http://www.reuters.com/article/us-health-bicycles-helmets-idUSKCN10...

No mention was made of of the context of the various accidents they analyzed. Perhaps the study authors considered that but the Reuters article failed to mention it. I can think of three pretty different contexts which might offer a better look into how and when helmets help:

• Collision with a motor vehicle
• Isolated crash at high speed (cyclist falls while riding at a good clip, say on a descent)
• Isolated crash at low speed (cyclist falls while -- for example -- riding to the grocery store)