http://www.cyclehelmets.org/papers/c2022.pdf
d) South Australia (SA, Fig 4) shows declining trends in hospital
admissions for concussion, but not other head/face injuries, and again no obvious effect of a law that increased helmet wearing from 40-90%.
The decrease in concussions was noted and explained: "it is understood
that, since helmet wearing became compulsory, the procedure for
patients with a short episode of concussion has changed in that such
patients are not now admitted routinely."
Das geht auf Thompson/Rivara zurück:
"Thompson RS, Rivara FP, Thompson DC A case control study of the effectiveness of bicycle safety helmets. N Engl J Med 1989; 320:1361-1367"
Die Studie ist mit so dermassen schlecht gewählten Samplegruppen
durchgeführt worden, das man mit der Auswertungsmethodik auch
die 79% Reduktion von allen anderen Verletzungen nachweisen konnte.
Prüfungen gibts unter anderem hier:
http://www.cyclehelmets.org/mf.html?1131
http://www.ingokeck.de/alte_seiten/verkehr/seattle89kritik.html