In case scientific study counts.
Netherlands research.
http://www.swov.nl/rapport/factsheets/uk/fs_bicycle_helmets.pdf
A recent study in Australia.
Helmets reduce head injury severity › News in Science (ABC Science)
"Bicycle riders without a helmet are almost six times more likely to suffer a severe head injury than a helmeted rider, a new Australian study shows.
The finding adds to growing evidence supporting mandatory helmet laws.
Australia is one of the few countries in the world with mandatory helmet laws for bicycle and motorcycle riders.
The research, published today in the Medical Journal of Australia looks at the protective effect of helmets by examining the link between severe head injury in an accident and helmet use.
Lead author Dr Michael Dinh from the University of Sydney and his team based their findings on a study of 348 patients aged over 15 years admitted to seven Sydney trauma hospitals in the 12 months from July 2008.
The team found cyclists without helmets were 5.6 times more likely to suffer any head injury than cyclists wearing a helmet and 5.5 times more likely to suffer a severe head injury.
For motorcyclists the benefit of helmet wearing was not as marked as those without helmets were only 2.2 times more likely to suffer any head injury than motorcyclists with helmets and just 3.5 times more likely to suffer a severe head injury.
"The protective effect of helmet use with respect to head injury prevention therefore appears to be greater in pedal cyclists compared with motorcyclists," the team writes."