[SystemSafety] Bicycle Helmets

DREW Rae d.rae at griffith.edu.au
Mon Oct 13 10:19:30 CEST 2014


Peter,
I looked into this one in detail for a podcast episode (disastercast.co.uk,
episode 22 :-). I've got some references in the show notes, transcript at
http://disastercast.co.uk/transcripts/episode-22-transcript/

The research is messy, because it often conflates helmet laws with helmet
wearing. Helmet laws tend to change the population that cycles as well as
whether that population wears helmets, so the causal effects are not self
evident from the statistics. It also conflates change in safety during a
crash wearing a helmet, and change in safety overall wearing a helmet.

There's a severity filter in that accidents below a certain threshhold
don't get reported. I think this is what is happening in the British study
-  if you define the size of the accident by the size of the injury, then
of course the same sized accidents have the same sized injury regardless of
whether you are wearing a helmet.

As you say, the causal reasoning is backwards - you can't measure the
effectiveness of a helmet by counting how many of the dead people aren't
wearing helmets, or how many of the minor injuries are wearing helmets.

On top of that, people who wear helmets also tend to be more "professional"
cyclists, more obedient of road rules in general, and wear higher
visibility clothing.

One way you can do it is by using something other than head injuries to
measure the seriousness of the crash - for example, broken limbs. You then
case control the group with head injuries and the group without head
injuries, and check for helmet use in the two groups.

Doing it this way specifically measures whether, in the same severity of
crash, wearing a helmet makes a difference. These studies tend to vary, so
you need to be careful of cherrypicking, but overall there is a
preponderance of studies saying that helmets reduce serious head injuries
and fatal head injuries in severe crashes. The reduction is around 60%,
which isn't _huge_, and explains why a poor study design or even just noise
can create studies that conclude no difference.

Helmets, unfortunately, are purely a severity reduction (or more precisely,
reduction in likelihood of the worst severities). They don't do anything to
prevent the accident, which can be pretty bad even with a helmet. I've just
moved back to the big city and I'm seriously considering changing my
cycling habits.



My safety podcast: disastercast.co.uk
My mobile (until October 2nd): +44 7783 446 814
My mobile (from October 6th): 0450 161 361

On Mon, Oct 13, 2014 at 5:56 PM, Peter Bernard Ladkin <
ladkin at rvs.uni-bielefeld.de> wrote:

> Another oddity on the front page of our local paper today.
>
> The British experience on bicycle helmets is paradoxical. I have a short
> report by Mayer Hillman,
> Cycle Helmets: the case for and against, published by the Policy Studies
> Institute in 1993, which is
> quite well known. There are a lot of other studies. Basically, in the UK
> the proportion of serious
> head injuries amongst seriously injured cyclists wearing helmets does not
> appear to be statistically
> distinguishable from the proportion of serious head injuries amongst
> serious injured cyclists who
> were not wearing helmets.
>
> I know three people wearing bicycle helmets who have been in serious
> accidents which injured them,
> each one caused by a motor vehicle. One was not hospitalised; one was
> hospitalised with a broken jaw
> and various other injuries; one was seriously injured in a non-localised
> manner and spent a fair
> amount of time in hospital and took up to a year before being fit enough
> to get on a bike again. All
> three hit their heads. The second also had another accident in which she
> hit her head (while wearing
> a helmet), which was caused by a sudden wheel failure (overtightened
> spokes in a rear wheel) while
> travelling at some speed (?40-50kph?). She spent two days in hospital with
> suspected concussion and
> had significant headaches for long afterwards.
>
> I always wear one. It seems obvious to me that, if I hit my head, I would
> want it to be protected by
> appropriate cushioning. Also that hitting one's head is not unlikely if
> one is projected from the
> bike in an uncontrolled manner. (When I mostly rode a recumbent, I thought
> the chances were low that
> I would hit my head in a fall, but quite high that my head would hit the
> vehicle body in a
> collision, and when riding I would be overseen at least a couple times a
> month, maybe once a week in
> winter. That's why I stopped riding a recumbent within the city limits.)
>
> So "common sense" says wear one. But the numbers seem to say that it's not
> clear how it helps.
>
> I don't know that there are any numbers specifically for kids, but kids
> are lower down and heads are
> likely to hit vehicle bodies in a collision. Besides, kids are less likely
> to have developed falling
> strategies that protect the head; those often start with sports at age 11
> and over.
>
> The story in the paper, on the first page, shows kids taking
> bicycle-riding lessons (taught by
> police in every school for children aged 9-10).
>
> [begin translated quote]
>
> Only every seventh cyclist in Germany wears a helmet, but Science has
> determined it yet again:
> bicycle helmets reduce the risk of deadly head injuries. Tasked by the
> (Association for) Accident
> Research for Insurers (UDV), the Institute for Forensic Medicine (Instutit
> für Rechtsmedizin) in
> Munich analysed 543 bicycle accidents. Amongst 117 fatally injured
> cyclists, only 6 wore a helmet.
> More than half the victims died from craniocerebral trauma. UDV Head
> Sigfried Brockmann said "most
> of the cyclists had lived had they been using a helmet".
>
> [end translated quote]
>
> 6 cases against 111, with 50-60 dying from head injuries, is not something
> on which statistics has
> any purchase. It is, unfortunately, presented in such a manner as to
> persuade people without some
> modicum of experience that the conclusion is obvious fact.
>
> It is not, as given here. As stated it is counterfactual: somebody is
> looking at cases and judging
> that people would have lived. It is thus a value judgement rather than
> fact, and one would wish that
> the assumptions under which that judgement is made were explicit. The
> British numbers say,
> counterintuitively and in contrast, that people wearing helmets die of
> head injuries in similar
> proportion to those without. There are some differences between
> road-accident statistics in Britain
> and Germany, but they are not *that* stark.
>
> Maybe there are some facts behind the research which are not reported in
> the article and which
> justify the conclusion.
>
> Can anyone shed light on this?
>
> PBL
>
> Prof. Peter Bernard Ladkin, Faculty of Technology, University of
> Bielefeld, 33594 Bielefeld, Germany
> Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de
>
>
>
>
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>
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