DD follow the links on the CTC page and you will find some
– there are two main sources of data on frequency of head injuries. Studies of admissions to hospital looking at cyclists with head injuries and helmet wearing – these tend to show a reduction in head injuries with helmet wearing in the order of 50 %. Badly flawed in that there is no allowance made for the missing people – Ie those who don’t get head injuries. Therefore you don’t know if helmet wearers are more likely to have accidents. Also no allowance for helmets making injuries worse which can happen ( one study shows 30% of all head injuries are caused by helmets!!)
The other type of study is whole population studies looking at head injury rates as helmet wearing rates alters. These show no reduction in head injuries as helmet wearing rates increase.
Again flawed as there is no great agreement on what constitutes a head injury and it could be the same amount of injuries but the severity is lower. However fatalities do not decrease either.
Teh main thing I have learnt from looking at a lot of studies on this and reading the original papers as well as the various commentaries is that he quality of the research is rubbish, many researchers seem to have the conclusion in mind before the study and find the data to fit, huge assumptions are made.
We need better quality research and more of it.