I've been avoiding this one like the plague, as this is maybe the one single most heated topic among people who ride bicycles (at least in the US). However, I feel like the time has come to say my peace and be done with it. So, here goes.
I think there are two things I would like to specifically address with relation to bicycle helmets. First of all, I want to address the actual efficacy of them, whether they do really help protect you and how much. Secondly, I'd like to address ideas of culture, safety as a whole, and fear.
Bicycle helmets are fragile and it is not uncommon for them to fail prematurely. The simple fact that you are supposed to replace one if you so much as drop it on the ground, I think speaks volumes about how much protection they can actually give. When bicycle helmets do fail, they do so by breaking catastrophically (cracking, splitting apart, etc), thus minimizing any kind of protection they were giving you. As they are made of styrofoam with a very thin layer of plastic on the outside, they can only take a very moderate amount of force before simply transferring all additional force directly to your head. Modern bicycle helmets are actually less effective than the ones made originally in the 1970's because they have been made lighter and more lightweight with a much lighter shell in order to be more comfortable. In fact, while cycling helmets are supposed to meet the standards set by one of several international standards agencies, there is no third party testing to assure that they meet those standards, and independent testing has shown that many of them do not meet the standards they claim to. No existing standard requires testing efficacy of a helmet with relation to rotational, or diffuse, injuries.
So, we can see that helmets are designed for a very specific range of protection. Low-impact, direct impact collisions, which would typically result in scrapes, lacerations, bruises, maybe concussion. They are not designed however, and have not been shown effective to prevent rotational injuries (injuries where movement of the brain relative to the skull is the cause of injury). These types of injuries are generally the ones that cause death or permanent damage. There has actually been some concern in the medical community that bicycle helmets can have the effect of converting a direct force into a rotational force, therefore increasing the chance of an internal brain injury from a minor impact.
In a study conducted in 1988, 8 million cases of injury and death among cyclists over 15 years in the US were studied, with the conclusion that riders wearing helmets died more often in bicycle accidents than those not wearing helmets.
In 2001, a story was run by the New York Times in 2001, documenting with research and statistics from the Consumer Product Safety Commission, National Sporting Goods Association, C.D.C, and others that during the 1990's in the US, as bicycle helmet usage really started to be pushed by city and national officials, bicycle companies, advocacy groups, etc - the percentage of cyclists wearing helmets jumped way up, the number of people riding bicycles declined sharply, and the number of head injuries in cyclists actually increased by 10%. The exact reasons for that aren't apparent (though there are some very sensible suspicions), however, it is at least clear that bicycle helmets weren't doing a lot to prevent head injuries.
An analysis of cycling and pedestrian facilities in Canada from 1983 to 2003 showed that likelihood of death was similar between both modes of travel, and that both decreased during the time period - however, while helmet usage in Canada rose from almost nothing to about 50% during that time period, no notable change in rate of fatalities was observed.
In London, police have noted that over the period of time in which usage of bicycle helmets has risen, the severity of bicycle-related injuries has also risen.
In the UK as a whole, the rate of head injuries from 1995 to 2001 fell more drastically for children than for adults. Yet, while helmet usage in adults increased by more than 50% during that time, helmet usage in children declined.
So, in conclusion, bicycle helmets were designed for a very specific purpose, to mitigate minor head injuries in low-impact crashes, usually in crashes not involving more than one party. Whether they may or may not prevent more serious injury in more serious accidents on specific occasions, they have not been shown to do so with any kind of regularity, and they really are not designed to do so.
So, on to culture, safety as a whole, and fear.
In modern American culture, riding a bicycle without a helmet is viewed as almost a death wish. When a bicycle accident is reported in news media, the comment is made "and the cyclist wasn't wearing a helmet," even if the cyclist wasn't injured at all or didn't suffer any head injuries - as if by wearing a helmet they somehow would have avoided the accident altogether.
In fact, only about 6% of reported bicycle accidents in the US result in moderate or higher level head injuries. In reality that percentage is probably much smaller, as many bicycle accidents which don't result in any major injuries are never reported.
What all this irrational insistence on wearing a bicycle helmet does, along with the overwhelming perception that you are always in imminent danger of suffering a fatal head injury, is cause people to be afraid of riding bicycles.
Without fail, in places where they have mandated use of bicycle helmets, the number of people riding bicycles has fallen significantly. What is also clear, is that whenever a place has seen a significant rise in the number of cyclists, it has also seen a decline in the rate of bicycle-related collisions. That is, the more people cycling, the safer it is for cyclists.
We tend to look at the problem of bicycle safety as being equivalent with the wearing of bicycle helmets. That is, if you wear a helmet, you are safe. If not, you aren't safe. I think this is an extremely narrow view of things, and I think many other factors play a more important role in the safety of cyclists.
Quoting from the above mentioned New York Times article:
"Dr. Richard A. Schieber, a childhood injury prevention specialist at the Centers for Disease Control and Prevention and the leader of a national bicycle safety initiative, said public health officials were realizing that in addition to promoting helmet use, safety officials must teach good riding skills, promote good driving practices and create safer places for people to ride.
"We have moved the conversation from bicycle helmet use to bicycle safety," Dr. Schieber said. "Thank God that the public health world is understanding there is more to bicycle safety than helmets."
This is exactly the philosophy that the Dutch Cyclist's Union takes, preferring to make people feel as safe and comfortable about riding bikes as they can (through education, infrastructure, etc), so that as many people will ride bikes as possible, and therefore safety will increase. The Netherlands is consistently rated the safest country in the world to ride in, and also has the highest percentages of people cycling, with some cities approaching 60-65% of all trips made by bicycle. They are absolutely against mandatory regulation of bicycle helmets, and that clearly hasn't hurt the safety of the riders in the Netherlands. They view cycling as completely normal, safe and practical.
So, first of all, head injuries are not nearly as common as people think. You are not necessarily in imminent danger of a head injury on a bike any more than you are walking or swinging in a park or running up and down stairs (or driving, for that matter - as the majority of major head injuries in the US are motor vehicle related). Yes, things happen, but that's how the world is.
Secondly, I think if we are really concerned about the safety of cyclists, there are much better ways to go about promoting it than making people afraid of bikes, and then selling them bike helmets. If we focused on designing our cities so they were safe for non-automotive traffic (bicycles AND pedestrians), educating both cyclists and motorists on how to interact safely and use the roads and paths responsibly, I think it would go much further towards promoting the safety of cyclists than simply telling them "wear a helmet, or else!"
I tend to be skeptical of anyone trying sell anything by means of fear. To me, that says that they have some ulterior motive for persuading you to buy what they're selling.
So, all of that to say, I'm not opposed to bicycle helmets in and of themselves. If they make the difference between you getting on a bike or not, go for it. They probably do have some benefit for preventing minor injuries. I also think they make a lot more sense for sport cyclists than for your average everyday getting around town cyclist. The type of bike and the way you ride it make a big difference in your level of risk. What I am opposed to is "promoting" safety by making people afraid, not to mention with pretty slim and sketchy statistics, ignoring the problems of interaction with automobile traffic and then just labeling cycling as extremely dangerous. I'm opposed to mandating the usage of bicycle helmets, because it has been shown to have negative results with regard to the safety of cyclists in places where helmets have been mandated.
The two best places for cycling in the world, the Netherlands and Denmark, both have no mandatory helmet laws, but rather make cycling safe by educating people and by re-arranging their cities to accommodate more than just automobile traffic. They do this because they see the benefits to society of having large numbers of people cycling rather than driving, and so they want to encourage that positively as much as they can.
I know, some of this is opinion, and while there may not be scientific proof that bicycle helmets can prevent major head injuries, there is still a chance that they may in some cases, I just don't believe that riding a bicycle is a dangerous activity, in comparison with all the things we do on a normal day in our lives, and I have an objection to people who, intentionally or inadvertently, paint it as something to be feared. I think education and responsible behavior, and not helmet legislation, is the best way to encourage safety on the roads and bicycle paths.