Associations observed between relative risk of collision, cyclist growth, and mean cyclist density over time averaged under the HiDens scenario (source: Thompson et al)

The Safety in Numbers effect is one of the most powerful ideas in cycling advocacy. Writing in Vox last week, Joseph Stromberg claimed:

It’s been proven over and over again that the most important predictor of a city or region’s level of safety for bikers is the number of bikers on the road.

This is a very attractive proposition for cycling advocates. Its beauty is it promotes cycling on roads while simultaneously ameliorating the biggest deterrent to riding i.e. safety.

Although some of the research is a little flaky (it’s hard to get reliable numbers on how much cycling there is), the existence of the Safety in Numbers effect is well accepted.

The key explanation offered by researchers for the phenomenon is behavioural adaptation by motorists; they drive more carefully in places where there are large numbers of cyclists.

The underlying dynamic is less clear. It might be because there’s a greater likelihood in a cycling city like Amsterdam that motorists are themselves cyclists and so more conscious of riders’ vulnerability.

Another possible reason is that larger numbers of riders make drivers more aware of their presence. Or perhaps large numbers reinforce the legitimacy of cyclists as road users.

There are grounds though for thinking there might be more to it. As I noted before (Does cycling on roads put your health at risk?), cycling fatalities and serious injuries in London increased in recent years at a faster rate than the rise in cycling. So perhaps the effect doesn’t apply in all situations.

And behavioural adaptation doesn’t seem to be the only possible explanation either. As R J Smeed found in a 1949 study of 62 countries, motoring fatalities also decline as the level of driving increases. The reason is slower speeds due not to behavioural adaptation but to traffic congestion.

Another explanation might be that it’s due to changes in the types of cyclists. As the total number of riders increases, it’s likely the proportion who’re cautious and hence less likely to crash starts to dominate.

Or perhaps it’s a chicken and egg argument; places with large numbers of cyclists are also safe because they have better infrastructure. That in turn encourages more travellers to ride.

A new research paper published last month in Traffic Injury and Prevention, Reconsidering the safety in numbers effect for vulnerable road users (gated), offers a new perspective.

The authors cautiously conclude the Safety in Numbers effect is not really about ‘numbers’ per se. Rather, what seems to be the key factor improving safety is the density/dispersion of cyclists within a place.

Jason Thompson, Giavanni Savino and Mark Stevenson constructed a virtual traffic system comprised of bicycles and cars and controlled by simple decision rules. They were able to replicate the safety in numbers effect in the simulation without recourse to psychological explanations:

In simulated scenarios where bicycle density increased over time alongside increasing bicycle numbers, per-capita risk of collision decreased. Conversely, where growth in bicycle density was lower, per capita collision risk decreased more slowly with increasing bicycle numbers. Further, when bicycle density growth was low and remained relatively stable over time, the association between increasing bicycle numbers and collisions more closely resembled a linear relationship, effectively nullifying the (Safety in Numbers) effect.

Since it’s a computer simulation, the finding that density matters more than numbers has nothing to do with changes in behaviour arising from increased exposure. The authors offer two possible explanations:

Firstly, increasing bicycle density coinciding with increasing volume may have the simple consequence of reducing the proportion of surface area per cyclist exposed to danger from cars… a second, closely related explanation drawn from the biological sciences is the influential ‘selfish herd’ theory…(It) proposes that increased density brought about through aggregation (e.g., bird flocking, fish schooling, etc.) is an adaptive mechanism that reduces risk of predation by minimising high-risk exposure at the periphery of groups

Their findings suggest that even big increases in the number of cyclists on the roads might not improve safety if riders are dispersed across the road network.

Conversely, safety can be improved even when cycling numbers are static or falling, provided riders who cycle on roads are concentrated on a limited number of high density routes.

Segregated infrastructure is the gold standard but where cyclists share roads with motorists (as they undoubtedly will in Australian cities for many years yet), failure to plan with the objective of increasing rider density might have adverse consequences.

It is possible that cycling ‘activism’ and desire to reclaim territory from cars in cities where cycling has experienced a relatively recent cultural renaissance may inadvertently play a role in increasing exposure to risk.

While there’s no doubt cycling is safer in places with more cyclists, the reasons aren’t as straightforward as is commonly assumed. As Thompson et al point out, there’s little published evidence to support the contention that behavioural adaptation by motorists is a likely explanation.