A 12-year study (1999 to 2010) analyzed fatality reductions in bicycle-car collisions to determine the effect of mandatory helmet laws. 16 states had bike helmet laws in the beginning or the study. The researchers identified all relevant fatalities, totaling 1612, in states with and without bike helmet laws.

Relevance was determined by adjusting for factors previously associated with rates of motor vehicle fatalities (elderly driver licensure laws, legal blood alcohol limit and household income) and, among those, they found that the adjusted fatality rate was significantly lower in states with helmet laws. On average, 900 people die annually in bicycle-motor vehicle collisions — three quarters of those are from head injuries. 

The researchers analyzed data obtained from the Fatality Analysis Reporting System (FARS) — a census compiled by the National Highway Traffic Safety Administration which included information from all 50 states, including the District of Columbia and Puerto Rico.

The FARS database only includes injuries sustained during a motor vehicle collision that resulted in the death of at least one person within 30 days of the collision, so the authors believe the findings understate the importance of helmet laws. 

While data show that helmets mean an 88 percent lower risk of brain injury, the scholars concluded that bicycle helmet laws led to the 20 percent decrease in deaths and injuries for children younger than 16 who were in bicycle-motor vehicle collisions. They suggest that having the helmet laws influence parents to require their children to wear helmets more than simply concern about safety. A Canadian analysis found that the benefit of helmet laws was unclear because the rate of injuries and fatalities was dropping anyway due to parental concern about safety.
The cross-sectional study was conducted by William P. Meehan III, MD, Lois K. Lee, MD, MPH, Rebekah C. Mannix, MD, MPH of Boston Children's Hospital, and Christopher M. Fischer, MD, of Beth Israel Deaconess Medical Center and was published in the Journal of Pediatrics.

"Past research shows that laws can be an important factor in helping parents adhere to best practice guidelines," says Meehan. "For parents who feel like there is conflicting information related to child health, this evidence supports the fact that helmets save lives and that helmet laws play a role."

The American Academy of Pediatrics supports federal legislation that requires all cyclists to wear helmets.