Criteria to justify a helmet law

See also this related commentary: How much are helmet laws justified?

It is not the tradition in western democracies to introduce laws without good evidence that they are necessary, that they are the best solution to a proven problem and that they would be likely to result in their intended consequence. This should apply to cycle helmet laws as any other.

To establish a need for a cycle helmet law:

There must be clear evidence that cycling is especially dangerous, compared with other common activities.
There must be evidence that cycling is unusually productive of head injuries.
The injuries that might be mitigated by a helmet must be of genuine concern (i.e. life-threatening or likely to result in long-term disability)

To ensure that a helmet law is the best solution, the following criteria - put forward by Unwin 1996 - must be met:

There must be a high level of scientific evidence that bicycle helmets are effective in reducing the rate of head injury to cyclists.
The benefits to society and others of mandatory cycle helmets must be convincingly demonstrated; helmet laws cannot be justified simply to protect individual adult cyclists.
There must be widespread agreement, ideally by a large majority, that the potential benefits of compulsory cycle helmets outweigh the infringement of personal liberty and other disbenefits.
There must be good evidence to suggest that compulsory helmet wearing would not make the public health and safety benefits of increased levels of cycling harder to obtain.

To establish that a law would be likely to result in its intended consequence:

Where the same solution has been tried elsewhere, there must be unambiguous proof that it has worked
There must be no evidence of harmful side-effects from helmet use, such as a greater likelihood of being involved in a crash or an increase in the severity of some types of injury.

References

Unwin 1996

Unwin NC, 1996. Cycle helmets – when is legislation justified?. J Med Ethics 1996 Feb;22(1):41-5.