Interesting study on the correlations between the percentage of people wearing helmets while cycling versus the percentages of people hurt & killed while cycling. Although common sense would seem to dictate that more helmets = more safety and less injuries / fatalities, this is not actually the case! In fact, more cyclists = more safety, regardless of whether or not helmets are worn, and in fact compulsory helmet laws tend to reduce the number of cyclists, therefore making cycling in said location less safe.
Link:
Federation francaise des Usagers de la Bicyclette – FUBicy