Cycles of all sorts

There is a long-running argument over the use, promotion, and compulsion of cycle helmets. Most heated controversy surrounds laws making helmet use compulsory, particularly regarding the substantial disparity between claimed injury savings in small-scale prospective studies (e.g. Thompson, Rivara and Thompson, 1989) and later, more comprehensive studies, particularly from jurisdictions which have used compulsion to substantially raise helmet use over a very short period. Helmet use in New Zealand, for example, rose from 43% to over 95% in under three years, with no measurable change in head injury rates (Scuffham, 1997).

Controversy is fuelled by support given to the pro-compulsion movement by Bell Sports in particular, and by the fact that many of the most vocal proponents of helmets are not themselves cyclists.

Various organizations have taken up definite positions on the issue, not always based on a full review of the evidence. For example, the British Medical Association used to be against helmet compulsion, following an extensive review of the evidence in 1999. In late 2004 the BMA's Board of Science and Education adopted a 'position' calling on the UK government to introduce cycle helmet legislation, and this was confirmed at the 2005 Annual Representative Meeting following fifteen minutes of debate (transcript). The BMA's new position use statistics provided by the British political lobby group, the Bicycle Helmet Initiative Trust, and exclude from consideration the majority of conflicting evidence, including the BMA's own previous work. Several provably wrong figures were removed after initial publication, but the review is still viewed as distorted, excluding not only references included in the 1999 BMA study, but the 1999 study itself. Debate continues within the BMA.

The World Health Organization is currently awaiting the results of a Cochrane review of bicycle helmet legislation before forming a view on whether to support compulsion.

Overall, according to CTC, the UK's national cyclists organisation, "the evidence currently available is complex and full of contradictions, providing at least as much support for those who are sceptical as for those who swear by them."

Reduction in bicycle participation

Mandatory bicycle helmet laws lead to a reduction in the number of cyclists. For example, when mandatory bicycle helmet laws were enacted in Australia, slightly more than one third of bare-headed cyclists ceased to ride their bicycles frequently. The reduction in the number of cyclists is likely to have a greater negative impact on the health of a population, than would have resulted from any increase in injury. The long term health benefits of bicycle use are manifold and extensively documented, and so any reduction in bicycle activity will likely have a negative impact on the overall health of a population.

As well as being an extra expense, cycle helmets make cycling less convenient; they are bulky and often cannot be stored securely with bikes. They are incompatible with some hairstyles, forcing bicycle users to recreate their hairstyle after each journey.

Cycle helmet promotion or high levels of use may deter cycling by reinforcing the misconception that bicycling is more dangerous than traveling by passenger car.

Finally, others have argued that the wearing of bicycle "crash-helmets" has subjected children to ridicule. For example, in the 2006 film The Benchwarmers, the character Clark -- played by Jon Heder -- sports a bicycle crash helmet as an accessory prop to highlight his lack of social skills and physical coordination. If children are dissuaded from learning to cycle because of this, they will be considerably less likely to ride as adults.

All of these factors will lead to an increased risk for those cyclists remaining on the road, due to a "safety in numbers" effect. The probability of an individual cyclist being struck by a motorist declines with the 0.6 power of the number of cyclists on the road. This means that if the number of cyclists on the road doubles, then the average individual cyclist can ride for an additional 50% of the time without increasing her probability of being struck. It is thought that the increased frequency of motorist-cyclist interaction creates more aware motorists.

Helmets and risk of injury

Many believe that a helmet can save a cyclist's life, an idea which is repeatedly asserted in debate. There is no known evidential basis for this claim and there are no known cases where mass helmet use has actually reduced the number of cyclists' deaths or serious head injuries. Association with increased risk of death has been reported. It is likely that helmets could prevent a significant number of minor cycling injuries but the overall safety benefits are inconclusive; this is thought to be in part due to risk compensation behaviour. A cost-benefit analysis of the New Zealand helmet law showed that the cost of helmets outweighed the savings in injuries even taking the most optimistic estimate of injuries prevented.

While a helmet may mitigate the effects of a fall or collision, other factors (such as maintenance, road conditions, and driver behaviour) are more important for reducing the chance of such accidents in the first place. In general, the value of bicycle helmets has been systematically overstated.

Some studies have even suggested that helmets increase risk. Although the head injury rate in the US rose by 40% as helmet use rose from 18% to 50%, this does not necessarily mean that helmets themselves increase risk. In fact, a range of theories exist to explain the observed disparity, including:

• Risk compensation: helmeted cyclists may ride less carefully; this is well supported by evidence for other road safety interventions such as seat belts and antilock brakes. Additionally, a 2006 British study found that vehicles pass helmeted cyclists with measurably less clearance (8.5 cm/3.5") than that given to unhelmeted cyclists (out of an average total passing distance of 1.2 to 1.3 metres), indicating additional risk compensation by drivers too.
• Poor fitting
• Sampling bias in prospective studies: voluntary wearers may be more risk averse, skewing the results.
• Recent evidence from Walker (Accident Analysis and Prevention) suggests drivers drive closer to helmeted cylists.

No research has yet been published which adequately addresses the reasons for the disparity.