Bike helmets and safety: a case study in difficult epidemiology

Ben Goldacre and David Spiegelhalter have published a paper in the British Medical Journal called "
Bicycle helmets and the law
", exploring the complex epidemiological conundrum presented by research on safety and bike helmets. As Goldacre pointed out, this is a perfect teaching case about the difficulty of evaluating risk and its relationship to law and the behavior. The paper is short and very clearly written, and makes a great companion to Goldacre's excellent books, Bad Science and Bad Pharma.

People who are forced by legislation to wear a bicycle helmet, meanwhile, may be different again. Firstly, they may not wear the helmet correctly, seeking only to comply with the law and avoid a fine. Secondly, their behaviour may change as a consequence of wearing a helmet through "risk compensation," a phenomenon that has been documented in many fields.4 5 One study—albeit with a single author and subject—suggests that drivers give larger clearance to cyclists without a helmet.6

Even if helmets do have an effect on head injury rates, it would not necessarily follow that legislation would have public health benefits overall. This is because of "second round" effects, such as changes in cycling rates, which may affect individual and population health. Modelling studies have generally concluded that regular cyclists live longer because the health effects of cycling far outweigh the risk of crashes.7 This trade-off depends crucially, however, on the absolute risk of an accident: any true reduction in the relative risk of head injury will have a greater impact where crashes are more common, such as for children.8

The impact on all cause mortality, and on head injuries, may be even further complicated if such legislation has varying effects on different groups. For example, a recent study identified two broad subpopulations of cyclist: "one speed-happy group that cycle fast and have lots of cycle equipment including helmets, and one traditional kind of cyclist without much equipment, cycling slowly." The study concluded that compulsory cycle helmet legislation may selectively reduce cycling in the second group.9 There are even more complex second round effects if each individual cyclist's safety is improved by increased cyclist density through "safety in numbers," a phenomenon known as Smeed's law.10 Statistical models for the overall impact of helmet habits are therefore inevitably complex and based on speculative assumptions.11 This complexity seems at odds with the current official BMA policy, which confidently calls for compulsory helmet legislation.

Bicycle helmets and the law
[British Medical Journal]

(Image: 137/365: Tandem, a Creative Commons Attribution (2.0) image from bradleypjohnson's photostream)