European Council of Ministers of Transport:
National Policies to Promote Cycling
National Policies to Promote Cycling 
ECMT
ISBN 92-821-2325-1, Paris, December 2004
"Though helmets are widely accepted as reducing the severity of head injuries, the issue of mandatory requirements for helmet use has been controversial for a long time. PROMOSING [1], a research project commissioned by the European Union and coordinated by the SWOV Institute for Road Safety Research (2001), suggests that from the point of view of restrictiveness, even the official promotion of helmets may have negative consequences for bicycle use, and that to prevent helmets having a negative effect on the use of bicycles, the best approach is to leave the promotion of helmet wear to manufacturers and shopkeepers. The report entitled 'Head Injuries and Helmet Law for Cyclists' by Dorothy L. Robinson, Bicycle Research report No. 81 (March 1997) shows that the main effect of the introduction of the general helmet law for cyclists in Australia was a drop in bicycle use."
[1] This should read PROMISING. The error is in the original text.