I received the following question in the topic suggestions page:

I recently got into a debate about the efficacy of bicycle helmets and — as someone who wears a helmet religiously and credits one with once saving me from serious injury — I was amazed to find that the research into this subject is… well, I’m not sure if “messy” or “inconclusive” is the right word:

http://en.wikipedia.org/wiki/Bicycle_helmet#Science:_measuring_helmet_effectiveness

Any insight you could provide into this would be much appreciated.

As with many things, the question is more complex than it may at first appear. It might seem at first that the question is straight-forward – do bicycle helmets work? But what exactly is meant by this? Most people might assume that this means – if you are riding a bicycle and get into an accident, will the helmet reduce the severity of the resulting head injury?

We could, however, ask several other reasonable questions related to using a bicycle helmet:

Is a rider more likely to get into an accident if they are wearing a helmet? Does wearing a helmet affect the riskiness of the cyclists behavior, or does it affect the behavior of vehicle divers that might threaten cyclicsts?

What are the net effects of bicycle helmet laws? Do they discourage bike riding, and if so what is the net health effect of decreased riding?

Are there other more effective methods of protecting the safety of bicycle riders?

The first question is perhaps the easiest to answer, and the answer is a resounding, yes! Helmets do work in that they reduce the severity of head injury if you are involved in an accident. This is the common sense result as well. If your head is going to smash into something hard, you want a protective cushion between your skull and the hard object.

A 2000 review concluded:

No randomized controlled trials were found. This review identified five well conducted case control studies which met our selection criteria. Helmets provide a 63%-88% reduction in the risk of head, brain and severe brain injury for all ages of bicyclists. Helmets provide equal levels of protection for crashes involving motor vehicles (69%) and crashes from all other causes (68%). Injuries to the upper and mid facial areas are reduced 65%.

The best data we have shows that helmets are pretty effective. The lack of controlled trials means that no one has done a study where cyclists were randomized to either wearing a helmet or not wearing one, but that seems like a minor problem for the question at hand – if you are in an accident, helmets reduce the risk of serious head or facial injury.

It could, however, mean that cyclists who choose to wear helmets also engage more generally in safer behavior, leading to fewer or less serious accidents. This does seem to be the case. Some feared that cyclists who wore helmets would engage in riskier behavior because of a sense of security, but the opposite is true – helmet wearers are safer. Some of the observational data, however, looks at those who are in an accident, not the risk of being in an accident, so this does not take away much from the data showing that helmets are effective.

Vehicle drivers, however, appear to give bicycle riders without helmets more room than those with helmets. So wearing a helmet may make it more likely to be hit by a car while riding a bike.

This brings us to bicycle helmet laws, which remain controversial despite the fact that helmets themselves seem to work. A 2006 review concluded that in locations where bicycle helmet laws are enacted compliance with helmet use does go up, especially among adults, but has little effect on trends in head inuries.

Other researchers disagree, however. In a 1998 study helmet use increased significantly, 13 fold, after a helmet law was passed, and this also decreased the incidence of head injuries.

A 2007 systematic review concluded:

Bicycle helmet legislation appears to be effective in increasing helmet use and decreasing head injury rates in the populations for which it is implemented. However, there are very few high quality evaluative studies that measure these outcomes, and none that reported data on an possible declines in bicycle use.

Regarding decline in bicycle use, this remains controversial as well. The Australia study cited above showed a decline in bicycle use after helmet laws were passed. A study in Canada, however, showed no decline in ridership. Perhaps climate is a factor. Also, the Australian study was challenged for not showing the duration of the decline in riding – perhaps it is very transient.

This is an important issue because bike riding is associated with a decreased risk of cardiovascular disease. A shift from riding bikes to driving cars may have a greater negative effect on health than the benefit from wearing helmets. But this remains an open question needing further research.

There are also other measures, other than helmets, that can improve the safety of riders. In countries that have many bicycle riders, like Holland, riding safety is much greater. This is likely for two reasons. When there are more riders, car drivers are forced to be more aware and careful about those riders.

Also, Holland has a significant bicycle infrastructure – special paths just for bikes that separate them from vehicle traffic.

Conclusion

Bicycle helmets work. The overall evidence suggests that if you ride a bicycle, especially on the streets, you should wear one.

Bicycle helmet laws increase compliance with wearing helmets, but it is currently unclear if they reduce head injuries and if they reduce overall bicycle use. Further study is needed to answer these questions.

Having bicycle paths that separate bikes from cars is effective in reducing accidents with no apparent health downside. If you drive in a city or town where there are bicyclists, then be especially careful and give them a wide berth, even if they are wearing a helmet.