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Nov
05
2009

Seat belts: another look at the data

I am grateful for a question posted today by Carsten Jasner in response to an earlier post of mine – Seat belts again. It has prompted another look at the data:

“Very interesting! But when the number of car occupant deaths increases while the number of all road user deaths decreases – how can the number of pedestrians and cyclists [deaths] also increase?”

Figure 1, all road accident deaths (excluding motor cyclists), shows that a well-established downward trend was interrupted (by the seat belt law?) and replaced by a slightly rising plateau. After the seat belt law (arrow) total deaths did not fall below the 1983 level until 1991.

Figure 1

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(Source of statistics: http://www2.dft.gov.uk/adobepdf/162469/221412/221549/227755/rrcgb2009.pdf)


For many decades, as car ownership increased in Britain the number of people moving about in cars also increased while the numbers moving about on feet or bicycles, and exposed to the risk of road accidents, fell sharply. Part of the decline in walking and cycling can be explained by the shift to car travel; another part by the fact, that as the volume of metal in motion increased, children were withdrawn from the threat, while vulnerable adults, especially cyclists, withdrew themselves. Figure 2 shows a dramatic decline since 1930 in the ratio of pedestrians and cyclists killed to car occupants killed – from 5.95 in 1935 to 0.47 in 2006.

Figure 2

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Figure 3 zooms in on more recent years. Between 1970 and 1982 the ratio dropped from 0.96 to 0.81. In 1983, the first year of the seat belt law, the ratio jumped sharply to 1.00, before resuming its historic downward trend, but it did not drop below 0.81 until 1989. This sharp jump is of course exactly what one would expect in the light of the decrease in car occupant deaths and increase in pedestrian and cycling deaths coinciding with the seat belt law noted earlier. The step change in the trend suggests that each year since 1983 the seat belt law continues to deserve credit for the deaths of vulnerable road users, who but for the law would still be with us.

Figure 3

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4 comments

2 pings

  1. Carsten Jasner says:

    Thanks for your reply! There is something I haven´t understood yet: the historic downward trend of all road accident fatalities since the 1970s (from 7,000 to 2,500). Though more and more people have been moving around in cars. What causes this trend? I can imagine that parents withdraw their children from the streets. But that can´t be the only explanation.

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    Carsten
    Ruben Smeed first observed this phenomenon in an article in 1949. He found it to be universal. Every country for which he could find data experienced a decrease in deaths per vehicle as vehicles per capita increased. (See my discussion of the Smeed Law in Risk and Freedom Chapters 2 and 7.) There are myriad adjustments to the growing threat of traffic. Anyone who has lived in a country at the early stages of motorization will be familiar with a very different driving style. Car owners tend to be rich and powerful and drive with disdain for the chickens and pigs and peasants in their path. If you live in a village with little traffic you do not spend a lot of time drilling your children on the Green Cross Code. The withdrawl of children is a significant part of the explanation. In 1922 in Britain there was very little traffic and a nation-wide 20mph speed limit – and there were more than three times as many children killed in road accidents than today. The Smeed Curve might be described as a social learning curve.

  2. David Hembrow says:

    I think the “withdrawal of children” as you put it is indeed a large part of the reason. There are simply fewer vulnerable road users on the streets of the UK for cars to crash into.

    This is one of the things which is very different about the Netherlands. Dutch roads are overall the safest in the world and they are so despite a much larger percentage of vulnerable road users.

    However, over here it is still the case that it’s extremely difficult for drivers of cars to crash into cyclists. As you can see in this video taken near our home.

    The Dutch car ownership rate was overtaken by the UK some years ago.

  3. Steve Jones says:

    I should say that I’m far from convinced by your use of statistics.
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    I am unclear as to whether this comment applies to this post only or also to my 16 other posts on the subject plus the essays and books listed on my website. Some of your complaints have been addressed elsewhere.
    JA
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    The use of epidemiological techniques in complex situations such as road safety, which are subject to huge numbers of social, legal and technical variables is tricky, to say the least, and very open to special argument. It seems to me that you’ve had to resort to all sorts of special explanations as to why several statistics (like the disproportionate drop in pedestrian deaths) don’t conform with the theory you espouse. I’m also not wholly comfortable with the use of trends, often without causative explanations (decreasing road fatality rates don’t just happen – it is due to a complex mixture of technical, legal and social issues which overlap).
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    Agree. Please see Smeed’s Law: some further thoughts

    The challenge is to isolate the effect of specific safety interventions from all the other variables contributing to the trend. In cases where the effects of interventions take place over an extended time period this can be very difficult. But occasionally “natural experiments”, in which interventions promising immediate large effects are introduced in some jurisdictions and not in others, make such isolation possible. One can look for the promised “step effect” in the trend and one can compare statistics in jurisdictions that introduced the safety measure with those that did not. Seat belt laws and motorcycle helmet laws have provided such opportunities. See Risk and Risk and Freedom
    JA
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    However, leave aside this for the moment and let us grant you your theory for argument’s sake. I want to know where the logic of this takes you. Unless I’m misunderstanding it, you would argue that any technology which makes the car driver feel safer will lead (on average) to those drivers taking more risks at the expense of others (maybe at their own risk to, but presumably the net effect is still beneficial to them). Most obviously this applies to seat belts, but presumably other overt safety technologies such as air bags, obviously better crash protection, cushioned dashboards/steering wheels and so on would have the same effect. Indeed I suspect we could extend that thought to proxy effects (like a family’s children being strapped into child seats compared to be unrestrained). Many of these safety measures are also mandated by various legal systems.
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    “I suspect we could extend that thought to proxy effects (like a family’s children being strapped into child seats.” Indeed I have; see The Failure of Seat Belt Legislation pp 14-15.
    JA
    ***************************************
    So, given that you are against the mandatory wearing of seat belts, does this also extend to other mandatory safety rules which you might also believe would lead to drivers taking compensatory risks? Take the logic a bit further, and if it is the fate of other road users you are concerned about, wouldn’t the logic be to reduce the overt protection of car occupants? I’m trying to work out what you would do if the seat belt law was repealed. Are you suggesting we would take measures to discourage seat belt wearing on the basis that it endangers other road users? If seat belt wearing continued at the same rate on a voluntary basis, presumably we would not realise the social benefits that you claim.

    Note this is completely different to the libertarian argument for not enforcing seat belt wearing that you also put forwards. However, we should be aware that such freedom is not cost free for the rest of us. In the UK at least, the state pays for many of the medical and social consequences of car occupants who are seriously injured, maimed of killed (not to mention the effect on insurance costs for the rest of us as, for sure, there will be increases in personal injury claims by the unrestrained where they are not to blame – indeed your argument is that they are less likely to be the risk takers in accidents with the restrained). Then there is also the issue of minors to be considered – are their parents to be free to carry their children around unrestrained in cars? Presumably the parents will drive that much more carefully.
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    “Presumably the parents will drive that much more carefully.” Apparently they do; see reference given above. Considering “the issue of minors” further – see One False Move …

    “wouldn’t the logic be to reduce the overt protection of car occupants?” Yes if can be shown a “safety” measure increases the overt danger to more vulnerable road users. Over many decades “road safety” has focused overwhelmingly on measures that make vehicles more crashworthy and roads more forgiving of heedless driving. On occasions when it has focused on those not in cars it has favoured measures that demand deference to traffic – forcing pedestrians through tunnels or over foot bridges or obstructing their routes with pedestrian barriers – promoting, and in some jurisdictions compelling, the use of cycle helmets and generally through education programmes and advertising campaigns fostering the idea that streets are no longer safe for pedestrians and cyclists, especially children. The result has been large reductions in walking and cycling and in children’s independence.
    JA

  4. Steve Jones says:

    I’m not sure that the position can be taken that road safety has focused overwhelmingly on the matter of technical safety issues.

    There have been a considerable number of changes to the legal system too, and not a small amount on changing the social acceptability of irresponsible driving. Off the top of my head, since the 1960s, we have seen the following initiatives to change driving behaviour including the imposition of national speed limits, the gradual reduction of such speed limits and more rigorous enforcement, the introduction of speed and traffic light cameras, legislation on drink driving, the introduction of tachographs into lorries and any number of campaigns on road safety awareness (targeted at both drivers and pedestrians).

    I seem to recall that RJ Smeed’s interpretation of his own empirical law on road fatality rates was that there was some sort of acceptable risk level to which drivers would somehow adjust. There’s probably a lot in that, but what also has to be allowed for is changes in public perception of risk and changed attitudes of what is socially acceptable behaviour (not just driving of course). It’s also fairly clear that social factors must surely be at the bottom of why there is such a wide disparity in road fatality rates in different countries with similar levels of development. It’s easy to find examples, such as Belgium (where, in 2005, the road fatality rate) was about 90% higher then the UK’s (or Switzerland’s ow Sweden’s), which surely can’t be explained in terms of the safety standards of cars and road environments. Social attitude differences must surely play the major part in such discrepancies. Personally, I rather think that the widespread use of such things as seat belts help builds up a sort of safety-awareness culture. Of course this is unprovable, but what is blindingly obvious is that something has surely changes the public’s perception of acceptable risks – at least in the UK and some similar countries.

    I’ll certainly accept the point that children’s freedom has been constrained with regard to reducing their exposure to traffic. However, that is hardly confined to traffic hazards – it can also be seen as a considerable attitudinal and social change to child risk in general. We see this with the various measures taken in the areas of child safety with regard to any number of matters, whether it is playground safety, the perceived dangers of predatory paedophiles and the reaction to the MMR scare. I rather suspect that this sort of increased social sensitivity to the danger to children is a factor (and in terms of children’s safety it has, generally, worked across many areas albeit with an obvious reduction in their freedom. It also has to be born in mind that children (at least among the relatively affluent) also get a lot of social benefit from private transport. These things are a complex series of trade-offs and fixating on one measure (child pedestrian deaths, and when that doesn’t work out, the reduced freedom of children in one context) strikes me as possibly losing sight of the bigger picture.

    Also, when it comes to the wider picture of danger to more vulnerable road users, among them I would assume you would include motorcycles, then there is the obvious contradiction issue that they are disproportionately responsible for killing pedestrians (over three times the rate per mile of cars). So, quite apart from the truly appalling carnage among motorcyclists themselves, this very same victim group (by the vulnerability definition you use) are also more likely to kill a pedestrian.

    Now I’m deeply suspicious of pure utilitarian arguments with regard to social matters (or we would ban at least high-powered motorcycles outright), but I can’t help but think that the overall trend on road safety, compared with the social benefits, have served us reasonably well – certainly by international comparisons. The simple fact is (that with a exceptions over the odd year or two), road accident rates have dropped among pretty well all road users, and among many of the most vulnerable, most of all.

  1. Selective Application of Smeed’s Law | Pedestrian Observations says:

    […] over time. Safety improvements do not bend or break the general trend. Quoting Adams again, the introduction of seat belts caused no reduction in traffic fatalities, and on the contrary caused pedestrian fatalities to temporarily inch up, as drivers felt safer and […]

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