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Does wearing a helmet while bicycling make an individual cyclist safer?

That is, in the case of an accident, is one safer wearing an helmet? If yes, are the odds of an accident lower, higher or identical if one is wearing a helmet? If the odds of an accident are higher when wearing an helmet, does the added safety of an helmet in the case of an accident makes the practice worthwhile?

To substantiate this claim being in the wild, Mikael Colville-Andersen casts doubt at a TEDxCopenhagen talk saying they make things worse:

"To my surprise, it didn't take me very long to figure out that the bicycle helmet doesn't have a very impressive track safety record, scientifically. The scientific community has been completely split for years on the subject 50/50 down the middle. If you look at this way, if the bicycle helmet was a vaccine or a medicine there is no way it would be anywhere near getting approved by a ministry of health. There is simply not enough proof. [...] There are actually scientific studies that show your risk of brain injury is higher when you wear a helmet. You have a 14% greater chance of getting into an accident with a helmet on."

While The League of American Bicyclists' contradicts that in their Helmet use when Cycling:

Helmets are safety devices which prevent or mitigate head injuries in a crash or fall, not substitutes for education which is aimed at the prevention of crashes and falls.

Evan Carroll
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Goodbye Stack Exchange
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    Amsterdam is on of the most bike-friendly cities on earth. There are 465,000 bikes there and no one wears helmets. – MSpeed May 27 '11 at 09:08
  • @billynomates: What's your point? – Beska May 27 '11 at 14:02
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    @Beska. No point, really. Just pointing out that they see little need for helmets even though they all ride bikes. Or maybe it's because of the abundance of bikes that makes it safer, whereas a mixture of cars and bikes makes cycling dangerous. /brainfart – MSpeed May 27 '11 at 14:05
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    There is also the associated factor in the larger discussion that when New Zealand instituted mandatory bike helmet laws for the entire population, the number of trips and the distance of those trips dropped sharply. http://www.cycle-helmets.com/zealand_helmets.html -- which suggests that there may be benefits to not mandating helmet use regardless of safety concerns. – Martha F. May 27 '11 at 16:04
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    @billynomates Amsterdam, and the Netherlands in general, have excellent bike infrastructure with generous bike lanes etc, and the extreme number of bikers means that car drivers are sufficiently aware of their existence. This makes it safer for cyclists than countries without special bike lanes and so few bikers that cars don't expect them and don't know how to behave in their vicinity. – Lagerbaer May 27 '11 at 22:29
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    Anecdotally, I've lived in Germany, New Zealand and Canada. The helmet laws in the latter two countries haven't kept me from using my bike for most errands. However, I found that drivers in NZ were least considerate of bikers, leaving only extremely narrow space between the biker and their vehicle. Comparing bike fatalities among different countries, therefore, is inherently biased. – Lagerbaer May 27 '11 at 22:30
  • @Lagerbaer: Is extremely narrow space between the biker and their vehicle a proven problem? Or do only fearful bikers feel uncomfortable, if passed closeley by a car? Do cyclists in NZ often suddenly fall from their bike while riding? And how is it related to questions of helmet wearing? Are there helmets, big enough, to keep car drivers on distance? – user unknown May 28 '11 at 14:43
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    @user unknown - A vehicle in motion moves air, and can blow a cyclist over when passing too closely. Anyone have a citation for this important fact? – Goodbye Stack Exchange May 28 '11 at 14:47
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    It's the same reason there's this "please stand back" sign at train stations. And even if it was a problem only for "fearful" bikers, that would be a sufficient reason to tell cars to leave sufficient space. – Lagerbaer May 30 '11 at 02:10
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    @Lagerbaer: Not long ago, a girl was killed here, near Berlin, from a train passing close to her. But I don't remember whether they explained at which speed the problem starts. From daily cycling I can tell, that I'm not confronted with moving air in the city (50 km/h Trucks and Busses), but outside, at 100 km/h, it might vary. `And even if it was a problem only for "fearful" bikers, that would be a sufficient reason to tell cars to leave sufficient space.` A nettiquette for car drivers isn't the topic, but accidents. – user unknown Jun 01 '11 at 02:25
  • Those interested in the relative efficacy of our safety laws and the issue of risk-compensation may note with interest the chart of the data from western Australia that begins in 1971. All the trends in accidents from the different groups of road users are rising and continue to rise for a couple of years. Particularly the pedestrian deaths. What was happening in 1971? Seatbelt law had been introduced right across the country. –  Oct 13 '11 at 02:09
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    @userunknown Rule from my driving school: a bicycle is 3m wide and 5m long. :D – Stefan Jan 29 '13 at 14:27
  • @EvanCarroll - Thanks for the reference, but it partially refutes the claim rather than simply documenting that the claim exists. Have added a more appropriate one. The link you posted could be part of a good answer, though. – Goodbye Stack Exchange Jan 29 '13 at 18:53
  • @neilfein take it to meta. – Evan Carroll Jan 29 '13 at 18:55
  • @EvanCarroll - [Done](http://meta.skeptics.stackexchange.com/q/2143/408). – Goodbye Stack Exchange Jan 29 '13 at 20:24
  • It's not true that no one wears helmets in Amsterdam. Other day I saw American tourists on bikes. Wearing helmets :-P – vartec May 19 '13 at 23:33
  • Your question doesn't take into account the context, which is policy making (should helmets be encouraged/enforced)? The real question is not "are they individually effective", but _how much_ effective, relative to other safety measures (quality of infrastructure, number of cyclists, cyclist/motorist awareness etc), considering the _negative_ effects of such policies (reduced cycling, risk compensation etc). I speculate that the net effect of helmet promotion on society is hugely negative, which is why I think Mikael attacks them so vigorously, even though his claims are not that sound. – BKE Mar 09 '14 at 09:03

3 Answers3

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Take this study:

Abstract

Objectives. —To examine the protective effectiveness of bicycle helmets in 4 different age groups of bicyclists, in crashes involving motor vehicles, and by helmet type and certification standards.

Research Design. —Prospective case-control study

Setting. —Emergency departments (EDs) in 7 Seattle, Wash, area hospitals between March 1, 1992, and August 31, 1994.

Participants. —Case subjects were all bicyclists treated in EDs for head injuries, all who were hospitalized, and all who died at the scene. Control subjects were bicyclists treated for nonhead injuries.

Main Results. —There were 3390 injured bicyclists in the study; 29% of cases and 56% of controls were helmeted. Risk of head injury in helmeted vs unhelmeted cyclists adjusted for age and motor vehicle involvement indicate a protective effect of 69% to 74% for helmets for 3 different categories of head injury: any head injury (odds ratio [OR], 0.31; 95% confidence interval [CI], 0.26-0.37), brain injury (OR, 0.35; 95% CI, 0.25-0.48), or severe brain injury (OR, 0.26; 95% CI, 0.14-0,48). Adjusted ORs for each of 4 age groups (<6 y, 6-12 y, 13-19 y, and ≥20 years) indicate similar levels of helmet protection by age (OR range, 0.27-0.40). Helmets were equally effective in crashes involving motor vehicles (OR, 0. 95% CI, 0.20-0.48) and those not involving motor vehicles (OR, 0.32; 95% CI, 0.20-0.39). There was no effect modification by age or motor vehicle involvement (P=.7 and P=.3). No significant differences were found for the protective effect of hard-shell, thin-shell, or no-shell helmets (P=.5).

Conclusions. —Bicycle helmets, regardless of type, provide substantial protection against head injuries for cyclists of all ages involved in crashes, including crashes involving motor vehicles.

(My emphasis)

The abstract can be found here: http://jama.ama-assn.org/content/276/24/1968.short

In addition, this should be a basic physics question: The severity of head injuries clearly depends on the force of the impact. If that force is reduced in any way, this will mean less severe injuries. In a way, this question is similar to the question if there are studies regarding the usefulness of parachutes...

EDIT There was the added question of increased odds of accident when wearing a helmet. This is the theory of risk homeostasis. According to that theory, each individual has a personal target level for the risk it takes. If safety measures reduce the risk, behavior is adapted to bring it to the higher level again. A discussion of this effect regarding bicycle helmets is found here: http://injuryprevention.bmj.com/content/7/2/89.full However, there is apparently no consensus and no study regarding the specific question of risk homeostasis for bicycle helmets. Subsequent studies proved to be inconclusive as well, and there are many more effects to take into account.

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Lagerbaer
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    "In a way, this question is similar to the question if there are studies regarding the usefulness of parachutes." Full ACK:-) – Martin Scharrer May 26 '11 at 17:51
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    Not really. For example, in the end of the 70's a law that required you to have a helmet when riding a moped was put through. The amount of mopedists that dies per km of travel didn't go down. So it *sounds* obvious, but it's not that simple. :-) – Lennart Regebro May 26 '11 at 18:52
  • That's what I'm addressing with the paragraph on risk homeostasis. – Lagerbaer May 26 '11 at 19:26
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    I have seen studies coming to the opposite conclusion: that more cyclists die or are injured without helmets than with. I think the effect was largely attributed to care on the part of _drivers_ rather than _bicyclists_. – Charles May 26 '11 at 19:47
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    Though FWIW, I think that "risk homeostasis" is a silly term, because it suggests that people act to keep risk the same, rather than utility-maximizing. Yes, activities that become less risky will be performed more, _ceteris paribus_, but not automatically to the same risk level. Often that level is such that the activity is more frequent but the risk is lower, but sometimes it's sensible to do it so much more that the risk from that activity rises -- the so-called Jevons paradox. – Charles May 26 '11 at 19:52
  • Risk homeostatis, afaict, refers to risk that's associated with one's own behavior. The main risk to bicyclists is in fact the behavior from others, in particular motor vehicle drivers (see the quoted study in the answer). – MSalters May 27 '11 at 08:51
  • @MSalters - For that reason, I always wear a helmet: except when I'm only going to be riding on a bike path, away from traffic! I.e. my helmet-wearing is associated with my higher-risk (riding in traffic) behaviour. – ChrisW May 27 '11 at 11:25
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    Interestingly, I think that the risk homeostasis can be ignored, for the sake of this question. Do helmets make you safer? The study and the basic physics indicate yes. Could risk homeostasis bring that benefit back down to 0? Possibly yes, but this is unclear and unknown. Until there is *something* to indicate that homeostatis does make some kind of difference, I think the null hypothesis indicates that the helmets are a net win. – Beska May 27 '11 at 14:09
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    @Beska I share your sentiment. Add to this that risk homeostasis is something in a person's control: I choose how reckless I ride my bike. But I cannot control everything else, and for this, a helmet certainly is useful. – Lagerbaer May 27 '11 at 20:36
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    The trouble with the reported results is that it doesn't address the right sample of cyclists. We should be want two comparable populations of cyclists: helmet wearers, and the helmet free. Which group of these has the highest *rate* of injuries and which injuries are more serious? This sample would automatically allow for any confounding factors and risk homeostasis. Sampling *people who have had accidents* allows every possible confounding factor: it might not be *helmets* that cause the differences but the fact that people who choose helmets *behave* differently. – matt_black Oct 02 '11 at 15:29
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    The study quoted (along with many similar helmet studies by Thompson, Rivara & Thompson) has been critised at http://www.cyclehelmets.org/1159.html – Qwerky Feb 15 '13 at 12:34
  • Cycle helmets are a matter of policy. While the OP asked for _individual effectiveness_, which your answer covers, the real underlying question is should cycle helmets be encouraged or enforced. Individual effectiveness does not imply social effectiveness, which is the real interesting question in this case. – BKE Mar 08 '14 at 19:00
  • What the sole cited study in this answer shows is akin to showing this: "Of people who have ever dropped out of a plane, those with a parachute are more likely to survive than those without a parachute." But it is very likely that those on a plane without a parachute tend to die less often than those on a plane with a parachute, because the latter are likely on the plane for very different circumstances. –  Oct 20 '14 at 14:04
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    @ChrisW That is a good point! If there is a high number of people which only wear helmets when they drive in dangerous conditions, then the study would be skewed to more accidents with helmets on. But not because helmets distract, but because you will only wear a helmet if you drive fast/in dangerous traffic. – Falco May 10 '16 at 12:11
  • *Helmets were equally effective in crashes involving motor vehicles (OR, **0.** 95% CI, 0.20-0.48)* Some digits here are missing (is it 0.20 or 0.60 or what ?). Too bad because it is one of the interesting figures of the study. – Evargalo Nov 06 '17 at 08:48
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Many of the frequently quoted statistics don't convincingly show a protective affect from helmets.

Cycling is an inherently safe activity where serious accidents are rare. This makes it harder to judge the effectiveness of safety interventions. Small behavioural changes can add significantly to the rate of accidents and confounding factors can easily sway the results. Care is required when drawing conclusions form the reported statistics.

There are, in principle, several ways for the reported statistics to go astray.

If we compare accidents (eg by hospital admissions) for helmet wearing and non-wearing groups we need to be sure the groups are comparable. If helmet wearers are significantly different (eg very safety conscious) some of the apparent protective effects may come from their (unrelated) risk aversion and not actually from helmets.

If helmets are worn by most people (usually because they are mandatory) then we no longer have a (useful) control group of non-wearers. And we have the possibility of risk compensation (where behaviour becomes riskier because the users feel safer). The extent of risk compensation is impossible to judge reliably in either sample.

If we bear these observations in mind we can assess the value of published studies.

Some studies are very weak. This BMJ report from 2000 argued that:

The number of serious head injuries among cyclists fell markedly during a period of increasing helmet use, suggesting that helmets offer protection.

No proper comparisons for different groups and correlation=causation assumed.

This much better study published in the BMJ in 1994 is typical of case control studies in that it compares injuries in those wearing helmets to those not wearing them, concluding:

There were no significant differences between the two groups with respect to type of accident or nature and distribution of injuries other than those to the head. Head injury was sustained by 4/114 (4%) of helmet wearers compared with 100/928 (11%) of non-wearers (P=0.023).

This is a much more robust result. But statistically suffers from the problem of confounding. We just can't know whether the people choosing to wear helmets behave like those who don't. The very choice to wear might suggest not, though some of the more detailed statistics in the paper might draw the opposite conclusion.

One critic writing in the BMJ summed up the problem with one study thus:

the study's serious flaw was that it did not discount the possibility that the cyclists with head injuries were very different from those with other injuries. An observational study in the same city showed that children wearing helmets were much more often white than black, riding in parks and on bicycle paths than on city streets, and riding with adults rather than riding alone.' Without proper matching for risk taking behaviour, the hospital study is useless as policy

Thought his point was specific, the problems he highlights may ocurr in all case-control studies.

If the protective effect of helmets that is derived from the case-control studies is right, we would expect to see big changes in head injuries when helmet use becomes very common or compulsory. This is where it gets controversial. Those population studies have been done and frequently don't show the expected gains in injury prevention.

This BMJ review in 2006 summarised much of the evidence like this:

I reviewed data from all jurisdictions that have introduced legislation and increased use of helmets by at least 40 percentage points within a few months: New Zealand, Nova Scotia (Canada), and the Australian states of Victoria, New South Wales, South Australia, and Western Australia. To avoid confusing reductions in injuries (from safer roads or less cycling) with benefits of helmets, I have focused on percentages of cyclists with head injuries. Head injuries were most commonly classified as admissions to hospital with head wounds, skull or facial fracture, concussion, or other intracranial injury. The data include 10 504 head injuries, and in most cases were available as percentages of all cyclist injuries.

and concluded (my emphasis):

In contrast to the fall in all road injuries in South Australia coinciding with helmet legislation, percentages of cyclists with concussion and other head or face injuries show generally declining trends, especially for concussion, but no clear response when helmet wearing increased substantially

Here is the chart of the data from Western Australia

from bmj review: head injury rates before and after helmet law in w austraila

Similar evidence from South Australia:

enter image description here

and New Zealand:

new zealand chart

Overall conclusion:

Case-control studies suggest cyclists who choose to wear helmets generally have fewer head injuries than non-wearers

Before and after data show enforced helmet laws discourage cycling but produce no obvious response in percentage of head injuries

This contradiction may be due to risk compensation, incorrect helmet wearing, reduced safety in numbers, or incorrect adjustment for confounders in case-control studies

It is only fair to note this is a highly controversial conclusion (see the BMJ discussion).

The reason why the studies about what happens when most people wear helmets is relevant to the question of whether helmets protect the individual helmet wearer is simply that they represent the only studies where the sample avoids confounding errors and takes into account any risk adjustment. We know there is some degree of risk adjustment (see this Skeptics question: Do cars pass with less space for bicyclists who wear helmets?) but we can only know the significance of that adjustment from a population study.

I'd say that, despite their intuitive benefits helmets provide, the population studies cast enough doubt on the theory advocated by the case-control studies that we cannot be certain helmets give a significant benefit to the user.

Update

A recent (feb 2013) study from the National Bureau of Economic Research (pdf here) looked at the effect of laws on childrens' cycling accidents and behavior. The study showed a significant reduction in injuries associated with the laws but tried to examine some of the effects of confounding. They conclude (my emphasis):

Cycling is popular among children, but results in thousands of injuries annually. In recent years, many states and localities have enacted bicycle helmet laws. We examine direct and indirect effects of these laws on injuries... ...we find helmet laws are associated with reductions in bicycle-related head injuries among children. However, laws also are associated with decreases in non-head cycling injuries, as well as increases in head injuries from other wheeled sports. Thus, the observed reduction in bicycle-related head injuries may be due to reductions in bicycle riding induced by the laws.

And:

The findings from this paper indicate that while bicycle helmet laws are widespread and thought to be effective, the net effect of these laws on health outcomes is actually not straightforward. It is clear that there are offsetting behaviors and unintended consequences of these laws, and these effects need to be considered by policymakers.

In short, the apparent success of compulsory helmet laws may not result from the protective effects of helmets from from the (unintended) effect of discouraging cycling. It isn't that helmets protect the cyclist. Public health may well, therefore, not benefit from such laws.

Update 2

A recent study of the effect of the range of different Canadian helmet laws on head injuries was recently published in the British Medical Journal (BMJ). this paper took into account injury trends pre and post legislation and used the variety of different rules or the lack of them across different canadian provinces to provide useful comparisons. While the study doesn't assess all the possible confounding factors, the results are fairly clear: legislation doesn't seem to have a significant effect.

It is worth quoting the results and conclusions in their entirely for context (my emphasis):

Results Between 1994 and 2008, 66 716 hospital admissions were for cycling related injuries in Canada. Between 1994 and 2003, the rate of head injuries among young people decreased by 54.0% (95% confidence interval 48.2% to 59.8%) in provinces with helmet legislation compared with 33.1% (23.3% to 42.9%) in provinces and territories without legislation. Among adults, the rate of head injuries decreased by 26.0% (16.0% to 36.3%) in provinces with legislation but remained constant in provinces and territories without legislation. After taking baseline trends into consideration, however, we were unable to detect an independent effect of legislation on the rate of hospital admissions for cycling related head injuries.

Conclusions Reductions in the rates of admissions to hospital for cycling related head injuries were greater in provinces with helmet legislation, but injury rates were already decreasing before the implementation of legislation and the rate of decline was not appreciably altered on introduction of legislation. While helmets reduce the risk of head injuries and we encourage their use, in the Canadian context of existing safety campaigns, improvements to the cycling infrastructure, and the passive uptake of helmets, the incremental contribution of provincial helmet legislation to reduce hospital admissions for head injuries seems to have been minimal.

Some of the core data is summarised in the charts below:

BMJ article charts

Annual rate of hospital admissions for cycling related head injuries, 1994 to 2008, in Canadian provinces and territories, and in Canadian provinces and territories grouped by bicycle helmet legislation status. Rates are connected by a LOESS regression line. Vertical bars indicate year legislation was enacted. Legislation in Ontario and Alberta targeted only cyclists aged less than 18 years.

In short: cycling injuries are reducing but detailed analysis of the statistics doesn't show that legislation independently contributes to that improvement. While the study is addressing the effect of legislation not helmet wearing it also notes:

Laws that mandate the use of helmets increase the chance that cyclists will wear a helmet, especially when they apply to all cyclists...

...helmet use rose dramatically in provinces immediately after the enactment of legislation...

So, though indirectly, it lends support to the idea that increased helmet use is much less helpful than expected.

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matt_black
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    The “where serious accidents are rare” seriously needs a reference. In fact, cycling seems relatively accident-prone to me. I hardly know a person who *hasn’t* been involved in some kind of cycling accident, and I have witnessed a lot of extremely close shaves between cyclists and other road users. Add to that the high velocity, the complete lack of protection and some basic physics. Of course I realise that this assessment suffers from various forms of biases so I wouldn’t offer it as a counter-argument, merely as a strong incentive to demand evidence for your claim. – Konrad Rudolph Feb 21 '12 at 13:31
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    @KonradRudolph - However, part of the catch-22 with the helmets is that they only protect you against head trauma. If a cyclist is hit by a car they might suffer fatal abdominal trauma which would render the helmet moot. – rjzii Jan 29 '13 at 14:14
  • Maybe I'm missing something, but one thing that strikes me about these statistics is (on the face of it) they don't make any mention of how the rate and absolute number of cyclist has changed over time - legislating for helmet use might have an impact on the number of people cycling. Maybe they mention something in the studies referenced. Ideally I'd want to see injuries as a proportion of cyclists, but the stats don't appear to give this. – Bamboo Jan 21 '21 at 04:48
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What I want to point out is that there are also some influences coming from the fact of wearing an helmet. I'm thinking about 2 aspects have heard of :

  • It has been proven by metric analysis (reference) that the vehicule pass closer by a bike if the cyclist wear an helmel. As to say "He is protected, so there will be less damage in case of accident"

  • The cyclist may feel "protected" in a way he/she drives with less attention.

*Ok, it not really an answer in itself, but this is an aspect of the question that should be mentioned

Rabskatran
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    Your second point was mentioned by Lagerbaer as “risk homeostasis”. Your first point would be very interesting *with a reference*. – Konrad Rudolph May 27 '11 at 10:45
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    @Rabskatran: You can also argue the opposite way (bullet 2): A helmet-wearing cyclist always gets conscious of the risks of cycling when pulling the helm on. – user unknown May 27 '11 at 13:45
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    @Konrad : I will ask the friend that told be about this and add a link is available. – Rabskatran May 27 '11 at 15:42
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    Some references on passing distance with or without helmets: http://www.bhsi.org/walkerstudy.htm and http://www.sciencedirect.com/science/article/pii/S0001457506001540 – Martha F. May 27 '11 at 16:02
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    I seriously question whether Ian Walker's result is reproducible. When I pass a cyclist in a car, I need to pay full attention to the road condition. I have zero impression on whether the cyclist wear a helmet or not, or what kind of bike they are riding. Let alone can I alter the pass distance. Someone setup a road side camera can probably verify his result. – Wai Yip Tung May 28 '11 at 06:12
  • On your second point, you can also argue the other way. People who wear helmet are more safety conscious and less incline to risky behavior. – Wai Yip Tung May 28 '11 at 06:16
  • A vehicle which passes close doesn't hit the cyclist, but passes - by definition. Closely passing cars might be annoying and disturbing, but I can't see how they should be a risk. Most car drivers fear about the proper surface of their car, and aren't interested in accidents themselves - whether helmet-wearing biker or naked head. – user unknown May 28 '11 at 15:19
  • The annoyance and disturbance has the potential to throw you off balance. – Lagerbaer Jun 01 '11 at 16:07
  • The wind of the vehicle can also throw you off balance, the car may not be considering their mirror or other above-wheel protrusions in their width estimate, and the car may be overestimating how straight a line the cyclist is riding in. There's a basic human startle reflex that makes keeping a straight line difficult when several tons of steel rush by at 50 miles an hour inches away. – freiheit Jun 01 '11 at 22:19
  • I do not have any statistics on either of these two points. I can only add my experience from two large cities: In Prague, I _always_ wear a helmet, because the drivers are very aggressive whatever you do (and their behaviour doesn't really change). In Paris, I _do not_ wear a helmet, because my experience and experience of others is that drivers treat you much more badly when you wear a helmet. My conclusion is that answer to this question depends a lot on where you ride your bike and what are the local habits there. – yo' Feb 01 '13 at 23:29