It's somewhat dated, but this: http://www.kenkifer.com/bikepages/health/risks.htm has a summary of most of the studies done on the subject, along with an assessment of their inadequacies. The hourly rates are about halfway down the page, the distance rates are closer to the 2/3 mark.

Ironically, the guy who wrote it did end up dieing in a bike accident, which is why it hasn't been updated in the past decade.