Over 20 states have adopted laws requiring youths to wear a helmet when riding a bicycle. We confirm previous research indicating that these laws reduced fatalities and increased helmet use, but we also show that the laws significantly reduced youth bicycling. We find this result in standard two-way fixed effects models of parental reports of youth bicycling, as well as in triple difference models of self-reported bicycling among high school youths that explicitly account for bicycling by youths just above the helmet law age threshold. Our results highlight important intended and unintended consequences of a well-intentioned public policy.The estimates indicate that bicycling fatalities among youth (age 0-15) decreased across states by about 19 percent. It also decreased youth bicycling participation by 4 to 5 percent.
These percentages look somewhat lopsided, but you have to consider the magnitudes of the levels of the variables. I can't find the average number of youth fatalities by state in the above paper, but inferring from the NHTSA, there were about 105 total youth fatalities in the entire United States. So in total, we are probably talking about saving lives by the tens or twenties.
On the other hand, the volume of participation is probably quite large. In the paper above, participation was 84 percent of a 115,000 person sample. A 4-5 percent reduction for the population at large is probably in the hundreds of thousands. To some extent, these participation reductions likely account for some of the reduced fatalities being reported.
The question becomes: How many forgone participants equal a life saved, knowing that the only 100% safe bicycling is no bicycling at all?