Taken from Cycle Training Instructor's Manual
* Helmets are know to reduce the severity of head injuries resulting
from a direct low speed blow to areas of the head, such as might
result from a fall or glancing collision.
* The actual protection offered is much lower than is commonly
thought. Helmets provide no protection to significant regions of the
head nor to any other parts of the body, and do nothing to prevent
torsional forces which usually cause more severe brain damage than do
concussive forces. Helmets increase the diameter of the head making
it more likely to be contacted and sent into a spin during a fall or
collision, leading to possible torsional brain damage and damage to
the neck and spinal cord.
* Helmets can cause the wearer's head to overheat, especially in hot
weather, leading to a reduction in concentration.
* Wearing a helmet can give the wearer undue confidence and affect
their risk-taking behaviour so as to make a fall or collision more
likely. It is thought that drivers may also increase the risks they
take with cyclists they perceive as protected because of helmet use.
Following the introduction of a mandatory helmet law in the Australian
state of New South Wales the rate of cycling related head injuries
reduced only slightly and the overall rate of cycling injuries
increased.
* Wearing a helmet is recommended in the Highway Code and not doing so
may leave a cyclist vulnerable to claims of contributory negligence if
they are injured, although such claims failed in a recent notable
case.
* The countries with the highest level of cycle use and the lowest
risks per kilometre have chosen to create safer road conditions rather
than promote the wearing of helmets. Any emphasis placed on the use
of protective equipment for vulnerable road users necessarily detracts
from emphasis placed on all road users to take responsibility for
their actions in preventing harm to vulnerable road users, as
evidenced by claims of contributory negligence such as mentioned
above.