Cycling deaths
The New Zealand Medical Journal has this month published a review of cycling deaths in New Zealand, with the key finding being that “the helmet law has failed in aspects of promoting cycling, safety, health, accident compensation, environmental issues and civil liberties”. This is a bold claim which should be held to high scrutiny.
The journal article (accessible only by subscription, which we at the University of Auckland are fortunate enough to have) is available at the journal’s website, but for those without subscription access can only be to the media reports of it such as on Stuff.
The article itself is jam-packed full of statistics from various sources, so please bear with me.
The most important is probably Table 1, which shows that whereas pedestrian hours have remained relatively constant from 1989 to 2009, cycling hour have decreased by half, and Table 2, which shows that both pedestrian AND cyclist deaths have decreased from 1989 to 2009. Whereas both have gone down by half, the ratio has remained constant at about one quarter. However these statistics are then ‘corrected’ for the number of hours walked or cycled.
Given that cycling hours have significantly decreased by about 50%, as have the number of cycling deaths by 50% over the same period, the stark result is that cycling deaths per hour cycled have remained about constant over the study period – and certainly not evidence that the introduction of the helmet law, or any other event, has increased the accident rate. Something else is going on with pedestrian deaths altogether, which have encouragingly decreased substantially per walking hour over 1989-2009.
However, the author places emphasis on a new statistic – the ratio of cycling to pedestrian deaths. Whereas pedestrian deaths per hour have markedly gone down, cyclist deaths per hour have not. The ratio of the two means that cycling deaths have apparently increased (but importantly, only relative to pedestrian deaths).
The pedestrian deaths trend is actually a red herring, as we could well compare cycling deaths to any number of trends. According to Statistics New Zealand crime has also gone down since 1994. We could equally posit that the number of cycling deaths relative to crimes has increased, but would this be an alarming statistic? (are the criminals using bicycles as getaway vehicles?).
The article is also loaded with other fascinating statistical statements such as “that life years gained by cycling outweighed life years lost in accidents by 20 times”, which I will not cover the moral implications of here, but is that supposed to be some solace?
The key result from this study seems to in fact be that the rate of accidents for pedestrians has declined significantly over the period of the review, which has to be good news, especially prior to correction for population growth. That cycling hours have halved may well reflect increased awareness of the dangers of cycling in New Zealand.
Regardless of that main finding, the article commits one of the deadly sins of statistics, implying causation from correlation. That the helmet law was introduced in 1994 is about as relevant as TV2 beginning 24 hour programming, or the Winebox enquiry, both in that same year. We could compare trends before and after but with no experimental relationship between the process and the pattern, as tantalising as a relationship between bike helmet laws and accidents might be, it is only a correlation.
James Russell is a quantitative ecologist jointly appointed in the School of Biological Sciences and the Department of Statistics. He was the 2012 Prime Ministers Emerging Scientist prize recipient. See all posts by James Russell »
I heard this report on TV and wondered about the extent to which all the relevant factors had been considered before concluding that helmets were the problem.
For instance
(1) commuting times have been increasing (particularly in the greater than 11 Km range) and so it may be that the number of people who live in the bike-to-work distance may have decreased (bicycling being concentrated in the 2 – 5 km distance in the census)
(2) the census shows that of those using personal motive power, older workers tend to walk in preference to cycle — and the population has been aging
http://www.stats.govt.nz/publications/populationstatistics/commuting-patterns-in-nz-1996-2006.aspx
The health impacts also depend on what people are moving to. If they are moving to walking or jogging, it would be hard to see that there is a negative health impact at all.
It rather felt like the “it must be helmets” answer was predetermined in the minds of the researchers.
12 years ago
James,
I agree that the use of the cycle/pedestrian death rate ratio is dubious but disagree that it is the focus of the article — the author does not mention it in the abstract nor the summary, nor the conclusion.
The use of the ratio is clearly trying to get at the idea that we should expect cycle deaths to fall roughly at the same rate that other road deaths fall, due to better road design, better road user education, safer vehicles/bikes, more helmet use etc. That we haven’t seen that drop in cycle deaths but have seen an increase in overall injuries to cyclists is a cause for concern.
What we have also seen is a massive drop in the number of cyclists over period. This is concerning due the well documented benefits of cycling on population health and the environment (both natural and urban). You are right that we can’t draw a causal link between this drop and introduction of the law (and even the correlation is hard to pin down given that the effect on cycle use was not studied around the time the law was introduced) but it is basic economics that imposing a new cost on an activity will see a reduction in that activity, all else being equal.
A priori, it is reasonable to argue that the imposition of compulsory helmet laws reduce the number of cyclists; when there are fewer cyclists on the road, the ones who are there face a greater danger; and even that cyclists wearing helmets are more prone to accidents. Studies like this do not prove those arguments but do lend some support to them.
The author’s conclusion is that “road safety and cyclist’s safety should be improved by coherent policies, which support health, the environment, and without the legal requirement to wear a helmet.” Given the evidence presented in the article, if the last condition were changed to “…and the legal requirement to wear a helmet should be reviewed”, it would be hard to argue with, no?
12 years ago
James, I disagree with your analysis. I remember reading somewhere (I know, that carries weight don’t it?) that the main statistical consequence of the mandatory helmet law was a halving of the population of cyclists (as a consequence of it being thereafter considered as a dangerous activity). Not a reduction in the rates or the seriousness of injuries, or anything like that. In fact it is shown that the mandatory helmet has had no statistical impact whatsoever there.
12 years ago
The key point here is that the helmet law was not needed and is a total failure. The message it sent out to the public was that cycling is a dangerous sport and people shouldn’t participate in it. The courts in New Zealand in turn take the view that if a cyclist is killed by a careless driver, the cyclist is at fault for participating in such a dangerous sport – this is evidenced by the light (or no) sentences handed out to drivers who kill cyclists in NZ. If anything the helmet law has made cycling more dangerous, fewer cyclists on the roads means drivers take even less notice of anything besides other cars.
12 years ago
The belief that the helmet law was not needed and is a total failure would be better served by a social sciences type analysis for the causal pathway suggested (i.e. public views, court rulings, driver behaviour). A correlative statistical analysis such as that presented does not support that argument. Importantly, it should be recognised that the helmet law is not intended to prevent accidents (a car will still crash in to you whether you wear a helmet or not), but to reduce the likelihood of a traumatic head injury conditional on you having an accident. I suspect whether cyclists wear helmets or not will not change the behaviour of drivers or rulings of courts. However if a cyclist chooses not to wear a helmet but has a major head injury do they expect the state to cover their health costs? Driving accidents (regardless of whether other drivers, cyclists or pedstrians), like hunting accidents, are treated differently to manslaughter charges by virtue of legal history and cultural context – cycling is not special in this regard.
12 years ago