It is likely that passing a law that makes bicycle helmets mandatory for all riders would improve safety and public health. However, if you want to test this, there are two paths that you could follow:
- You could look at the number of accidents that took place before the law was passed and compare them to the number of accidents that take place now. This would indicate whether the law made a difference in terms of absolute number of accidents.
- You could also look at the accidents that did happen and measure what percentage of those accidents were less serious because the people involved were wearing helmets.