J R Soc Med 2004;97:409-410
© 2004 Royal Society of Medicine

J R Soc Med 2004;97:409-410
© 2004 The Royal Society of Medicine

Effectiveness of cycle helmets and the ethics of legislation

Malcolm Wardlaw

6 Panmure Place, Edinburgh EH3 9JJ, Scotland, UK

[email protected]

Professor Sheikh and his colleagues (June 2004
argue for compulsion in the use of helmets by cyclists, referring to a
previous paper of
theirs2 claiming
that the rate of head injury amongst serious casualties fell 3.4 percentage
points (PPs) more for cyclists than pedestrians during a period in which
measured on-road helmet use increased by 5.8 PPs, a rate of increase of less
than 1 PP per annum. It was assumed that all of the advantage for cyclists was
due to increasing helmet use. They concluded that cycle helmets prevent 60% of
serious head injuries. Clearly they miscalculated. With a prevailing rate of
head injury amongst serious casualties of about 30%, as in this case, and
helmets 60% effective, a 6 PP increase in helmet wearing would reduce the head
injury rate by only 6x0.18 PPs=1.09 PPs, not the 3.4 PPs assumed to be a
‘helmet effect’. If all the observed improvement were due to helmet
use, then the effectiveness would be around 190%.

The authors’ assumption, in their JRSM paper, of a linear
relationship between fractionally rising helmet use and population level
injuries is speculative and is contradicted by experience in countries where
helmet use increased at more than ten times the rate in Britain. In both
Western Australia and New Zealand, helmet use increased from negligible levels
to more than 80% in around eight years, yet follow-up
did not show long-term benefits for the cyclist populations relative to
control groups. Study of injury trends in each state of Australia for the
period when helmet laws were passed shows stable characteristics, revealing no
evidence of extra prevention due to legislation coming into
force.5 Thus
international evidence indicates that the authors’ interpretation of
British data is in error. It must be stressed that hospital-based injury data
include both off-road and on-road injuries. Road casualty data specifically
show that rising helmet use is associated with cyclists’ injuries getting
more severe relative to other road users. An alarming association with
increased risk of death has twice been
Risk compensation by helmeted cyclists is the most plausible explanation.

The case for only cyclists to wear helmets is weakly founded. Estimates of
risk8 may be
calculated from routinely collected casualty and use data. In pedestrians,
risk per mile travelled is about 60% higher than in cyclists. Pedestrians are
far more vulnerable than cyclists, facing a 2% risk of death in a reported
road accident, as against 0.7% for
cyclists.9 A
scenario of the disastrous consequences of promoting walking helmets has been
Comparison with risk in driving requires certain adjustments to enable a
semblance of like for like comparison. Despite the marginalized condition of
cycling in Britain, the risk per hour travelled may be as low as the EU
average for drivers. There is no case for distinguishing cyclists as a
high-risk group.

The one clear population-level effect of helmet laws that has been widely
reported is the deterrence of cycling. In every case where data are available,
cycle use has fallen by 25–50% when a helmet law was
This has a direct consequence on the risk of death in cycling. Study of
international evidence points to a reliable relationship between the amount of
cycling and the risk in
power–law relationship with an index value of around 0.4. A fall in
cycle use of 50% would increase risk per cyclist by more than 50%, whereas an
increase in cycling of 100% would reduce the risk by almost 40%. Public health
would benefit
substantially.13 A
report by the Commons Select Committee on Health specifically cited a
resurgence in cycling as ‘probably the most effective response’ that
could be made to address the obesity ‘time bomb’. It is most likely
that road deaths would fall overall; even in Britain one hour of cycle use is
not more likely to result in a road death than one hour of driving, because
the third-party risk from cycling is so
low.8 With an
increase in cycling, the advantage would swing to the bicycle.

Tripling the level of cycle use by 2012 in line with Government policy
would dramatically reduce the risk in cycling, improve public health and most
likely reduce road casualties overall. Helmet laws never achieved anything
positive elsewhere; why should the British experience be any different?


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