[...] motorcycle riders should be exempt from helmet laws as long as they agree to be organ donors.
Read the whole summary of Charles Wheelan's favourite session, "Economics of Traffic Safety: Children, Teenagers and the Elderly", from the AEA meeting this year. And if you want more details, 3 of the 4 papers in that session can be downloaded here (direct links: Levitt and Doyle on child safety seats and seat bealts; Karaca-Mandic and Ridgeway on graduated driver licensing and respectively, Laughran and Seabury on accident risk of senior drivers). The 4th paper in that session, by Thomas Dee, on traffic safety and organ donations---connected to the phrase of the day--- is seemingly not yet available for download, so you'll have to stick to Wheelan's appraisal for now.
PS. Since I mention Wheelan above, remember that a while ago I recommended his book to all those that have no clue about economics, but would like to learn something fast and without much effort :-).