You knew it was dangerous. But you let people go swimming anyway.
Let us imagine a situation on a holiday resort island – call it ‘Amity Island’ – which is subject to increasing numbers of shark attacks on its numerous beaches. The authorities are rightly concerned, and commission a study, which discovers a correlation between the numbers of people swimming at different beaches, and the number of shark attacks.
Namely – it is found that there are more shark attacks, per person swimming in the sea, at those beaches were there are fewer people swimming. And, conversely, it is (relatively) safer to swim at beaches where there are plenty of people swimming. On these beaches the rate of shark attacks, per person swimming, is lower.
It is then hypothesised that perhaps it is actually the numbers of people swimming at these more populated beaches that is somehow responsible for the lower rate of shark attacks. Perhaps it is something connected to the numbers of people in the water that discourages the sharks from attacking? In other words, the reason might be that the sharks’ behaviour is modified by the number of people in the water – lots of people mean they are less likely to attack.
No hard and fast evidence for this change in the sharks’ behaviour is discovered, but nevertheless a plan is put forward to encourage more people to swim in the water on those beaches where people are more likely to suffer a shark attack. After all, there is a proven link between the risk of shark attack, and the numbers of people swimming. Thus, if you can simply get more people swimming on the more dangerous beaches, then swimming becomes (relatively) safer for all. The more people that swim, shark attacks become less likely (for each person swimming).
Except, of course, it isn’t. This isn’t a very sensible way of dealing with the problem of shark attacks, for one particular reason. There might be confounding variables, which make the best beaches for swimming – hence the most popular – simultaneously the least likely to suffer shark attacks (per person swimming).
Another problem is that local knowledge of the beaches, and where sharks are likely to attack, might explain why fewer people are found swimming at the more dangerous beaches. In other words, sharks are keeping people away from certain beaches, rather than people keeping sharks away from others. Put simply, the correlation could run in the other direction.
If either of these explanations are true – and they could well be – then simply encouraging more people to swim on the most dangerous beaches is hopelessly misguided, perhaps even immoral.
It’s immoral in the further sense that it doesn’t actually reduce the absolute risk of shark attacks – which could be achieved by, for instance, erecting barriers to keep sharks away from the beaches completely, or similar measures. The ‘Safety in Numbers’ policy only aims to reduce the relative risk of people swimming. Even if it works, more people could actually die on beaches as a result, although each person swimming would be relatively safer.
Yet this kind of approach to the safety of cyclists and pedestrians has gained a worrying degree of traction over the last decade, despite a similar lack of evidence behind the theory. One of its exponents has been the Mayor of London, Boris Johnson, who by coincidence or otherwise has written
My hero is the mayor in Jaws. He’s a fantastic guy, and he keeps the beaches open, if you remember, even after it’s demonstrated that his constituents have been eaten by this killer fish. Of course he was proved catastrophically wrong in his judgment, but his instincts were right.
‘Safety in numbers’ is attractive as a theory because it requires very little financial or intellectual investment in trying to improve conditions for the most vulnerable users. All that needs to be done is to ‘encourage’ more walking and cycling, without the hard effort required of improving the environment itself (which, incidentally, does have a proven record of making walking and cycling safer).
The modern history of this idea can be traced back to a paper by P L Jacobsen, published in 2003 in the journal Injury Prevention. Looking at various data sets at a variety of scales (the amount of walking and cycling in 68 different Californian cities, the distance walked and cycled in Danish towns, walking and cycling rates in a number of European countries, and the total amount of annual cycling in the UK and the Netherlands) and comparing them with fatality/injury rates, Jacobsen argued
A motorist is less likely to collide with a person walking and bicycling if more people walk or bicycle. Policies that increase the numbers of people walking and bicycling appear to be an effective route to improving the safety of people walking and bicycling.
The one graph in Jacobsen’s paper that might be of particular interest to UK cycling campaigners – who are often deeply attached to the Safety in Numbers concept– is this one –
Does “increasing the numbers of people walking and cycling” mean that fatalities are less likely, on the evidence of this graph? Well, not really. The fatality rate in 1950 is virtually identical to that in 1999, despite a vastly greater amount of cycling in 1950. Likewise cycling was much safer, relatively, in 1999 than it was in 1972, despite a virtually identical amount of cycling being carried out in the UK. And perhaps most damningly of all, from 1983 to 1999, people cycled less, and yet it became safer to do so (the graph runs down towards the origin).
Those who think that the best way to create safer cycling is just to get more people cycling should take a long hard look at this graph, because frankly it’s a complete mess, that shows no clear relationship at all between the amount of cycling, and relative safety.
However, Jacobsen himself is undaunted, stating that the relationship is ‘complex’, and ties himself in knots coming up with intervening variables that account for the way the graph shoots all over the place (intervening variables like, for instance, the amount of driving, or even seatbelts, which are not accounted for by the ‘Safety in Numbers’ hypothesis).
No such problems with another graph Jacobsen includes, comparing cycling rates with safety, but this time in the Netherlands, from 1980-98 –
By comparison, this relationship is more straightforward. More cycling, from 1980 onwards, corresponds more or less directly with greater relative safety.
While Jacobsen concedes (somewhat vaguely) that, over this period,
the Netherlands has implemented a range of policies to encourage people to walk and bicycle and make them safer. These efforts have succeeded in increasing bicycle use and decreasing risk.
He then goes on to state that
it is improbable that the roadway design, trafﬁc laws, or social mores, all of which change relatively slowly, could explain the relationship between exposure and injury rates. The more plausible explanation involves changes in behavior associated with changes in the amount of walking and bicycling.
In other words, the reason cycling has become safer is not because of changes to the physical environment; rather, the “more plausible explanation” is that drivers are behaving differently now that they are surrounded by more cyclists.
I’ll leave you to judge whether that is indeed more plausible; what is interesting is that this explanation of improved driver behaviour as a consequence of being surrounded by more walkers and cyclists is only a hypothesis. It is not substantiated by Jacobsen. He cites research showing that motorists drive slower when surrounded by more pedestrians (not, in itself, an indicator of better or safer driving), and also speculates that
motorists in communities or time periods with greater walking and bicycling are themselves more likely to occasionally walk or bicycle and hence may give greater consideration to people walking and bicycling.
The operative word here being ‘may’. There isn’t any research cited; this is pure speculation.
the most plausible explanation for the improving safety of people walking and bicycling as their numbers increase is behavior modiﬁcation by motorists when they expect or experience people walking and bicycling.
And that’s it. It’s very thin stuff indeed for what has proved to be the foundation of nearly a decade of ‘safety’ promotion amongst transportation engineers, urban planners and walking and cycling advocates. Just last month I read Rob Penn arguing, in the Observer, that
Right now… we need to keep transmitting a positive message, a message that extols the utilitarian, democratic virtues of the bicycle, a message that encourages people to wheel their steeds out of the garage and into the sunshine. Which brings me back to my first point: if you increase the number of cyclists, it becomes safer to cycle. [my emphasis]
This argument has been repeated so often it is perhaps a little unfair to single one person out for parroting it – this is just the most recent example – but the causal relationship posited in bold simply isn’t substantiated. Jacobsen only identified a correlation (and a deeply shaky one at that, if we re-examine the graph of cycling and risk in the United Kingdom). He did not establish a causal relationship – only assumed one! – and on top of that only speculated about a possible cause that might explain that relationship, namely improved driver behaviour.
That cause has not been established. In a critical appraisal of the ‘Safety in Numbers’ concept by Rajiv Bhatia and Megan Wier, published last year in the journal Accident Analysis and Prevention, the authors write
A popular explanation of [the non-linear relationship between numbers of walkers and cyclists and the rate of injury or death] is that an increasing numbers of walkers or cyclists directly inﬂuences hazardous behaviors of motor vehicle drivers; hence, the term “Safety in Numbers” (Jacobsen, 2003). However, this causal inference has not been subject to a thorough critical examination in the peer-reviewed literature.
In other words, we still don’t have any evidence that driver behaviour is actually modified or improved by the presence of more cyclists to the extent that there might be pay-offs in terms of the safety of the latter group.
Beyond serious concerns about the lack of evidence for a plausible mechanism behind the ‘Safety in Numbers’ effect, the authors also provide a succinct discussion of the serious challenges to the relationship itself, namely Temporal Direction and Confounding (these two problems were touched upon in my hypothetical introductory discussion of shark attacks).
Clearly the temporal direction of the relationship needs to established; we need to see whether more pedestrians or cyclists on a street subsequently ‘create’ more relative safety, rather than safer conditions leading to more walking and cycling. However, just as with the ‘explanatory mechanism’, the evidence for the relationship running in this direction is lacking. The authors write
to our knowledge, there have been no experimental or prospective studies that have evaluated changes in pedestrian volume on changes in injury frequency or rates.
research demonstrates that walkers have a strong value preference for lower speeds, lower traffic volumes, and greater buffers between pedestrians and motorists.
(It is unstated in the paper, which largely focuses on pedestrians, but research into the preferences of cyclists quite obviously finds similar results). In other words, the best evidence suggests that the relationship runs in exactly the opposite direction to that commonly assumed – namely, it is safety that is producing numbers.
So if it turns out that people are staying away from beaches where there are shark attacks, quite obviously it shouldn’t be sensible policy to encourage more people to swim on those beaches. Yet that is what the Safety in Numbers concept, as so commonly and crudely interpreted, suggests.
Likewise on the subject of Confounding, Bhatia and Wier are quite clear that safer environmental conditions could result in both more walking and cycling, and fewer injuries; traffic law and enforcement, and social norms, could have similar (although less well-established) effects. This is the most plausible explanation for the relationship exhibited in the Netherlands; an explanation Jacobsen was quick to dismiss.
The authors further argue that
Since, at the population level, more walking typically means less driving, a reduction in collisions per walker could also potentially be explained by lower traffic volumes, an established and potent determinant of collisions
In other words, more people walking and cycling may indeed result in greater relative safety, but not because of the modified behaviour of drivers. Instead, more straightforwardly, the relative safety comes from the fact that there are simply fewer drivers on the road.
It is this explanation – alongside a failure to change the physical environment for cycling, like in the Netherlands – that I think most persuasively explains why the increasing numbers of cyclists on Britain’s roads has not seen a corresponding reduction in the rate of injuries and fatalities, Indeed in recent years, the ‘Safety in Numbers’ relationship has gone into reverse – despite more cycling, the rate of KSIs (Killed or Seriously Injured) has increased, which is the exact opposite of what the proposed effect suggests should happen. As The Times noted recently
The rate of cyclists killed and seriously injured measured as a proportion of distance travelled rose by 9 per cent in 2011. It was the third consecutive year in which the rate of death and serious injury amongst cyclists had increased. The data undermined Government claims that it is becoming ever-more safe to cycle as increased numbers of cyclists take to the roads in Britain.
This shouldn’t really surprise us – if we trace the graph of fatalities/billion km travelled against the amount of UK cycling presented in the Jacobsen paper, we could easily be heading back towards 1983.
The problem is two-fold. There are still plenty of motor vehicles on the roads, and the physical environment has not changed. Clearly if everyone stopped driving, then an unchanged road environment would become considerably safer. You would only see single person accidents, or cyclist-on-pedestrian, or cyclist-on-cyclist collisions, which are far less serious.
But despite more people cycling, motor traffic has barely declined at all. So there is still, approximately, the same volume of hazard that cyclists have to deal with, on roads that are just as dangerous for cycling on. It is deeply unrealistic, therefore, to expect the rate of injury and death to decrease.
At some distant point in the future we may hit a point at which sufficient numbers of people walking and cycling, and few enough are driving, that relative safety is achieved, without any change to the environment, but that is some way off, and the route to that point is unclear, and may be littered with the bodies of cyclists and pedestrians.
In a paper published in 2009, Rune Elvik argued that a doubling of pedestrian and cyclist volume, with corresponding mode shift away from driving, would not, alone, reduce the KSI burden, and may actually increase it. Indeed, he suggests that, without any change in the environment, it is only when the amount of driving is reduced by 50% (with corresponding mode shift to walking and cycling) that we may see a reduction in the total KSI burden. That is an enormously long way to go by ‘Safety in Numbers’ alone, with an increasing KSI burden in every year that we attempt to get people to switch to bicycles. Freewheeler parodied this attitude in a recent post, writing
In a lively question and answer session the speaker was asked about recent rises in cyclists killed and injured on Britain’s roads. Sam said this showed that more people were cycling, which was very good news indeed. However, numbers were not yet great enough for the safety in numbers effect to work, which is why we must get more people cycling by spreading the message that cycling is safe.
But unfortunately this parody is not very far from the truth.
This brings us to a final problem with the ‘Safety in Numbers’ thesis, again touched on in my ‘shark’ discussion. Namely, that it is not particularly concerned with the total burden of injury or death, only with the rate at which deaths and injuries are suffered by a particular group of road and street users. ‘Safety in Numbers’, crudely applied, is quite content to see an increase in the absolute numbers of people being killed or seriously injured, if the rate at which they are being killed or seriously injured decreases.
This has serious implications for the way the safety of pedestrians and cyclists might be dealt with. Just like our resort of Amity might have decided to forgo the expense of erecting shark nets to keep the sharks away – which would have provided a higher degree of both relative and absolute safety – opting instead for the cheaper option of encouraging ‘Safety in Numbers’, so transport and urban planners might similarly decide to forgo the expense of redesigning streets in order to lower both the relative and absolute KSI toll, likewise plumping for the cheapest, and politically easiest, option. As Bhatia and Wier write
A danger with the SIN [Safety in Numbers] policy prescription is that it potentially shifts the cause and responsibility of the problem to walkers – the victims of pedestrian-vehicle collisions – and shifts attention away from fundamental change in roadway design, revision of speed policies, increased investment in pedestrian and bicycle infrastructure, gross disparities in conditions that support safe walking and bicycling in our communities, and the high absolute numbers of people being killed or injured while engaging in these sustainable transportation behaviours in the United States and elsewhere.
There is, I think, an analogous situation in Great Britain, where very little attention has been paid over the last decade to adjustment of the physical environment to make cycling safer. (There are signs – small signs – that this is about to change). Most of the attention has instead focused on behaviour, particularly the behaviour of cyclists, with attempts to train them to become safer, and also on how they act and dress. How much of this is down to widespread adoption of the ‘Safety In Numbers’ hypothesis, I cannot say, but it has certainly provided an outlet for those in positions of power who might be less inclined to grasp the nettle and make hard decisions about changing the physical environment for cycling. Again, without wishing to single anyone out, I notice that Transport for London’s Ben Plowden was sticking to this script at the recent London Assembly Transport Committee on Cycling Safety –
I’ve been cycling in London for many years. You used to see hardly any other cyclists on the road, but it’s very different now. I think you can get a “virtuous circle” where you get more cyclists, and improved behaviour by road users, leading to a “tipping point” where things change significantly. Now I see in some places 40 cyclists at every cycle of the lights. That must be having an effect on people’s perception.
Quite where this ‘tipping point’ might lie is, as will hopefully be apparent from the discussion here, really quite unknown, and the effects of ‘Safety In Numbers’ in the meantime very questionable. (The report by David Arditti on this meeting is worth reading in full because it gives a real flavour of the pervasiveness of ‘Safety in Numbers’ thinking).
Bhatia and Wier argue that the environmental determinants of injuries to vulnerable road users have been studied in depth, and it is clearly and consistently motor vehicle volume and speed, and roadway design, that are the important predictors. This evidence suggests, in their words, ‘an urgent need’ to focus on policies that reduce exposure to risk. It does not – quite clearly – suggest that we should employ policies that simply ‘encourage’ more people onto the streets, when those streets have relatively undiminished numbers of motor vehicles on them, and that are still designed in ways that create conflict.
In other words, ‘Safety in Numbers’ is a serious distraction from the actual business of making cycling safer. It is deeply unfortunate that instead of talking about the real problems facing the safety of cyclists, that debate about danger – both actual and perceived – is often discouraged by people who fear potential new cyclists might be ‘put off’. (It is also a handy way – if we are being cynical – for politicians to silence criticism of the dangers on the roads they are responsible for).
It may or may not be true that people are actually discouraged from taking up cycling by how it is discussed (I personally believe any such effect must be negligible in comparison to the everyday perception people have of the streets and roads they walk and drive along, and how inviting they might be for cycling). But if the only reason we want more people to cycle is to diminish the relative risk faced by current cyclists, then frankly I don’t think we should really care about whether they are put off. Because that’s simply not the right approach, both morally, and practically.