Bike Snob hits the nail on the head, again.
I would also like to point out that beyond preventing nothing really beyond superficial and skull injury, helmets make the effective circumference and radius of your head much larger. Logically, this size increase may make your head hit things, especially glancing things, it might not otherwise. So, the possibility exists that your helmet actually causes a larger chance of “catching” a glancing surface which will cause your head to turn in a rotational fashion about the axis of your spine. This causes a severe brain injury called diffuse axonal injury.
From a study on glancing glow effects on helmeted dummies:
“The rotational effects of the tested helmets differ a lot. The shell helmets do not grip the asphalt layer at all and do not rotate, which implies that neither the head form rotates. The non-shell helmets grip the asphalt layer in each impact, rotate and transfer this rotation to the test dummy head form.
“The method used in this investigation is probably applicable as an oblique impact test and might prove effective for testing several properties of a bicycle helmet; whereas today’s recognized standards use different methods for testing different properties of the helmet. Different accident types can easily be simulated and extensive measurements could be carried out. “
Now it is true that all modern helmets should have slick shells, a large number of people (including me) put things that are not slick on their helmets. I see visors, stickers, lights mounts, lights, cameras, reflectors and other things. All will grip pavement. Technologies like MIPS are promising but the efficacy is largely untested.
And then there is this:
“It is concluded that: (a) the meta-analysis does not provide scientific evidence that bicycle helmets, not being tested for capacity to mitigate the main factors that cause serious injury to the brain, do reduce it; and (b) the Australian policy of compulsory wearing of helmets lacks a basis of verified efficacy against brain injury, suggesting a need for an independent and open review taking account of relevant scientific research.”
Everyone pushing helmets perpetuate the myth that cycling itself is inherently dangerous. Getting hit by cars is what is dangerous. The trauma sustained by getting smacked by a car at 30MPH will not be lessened by a piece of foam strapped to your head. Having proper bicycle infrastructure (to reduce solo cycling accidents, dooring and dangerous car/bike interactions) and getting more (educated) people out on bikes is what is the most important to protect us.
Yesterday, I was wrangling my bike to the street on my way to work. An acquaintance riding the wrong way on a one way street went by and said “You forgot your helmet.” I shrugged. We ended up at the stop light together. She turned and asked my where my helmet was. I told her I don’t wear one. She looked at me oddly. I said I often where one while mountain biking. Which wasn’t true; I always wear one mountain biking.
We pulled away from the light. She proceeded to ride on the sidewalk to avoid some congestion which puts you exactly where a motorist who is pulling out will not see you. She then jumped a stop light at a bad intersection. Up on the sidewalk again when there was lane closure and flagmen directing around some street work. She had her helmet on and put herself in some bad places. I followed the rules of the road, stayed in the flow of traffic, road defensively and stayed visible and predictable to motorists. I managed to stay just behind her. She was “prepared” to get smacked by a car and invited it; I road to avoid the same. I know that all sounds harsh. She is a nice, smart person, just uninformed.
For the record, there are times that a helmet is absolutely necessary for me. Helmets are great for fending of overhanging branches and tree trunks that jump out in the middle of the trail when mountain biking. They are a place to put your sunglasses and make great platforms for lights and cameras. Helmets are important for those competing at their limits in racing or other cycling disciplines. My next helmet will be a MIPS design.