12 percent of bicyclists younger than 17 were wearing helmets when injured. Bicyclists who were wearing helmets when accidents occurred generally had less severe injuries, spent less time hospitalized, including shorter stays in intensive care, and were less likely to have died as a result of the accident, according to researchers' analysis of the data, published in the journal Brain Injury.

The data did not include details on the type or design of helmets worn by the injured bicyclists, nor did it indicate why bicyclists were not helmeted. The researchers noted, though, that other studies have quoted bicyclists as saying that helmets were "annoying" or "uncomfortable" or that they did not wear one because they did not have one.

According to the Bicycle Helmet Safety Institute, there is no federal law that mandates wearing a helmet while biking. But as of this spring, the institute says 21 states and the District of Columbia, along with more than 200 local jurisdictions, have helmet laws - although most require helmets only for those younger than 18.