Here's a 2013 Australian study: "The effectiveness of helmets in bicycle collisions with motor vehicles: A case–control study": http://www.sciencedirect.com/sci

Here's a 2013 Australian study: "The effectiveness of helmets in bicycle collisions with motor vehicles: A case–control study": http://www.sciencedirect.com/science/article/pii/S0001457513000183 http://www.sciencedirect.com/science/article/pii/S0001457513... "Helmet use was associated with reduced risk of head injury in bicycle collisions with motor vehicles of up to 74%, and the more severe the injury considered, the greater the reduction." Is there some reason to doubt this?