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«Why I’m done wearing a helmet
I’m done wearing a bike helmet.
Now, don’t hold me to that. Maybe I’ll want one in the winter when the roads are icy. Maybe I’ll be required to wear one on a group ride. Maybe I’ll travel cross-country, planning to bike on deserted roads. But when it comes to casual riding, I’m done.
At the conference I attended last week, I mentioned, “I don’t wear a helmet, and I’m a public health professional,” to audible gasps and laughter. I invited people to ask me why later, and many of them did. I’ll share with you what I told them in just a moment, but let me preface this: I think the main problem around making an informed decision about whether to wear a helmet is hard because there isn’t great data around helmet use. Due to the confirmation bias, we’re all looking for information to back up the belief that we already have. Do your own research, use your own brain, and figure out what seems right and feels right for you.
It’s debatable whether helmets are effective.
A 1987 study of helmet use determined that helmets reduce the risk of serious injury by 85%. That’s a statistic you’ll still see all over the place. The problem is, that study was deeply flawed and has been refuted. Governmental agencies have stopped using this number due to issues with the study. Other more recent studies have investigated the question of helmet efficacy, and have found that the benefit is not nearly as high as we used to think. One well-done study that evaluated all the current studies out there (called a meta-analysis) found there to be no benefit to helmet use when you take into account all types of injuries. Helmets protect against certain kinds of injuries (those to the head) and increase the likelihood of other injuries (those to the neck). Any study about helmet use is very hard to do well. You can’t assign one group of people to use helmets and another group of people not to use them. All you can do is look at two groups of people and compare them. The people who wear helmets are likely more safety-conscious than those who don’t, which makes comparing the two groups very difficult and will make it appear that helmets are more protective than they actually are.
People will often put up photos on social media of obliterated helmets and say, “Holy crap, look at my helmet! It saved my life!” But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way. Most of the pictures I’ve seen are of helmets that have broken apart. It’s likely that the helmet did not protect someone from a severe injury.
Most cyclists don’t die from independently falling off their bikes, 92% of cyclists die because they are hit by cars. Bike helmets only protect against certain types of injuries to certain parts of the head, and the evidence is not compelling that they even do that well.
Helmets don’t protect you against crashes
Helmets don’t protect you against crashes, in fact they might make crashes more likely. It’s inevitable that after a motorist-cyclists crash you’ll see every media article mention, “the cyclist was/wasn’t wearing a helmet.” It doesn’t matter if a drunk driver sideswiped the cyclist and broke both of their legs, the news report will say “the cyclist was/wasn’t wearing a helmet.” It’s as if people think that wearing a helmet will save you from crashing in the first place. It won’t.
There is some evidence that cars may pass cyclists closer if they are wearing a helmet. This makes intuitive sense to me as cyclists appear more aggressive and protected when wearing a helmet which may make drivers feel safer giving them less room to ride. Some of the most severe and dangerous motorist-cyclist crashes happen when the driver gets too close and the car sideswipes the bike. I want as much room as possible, thankyouverymuch.
Just because someone wears a helmet doesn’t mean they’re a safer cyclist. It seems like a lot of people use helmet use as a proxy for caring about safety, and that’s just not true. Learning safe riding skills, being visible, and being attentive are the things we all can do to prevent a crash.
Helmet use may deter people from biking
The most protective factor for biking is having more bikers on the road. When there are more cyclists on the road, drivers are more used to seeing cyclists out, and are more likely to be looking for them. However, if potential cyclists see everyone else in their community wearing helmets while riding a bike, it communicates that biking is a dangerous activity that requires special protective gear. Someone may be deterred from riding a bike because they think it’s risky. Someone else may be deterred from biking because they think they have to wear a helmet and they don’t want to mess up their hair. Many more motorists and pedestrians die in traffic collisions per year than do bicyclists. We don’t see them wearing helmets.
The bike share systems are good evidence of how important casual cycling is. In over 23 million trips on bike share bikes, there haven’t been any fatalities and only 40 people have been hurt. Most bikers on bike share bikes do not wear helmets. In fact, mandatory helmet laws make it incredibly difficult for cities to begin bike share systems, even though these systems successfully increase casual cycling and getting more people biking. The people riding these bikes are usually slower than other cyclists and the upright orientation of the bike share bikes may lend to a more casual style of cycling.
The benefits of biking much outweigh the risks
Biking is good for public health, it increases physical activity and reduces air pollution. The long-term benefits of cycling on health outweigh the slight risk involved. When you take into account the long-term health benefits, it’s much more dangerous not to get enough physical activity than it is to ride a bike. If not wanting to wear a helmet deters someone from riding a bike, that sucks, because biking is healthy and awesome.
It’s humanizing not to wear a helmet
I am a human. I have skin that can get bruised and a head that can get bashed in and blood that can gush out in the event that a car runs me over. I don’t want to hide my head behind a helmet, I want drivers to pass me on the street and see that I am a person, a human, just like them. I think of my bare head as a sign that says: I am a living creature who wants to keep living, so please don’t hit me.
I was just in Europe, in Barcelona and Amsterdam, where cycling is pervasive. Hardly anyone wears a helmet, and most people have upright handlebars. This orientation feels safer to me. It makes it easier to see my surroundings and encourages me to take my time, look around, and bike a little slower. Since I got home, I replaced my road bike’s drop handlebars with swept back bars, for a more upright riding position, and I stopped using the helmet.
Besides, it’s glorious to feel the wind in my hair.»
(Herfra: http://www.bikinginmpls.com/im-done-wearing-helmet/ )
Dette!!!!!
I’m done wearing a bike helmet.
Now, don’t hold me to that. Maybe I’ll want one in the winter when the roads are icy. Maybe I’ll be required to wear one on a group ride. Maybe I’ll travel cross-country, planning to bike on deserted roads. But when it comes to casual riding, I’m done.
At the conference I attended last week, I mentioned, “I don’t wear a helmet, and I’m a public health professional,” to audible gasps and laughter. I invited people to ask me why later, and many of them did. I’ll share with you what I told them in just a moment, but let me preface this: I think the main problem around making an informed decision about whether to wear a helmet is hard because there isn’t great data around helmet use. Due to the confirmation bias, we’re all looking for information to back up the belief that we already have. Do your own research, use your own brain, and figure out what seems right and feels right for you.
It’s debatable whether helmets are effective.
A 1987 study of helmet use determined that helmets reduce the risk of serious injury by 85%. That’s a statistic you’ll still see all over the place. The problem is, that study was deeply flawed and has been refuted. Governmental agencies have stopped using this number due to issues with the study. Other more recent studies have investigated the question of helmet efficacy, and have found that the benefit is not nearly as high as we used to think. One well-done study that evaluated all the current studies out there (called a meta-analysis) found there to be no benefit to helmet use when you take into account all types of injuries. Helmets protect against certain kinds of injuries (those to the head) and increase the likelihood of other injuries (those to the neck). Any study about helmet use is very hard to do well. You can’t assign one group of people to use helmets and another group of people not to use them. All you can do is look at two groups of people and compare them. The people who wear helmets are likely more safety-conscious than those who don’t, which makes comparing the two groups very difficult and will make it appear that helmets are more protective than they actually are.
People will often put up photos on social media of obliterated helmets and say, “Holy crap, look at my helmet! It saved my life!” But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way. Most of the pictures I’ve seen are of helmets that have broken apart. It’s likely that the helmet did not protect someone from a severe injury.
Most cyclists don’t die from independently falling off their bikes, 92% of cyclists die because they are hit by cars. Bike helmets only protect against certain types of injuries to certain parts of the head, and the evidence is not compelling that they even do that well.
Helmets don’t protect you against crashes
Helmets don’t protect you against crashes, in fact they might make crashes more likely. It’s inevitable that after a motorist-cyclists crash you’ll see every media article mention, “the cyclist was/wasn’t wearing a helmet.” It doesn’t matter if a drunk driver sideswiped the cyclist and broke both of their legs, the news report will say “the cyclist was/wasn’t wearing a helmet.” It’s as if people think that wearing a helmet will save you from crashing in the first place. It won’t.
There is some evidence that cars may pass cyclists closer if they are wearing a helmet. This makes intuitive sense to me as cyclists appear more aggressive and protected when wearing a helmet which may make drivers feel safer giving them less room to ride. Some of the most severe and dangerous motorist-cyclist crashes happen when the driver gets too close and the car sideswipes the bike. I want as much room as possible, thankyouverymuch.
Just because someone wears a helmet doesn’t mean they’re a safer cyclist. It seems like a lot of people use helmet use as a proxy for caring about safety, and that’s just not true. Learning safe riding skills, being visible, and being attentive are the things we all can do to prevent a crash.
Helmet use may deter people from biking
The most protective factor for biking is having more bikers on the road. When there are more cyclists on the road, drivers are more used to seeing cyclists out, and are more likely to be looking for them. However, if potential cyclists see everyone else in their community wearing helmets while riding a bike, it communicates that biking is a dangerous activity that requires special protective gear. Someone may be deterred from riding a bike because they think it’s risky. Someone else may be deterred from biking because they think they have to wear a helmet and they don’t want to mess up their hair. Many more motorists and pedestrians die in traffic collisions per year than do bicyclists. We don’t see them wearing helmets.
The bike share systems are good evidence of how important casual cycling is. In over 23 million trips on bike share bikes, there haven’t been any fatalities and only 40 people have been hurt. Most bikers on bike share bikes do not wear helmets. In fact, mandatory helmet laws make it incredibly difficult for cities to begin bike share systems, even though these systems successfully increase casual cycling and getting more people biking. The people riding these bikes are usually slower than other cyclists and the upright orientation of the bike share bikes may lend to a more casual style of cycling.
The benefits of biking much outweigh the risks
Biking is good for public health, it increases physical activity and reduces air pollution. The long-term benefits of cycling on health outweigh the slight risk involved. When you take into account the long-term health benefits, it’s much more dangerous not to get enough physical activity than it is to ride a bike. If not wanting to wear a helmet deters someone from riding a bike, that sucks, because biking is healthy and awesome.
It’s humanizing not to wear a helmet
I am a human. I have skin that can get bruised and a head that can get bashed in and blood that can gush out in the event that a car runs me over. I don’t want to hide my head behind a helmet, I want drivers to pass me on the street and see that I am a person, a human, just like them. I think of my bare head as a sign that says: I am a living creature who wants to keep living, so please don’t hit me.
I was just in Europe, in Barcelona and Amsterdam, where cycling is pervasive. Hardly anyone wears a helmet, and most people have upright handlebars. This orientation feels safer to me. It makes it easier to see my surroundings and encourages me to take my time, look around, and bike a little slower. Since I got home, I replaced my road bike’s drop handlebars with swept back bars, for a more upright riding position, and I stopped using the helmet.
Besides, it’s glorious to feel the wind in my hair.»
(Herfra: http://www.bikinginmpls.com/im-done-wearing-helmet/ )
Dette!!!!!
«Why I’m done wearing a helmet
I’m done wearing a bike helmet.
Now, don’t hold me to that. Maybe I’ll want one in the winter when the roads are icy. Maybe I’ll be required to wear one on a group ride. Maybe I’ll travel cross-country, planning to bike on deserted roads. But when it comes to casual riding, I’m done.
At the conference I attended last week, I mentioned, “I don’t wear a helmet, and I’m a public health professional,” to audible gasps and laughter. I invited people to ask me why later, and many of them did. I’ll share with you what I told them in just a moment, but let me preface this: I think the main problem around making an informed decision about whether to wear a helmet is hard because there isn’t great data around helmet use. Due to the confirmation bias, we’re all looking for information to back up the belief that we already have. Do your own research, use your own brain, and figure out what seems right and feels right for you.
It’s debatable whether helmets are effective.
A 1987 study of helmet use determined that helmets reduce the risk of serious injury by 85%. That’s a statistic you’ll still see all over the place. The problem is, that study was deeply flawed and has been refuted. Governmental agencies have stopped using this number due to issues with the study. Other more recent studies have investigated the question of helmet efficacy, and have found that the benefit is not nearly as high as we used to think. One well-done study that evaluated all the current studies out there (called a meta-analysis) found there to be no benefit to helmet use when you take into account all types of injuries. Helmets protect against certain kinds of injuries (those to the head) and increase the likelihood of other injuries (those to the neck). Any study about helmet use is very hard to do well. You can’t assign one group of people to use helmets and another group of people not to use them. All you can do is look at two groups of people and compare them. The people who wear helmets are likely more safety-conscious than those who don’t, which makes comparing the two groups very difficult and will make it appear that helmets are more protective than they actually are.
People will often put up photos on social media of obliterated helmets and say, “Holy crap, look at my helmet! It saved my life!” But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way. Most of the pictures I’ve seen are of helmets that have broken apart. It’s likely that the helmet did not protect someone from a severe injury.
Most cyclists don’t die from independently falling off their bikes, 92% of cyclists die because they are hit by cars. Bike helmets only protect against certain types of injuries to certain parts of the head, and the evidence is not compelling that they even do that well.
Helmets don’t protect you against crashes
Helmets don’t protect you against crashes, in fact they might make crashes more likely. It’s inevitable that after a motorist-cyclists crash you’ll see every media article mention, “the cyclist was/wasn’t wearing a helmet.” It doesn’t matter if a drunk driver sideswiped the cyclist and broke both of their legs, the news report will say “the cyclist was/wasn’t wearing a helmet.” It’s as if people think that wearing a helmet will save you from crashing in the first place. It won’t.
There is some evidence that cars may pass cyclists closer if they are wearing a helmet. This makes intuitive sense to me as cyclists appear more aggressive and protected when wearing a helmet which may make drivers feel safer giving them less room to ride. Some of the most severe and dangerous motorist-cyclist crashes happen when the driver gets too close and the car sideswipes the bike. I want as much room as possible, thankyouverymuch.
Just because someone wears a helmet doesn’t mean they’re a safer cyclist. It seems like a lot of people use helmet use as a proxy for caring about safety, and that’s just not true. Learning safe riding skills, being visible, and being attentive are the things we all can do to prevent a crash.
Helmet use may deter people from biking
The most protective factor for biking is having more bikers on the road. When there are more cyclists on the road, drivers are more used to seeing cyclists out, and are more likely to be looking for them. However, if potential cyclists see everyone else in their community wearing helmets while riding a bike, it communicates that biking is a dangerous activity that requires special protective gear. Someone may be deterred from riding a bike because they think it’s risky. Someone else may be deterred from biking because they think they have to wear a helmet and they don’t want to mess up their hair. Many more motorists and pedestrians die in traffic collisions per year than do bicyclists. We don’t see them wearing helmets.
The bike share systems are good evidence of how important casual cycling is. In over 23 million trips on bike share bikes, there haven’t been any fatalities and only 40 people have been hurt. Most bikers on bike share bikes do not wear helmets. In fact, mandatory helmet laws make it incredibly difficult for cities to begin bike share systems, even though these systems successfully increase casual cycling and getting more people biking. The people riding these bikes are usually slower than other cyclists and the upright orientation of the bike share bikes may lend to a more casual style of cycling.
The benefits of biking much outweigh the risks
Biking is good for public health, it increases physical activity and reduces air pollution. The long-term benefits of cycling on health outweigh the slight risk involved. When you take into account the long-term health benefits, it’s much more dangerous not to get enough physical activity than it is to ride a bike. If not wanting to wear a helmet deters someone from riding a bike, that sucks, because biking is healthy and awesome.
It’s humanizing not to wear a helmet
I am a human. I have skin that can get bruised and a head that can get bashed in and blood that can gush out in the event that a car runs me over. I don’t want to hide my head behind a helmet, I want drivers to pass me on the street and see that I am a person, a human, just like them. I think of my bare head as a sign that says: I am a living creature who wants to keep living, so please don’t hit me.
I was just in Europe, in Barcelona and Amsterdam, where cycling is pervasive. Hardly anyone wears a helmet, and most people have upright handlebars. This orientation feels safer to me. It makes it easier to see my surroundings and encourages me to take my time, look around, and bike a little slower. Since I got home, I replaced my road bike’s drop handlebars with swept back bars, for a more upright riding position, and I stopped using the helmet.
Besides, it’s glorious to feel the wind in my hair.»
(Herfra: http://www.bikinginmpls.com/im-done-wearing-helmet/ )
Dette!!!!!
Studien det refereres til er fra 1987!!!! Lite seriøst å linke til en så gammel studie. Kildekritikk er også viktig.
Du leste den ikke gjorde du vel? Hun brukte ikke studien fra 1987 til å backe opp sin argumentasjon, hun sa at nyere studier har vist at hjelm ikke er effektivt, blant annet denne: http://www.cycle-helmets.com/elvik.pdf fra 2011
Hun lenket også til at det er vist at bilister viser mindre hensyn når syklister har på seg hjelm http://drianwalker.com/overtaking/overtakingprobrief.pdf
Jeg leste den, men ser ikke det som seriøs kildekritikk. Studier fra 2011 blir regnet som gamle i forskningsmiljøet btw
Uansett hvordan du vrir og vender på det og følger opp med linker osv er det helt idiotisk å sykle uten hjelm. Jeg ser hver uke pasienter med traumatiske hodeskader som har syklet uten hjelm. De ligger med deler av kraniet åpent for å hindre at hjernen ikke presses sammen. De er ødelagt for livet og som regel helt pleietrengende. Unge mennesker med livet ødelagt som må mates og bruke bleie. Det er argumentasjon nok for meg.
At du mener det ikke er nødvendig å bruke hjelm, får stå for din regning.
Jeg leste den, men ser ikke det som seriøs kildekritikk. Studier fra 2011 blir regnet som gamle i forskningsmiljøet btw
Jeg ser hver uke pasienter med traumatiske hodeskader som har syklet uten hjelm. De ligger med deler av kraniet åpent for å hindre at hjernen ikke presses sammen. De er ødelagt for livet og som regel helt pleietrengende. Unge mennesker med livet ødelagt som må mates og bruke bleie. Det er argumentasjon nok for meg.
At du mener det ikke er nødvendig å bruke hjelm, får stå for din regning.
Jeg er forsker (og har en phd), og jeg blir litt flau på dine vegne når du sier slike ting. Det spiller ikke en så stor rolle hvorvidt en studie er fra 1960 eller fra 2018. Det er metoden som er brukt og etterprøvbarheten som er viktig.
Det finnes mange mennesker som er ødelagt for livet av å ha blitt påkjørt fordi de gikk over veien på forgjengerfelt. Synes du det er argumentasjon for at det ikke bør være tillatt å gå over veien? Det er mange flere som har fått hodeskader i bilulykker enn i sykkelulykker. Hvorfor er det greit å kjøre bil uten å ha på hjelm? Motorsykkelhjelmer burde også brukes i bil, og det hadde ført til at hodeskader ved bilulykker hadde vært mye mindre utbredt. Hva tenker du om det?
Sykkelhjelmer er veldig små og fører til større sjanse for nakkeskade enn om man ikke har på sykkelhjelm. Om man blir påkjørt av en bil har 200 gram isopor på toppen av hodet svært lite å si. Ansiktet er ikke beskyttet. Det er heller ikke kroppen.
Det er svært mye vi gjør i det daglige som har høyere risiko enn å sykle uten hjelm. Slik er livet. Det er farlig å leve, det er farlig å gå i trapper. Farlig å lage mat. De fleste ulykker skjer i hjemmet.
Jeg synes det er fascinerende hvor viktig hjelmen gjøres, og litt skummelt all den tid det viser hvorfor det er en falsk beskyttelse. Den beskytter ikke mot de fleste typer ulykker og skader som skjer på sykkel, men bidrar til at man sykler farligere og tar flere sjanser. Den lille beskyttelsen man får av hjelm overskygges av at man selv tar mer risiko, og av at bilister viser mindre hensyn.
Jeg skrev ikke spesifikt deg, jeg skrev alle. At du tolker det som personangrep må være opp til deg.
Hvis du virkelig er forsker med phd, noe jeg ikke tror, burde du ha funnet den nyeste og mest relevante forskningen. Ikke linket til ett dårlig innlegg. Det har selvfølgelig noe å si hvilket årstall det er skrevet. Ting utvikler seg og forskningen går stadig fremover. Btw så rapporte du ett innlegg som du tolket at vedkommende kalte deg idiot og gikk på personangrep. Her gjør du det samme selv
Alt kan være farlig om du vrir og vender på det.
Kan du fortelle meg på hvilken måte forskningen på hjelmer har gått fremover?Hvis du virkelig er forsker med phd, noe jeg ikke tror, burde du ha funnet den nyeste og mest relevante forskningen. Ikke linket til ett dårlig innlegg. Det har selvfølgelig noe å si hvilket årstall det er skrevet. Ting utvikler seg og forskningen går stadig fremover. Btw så rapporte du ett innlegg som du tolket at vedkommende kalte deg idiot og gikk på personangrep. Her gjør du det samme selv
Alt kan være farlig om du vrir og vender på det.
Jeg forsker på noe helt annet enn sykkelhjelmer. Noe mye morsommere. Det vil si, de fleste synes nok ikke det er så morsomt, men jeg synes jo det, derfor har jeg valgt å forske på det. Om jeg sier hva det er vil du enkelt finne ut hvem jeg er, så jeg vil ikke være mer spesifikk. Du får tro på det eller la være
Jeg synes det er vanskelig å tro at du ukentlig ser folk som er ødelagt av ulykker der manglende hjelmbruk var utslagsgivende, men jeg tror ikke du lyver om det. Jeg velger å tolke det du skrev dithen at det er de samme pasientene du følger over tid, og ikke at det kommer nye pasienter som er skadet av manglende sykkelhjelmbruk hver uke?
TwinBaBe
Glad i forumet
Signerer denne
Om folk ikke har sett sykkelulykker hvor det ikke har blitt brukt hjelm, så er det bare å søke litt så kan man se hvor dramatisk det kan gå.
Så ja, vil påstå det er idioti å ikke bruke hjelm .
Risk of head injury per million hours travelled
( http://www.howiechong.com/journal/2014/2/bike-helmets )
Hvorfor mener dere ikke det er idiotisk å kjøre bil uten hjelm eller å jogge uten hjelm da? Sjansen for hodeskader er større.
- Cyclist - 0.41
- Pedestrian - 0.80
- Motor vehicle occupant - 0.46
- Motorcyclist - 7.66
( http://www.howiechong.com/journal/2014/2/bike-helmets )
Hvorfor mener dere ikke det er idiotisk å kjøre bil uten hjelm eller å jogge uten hjelm da? Sjansen for hodeskader er større.
There is some evidence that wearing a helmet may directly increase your chance of getting injured in the first place. In 2001, an article in the New York Times reported that the rate of bicycle head injuries had risen sharply — an increase of 51% — during a ten-year period when bicycle helmet use became widespread. This during a time when statistics showed an overall decrease in bicycling in the United States. No one knows for sure why head injuries among cyclists increased, but there are a few theories.
First, wearing a helmet changes how drivers perceive the cyclist. A University of Bath study showed that drivers, when overtaking cyclists, gave helmeted cyclists significantly less space than they gave cyclists who don't wear head protection. The study found that drivers were twice as likely to pass closely to a helmeted cyclist, and that drivers passed an average of 8.5 cm (3 1/3 inches) closer when the researcher was helmeted than when he was not. Not only does this increase the chance of being clipped by a vehicle, it leaves cyclists with far less maneuvering room to avoid other potentially injurious road hazards like potholes and icy patches.
Second, the design of the helmets themselves may increase the chance of some types of injuries when incidents do occur. Three separate studies have shown that bike helmets may increase the probability of certain types of neck injuries. There's some evidence that having an enlarged piece of plastic and foam on your head increases the probability of hitting an object that you'd be able to avoid in the first place, or that otherwise glancing contact with a surface becomes a full-on blow when the head is helmeted.
Finally, wearing a helmet may create a false sense of security and induce risk-taking that cyclists without head protection might not make. Those wearing helmets may take risks that they wouldn't otherwise take without head protection.
First, wearing a helmet changes how drivers perceive the cyclist. A University of Bath study showed that drivers, when overtaking cyclists, gave helmeted cyclists significantly less space than they gave cyclists who don't wear head protection. The study found that drivers were twice as likely to pass closely to a helmeted cyclist, and that drivers passed an average of 8.5 cm (3 1/3 inches) closer when the researcher was helmeted than when he was not. Not only does this increase the chance of being clipped by a vehicle, it leaves cyclists with far less maneuvering room to avoid other potentially injurious road hazards like potholes and icy patches.
Second, the design of the helmets themselves may increase the chance of some types of injuries when incidents do occur. Three separate studies have shown that bike helmets may increase the probability of certain types of neck injuries. There's some evidence that having an enlarged piece of plastic and foam on your head increases the probability of hitting an object that you'd be able to avoid in the first place, or that otherwise glancing contact with a surface becomes a full-on blow when the head is helmeted.
Finally, wearing a helmet may create a false sense of security and induce risk-taking that cyclists without head protection might not make. Those wearing helmets may take risks that they wouldn't otherwise take without head protection.
