I'll expand on this. What I am talking about here is, at its core, the questions you will need answers to if you are going to convince a non-cycling, general audience away from a helmet mandate. I had a bit of this conversation with my mother several years ago with me taking the side of individual choice. The issue was immediately drawn to shared societal costs; insurance especially. The conservative option, for a vast majority of people, is to mandate helmets. The only obstacle that usually carries any weight with anyone other than a fellow cyclist is the administrative cost of enforcement.

So you, in trying to define the cost/benefit analysis in favor of choice, will have to admit and quantify the costs and then convincingly show the benefits of free choice. Because, frankly, you have an uphill battle with convincing non-cyclists. The only thing in your favor is people tend not to embrace new legislation on principle in the US. You should relish the opportunity to test your arguments in an obscure forum under internet anonymity.