From: drobinso@mendel.une.edu.au (D.Robinson)
Newsgroups: rec.bicycles.soc
Subject: Re: Statistics concerning bicycle helmet use in Holland.
Summary: Risk ratios per km
Keywords: Injuries, risk ratios
Date: 15 Dec 92 00:35:49 GMT

In article <rlewis.722894661@muskwa.ucs.ualberta.ca>, rlewis@muskwa.ucs.ualberta.ca (Robert Lewis) writes:
> drobinso@mendel.une.edu.au (D.Robinson) writes:

>>A more interesting question is: are Dutch cyclists without helmets
>>safer than American riders who wear them?
>>From my limited experience, including cycling both in Holland and America, 
>>I'd guess the answer to that is yes.

>Table 2.  Percentages by age group and by gender of journeys made by 
>bicycle (i.e., of all cycle journeys, % made by each age group).  
		11-15	16-20	21-29	30-59	60-64	65+
>Males
>U.K.		47.3	20.8	 9.2	 7.4	 7.4	 7.8
>Netherlands	30.8	24.3	11.3	 9.3	10.9	13.4	

>Females
>U.K.		37.3	17.6	12.7	14.7	12.7	 4.9
>Netherlands	28.4	22.1	12.5	14.4	11.8	10.9

>Extrapolated from:  British Medical Association (1992).  Cycling 
>Towards Health and Safety.  Oxford:  Oxford University. p. 73.

>What this shows is that in the U.K. those groups which have the highest
>accident rate (males 11-15 and 16-20) also makes up a much larger 
>percentage of the cycling journeys than in the Netherlands....

>that difference be explained by the different cycling populations rather
>than to external causes (i.e., drivers, facilities, weather, etc.).  In

Try doing a few elementary calculations on your own data!

Mike Hudson et al (Bicycle Planning, 1982) quoted a Metropolitan Police
study which judged cyclists under 15 contravened a law or code in two
thirds of their accidents, compared with one third of accidents for cyclists
16 and over.  Assuming the probability of the motorist being at fault
is roughly independent of the cyclist, then we have:
P(adult accident) = 0.5Mf + Mf
P(child accident) =   2Mf + Mf
where the first term represents the probability of the cyclist being at
fault, and the second the (constant) probability of the motorist erring.
This gives us the figure that children are twice as likely to be involved
in an accident as adults (from the ratio 3Mf/1.5Mf = 2:1).  Motorists are 
nearly always involved in bicycle fatalities, so this approach seems valid.     

Now adjust for the age differences between UK and Holland, assuming under
16s are twice as likely to have an accident and 16-20s, say 1.25.  Ditto
for the over 65s, who also tend to have more accidents.
Risk(UK Males)    = 2*47.3 + 1.25*20.8 + 1* 9.2 + 1* 7.4 + 1.25* 7.8 = 155
Risk(Dutch Males) = 2*30.8 + 1.25*24.3 + 1*11.3 + 1*10.9 + 1.25*13.4 = 140
So, the adjustment factor for population demographics of Dutch vs UK
males is 155/140 = 1.09.  The figure for females is 1.04.  Leaning towards
the males who generally cycle more, suggests a figure of 1.07.
From personal experience of cycling both in UK and Holland, I'd be prepared
to bet that there were more occasional cyclists in Holland than the UK,
reducing the adjustment, but we'll leave it at 1.07.

Your figures for Holland implied 36 million km cycled between fatal accidents.
Adjusting to UK cycling population reduces that to 34 million.
My figure for UK (1974) is 11 million km.  Now in Australia, compulsory
helmets seemed to results in an 11% increase in deaths and serious injuries
both to child and adult cyclists, at a time when accidents to all other classes
of road users went down quite dramatically and might have been expected to
do so for cyclists if the legislation had not been introduced.  However,
assuming this was because the loss of cyclists counteracted the benefits of
helmets, we could use the figure, also from 'Bicycle Planning' that half of
bicycle fatalities are due to head injuries alone.  

If helmets prevent ALL of these, then the UK figure would have been 22 million 
km, still well short of the adjusted Dutch figure of 34 million km.
So, we can certainly conclude that a non-helmeted Dutch cyclists are safer
(in terms of fatalities) than a helmeted UK cyclist would have been in 1974.
Cycling in the UK may have become safer since then due to more 
cycling and better facilites for cyclists, so the current situation may be 
more finely balanced.  

As for USA, and Canada, let me mischievously requote John Forrester's
figure of 27.6 million km between fatalities for all American road users
in 1965, or the 'World Road Statistics' (International Road Federation) figure 
of 31 million km between fatalities for all Canadian road users in 1987.  
Given these figures, I would very much doubt if cycling in Canada is much
safer than it was in the UK in 1974, so the agrument still stands!  :(
D.Robinson "Bike it, you'll like it; .sigs waste bandwith!@"