Accident Analysis & Prevention Special Issue on Powered two-wheelers within the traffic system - NSW OZ
Elsevier paywall - http://www.sciencedirect.com/science/journal/00014575
Press Release review - http://medicalxpress.com/news/2012-1...ays-halve.html
No mention of confounding factors. http://wattsupwiththat.com/2012/10/0...ck/#more-71852"The Safe-System Approach adopted by all state and federal ministers in their national road safety strategy - involving multiple safety interventions - delivers significantly greater safety benefits than individual interventions alone. The study shows that cycling infrastructure doesn't simply get more people cycling, it makes it much safer for them to do so when combined with wearing a helmet."