The Insurance Institute for Highway Safety, Highway Loss Data Institute maintains a list of 'Helmet Use Laws' (http://tinyurl.com/ctlq) for motocycles and bicycles by US state. Whether unicycles count as bicycles in your state is not included, but the list can still be a handy guide.
That page doesn't list any of the local city our county laws that require bicycle helmet use. For example, King County and the city of Seattle now have a bicycle helmet law that covers all riders (minors and adults). Just because there is no statewide bicycle helmet law doesn't mean that local cities or counties haven't enacted such a law.
