A law requiring bicycle helmets for all riders would almost certainly increase safety and public health. However, if you want to put this to the test, you have two options:
You may compare the number of accidents that occurred before the law was passed to the number of accidents that occur now. This would show if the law had an effect on the absolute number of accidents.
You may also examine the incidents that did occur and calculate the proportion of those accidents that were less serious because the persons involved were wearing helmets.
