Monday, December 3, 2012

Helmets and Statistics

Some people say statistics have no meaning.  It is true that statistics can be twisted to prove anything.  It is also true that people who understand how numbers are collected, can learn valuable lessons from statistics.  So you may ignore statistics that are being used only for propaganda.  But some statistics have survival value.

So now let me discuss statistics, without really giving any statistics.  The statistics we are all looking for regarding helmets:  When wearing a helmet how much are your chances of surviving improved?  Also, how effective is a helmet in preventing different kinds of injury? e.g. brain damage, face damage to skin or bones, etc. And furthermore, what is the effectiveness of a helmet in different situations? e.g. sliding down the road, hitting an immovable object at speed, or resisting penetration when you strike a sharp object with your head, or if it strikes you.  For some questions, no data has been gathered yet.  For others, the same answers keep coming back.  Mainly, a helmet can save you from death about one third of the time.  That statistic is worth remembering. Use it to decide if you want to wear a helmet.  This statistic is also intuitive, in that common sense should tell you that wearing a helmet is going to provide more protection than not wearing one.  You can ignore it if you like, but don't make up any bogus argument like "Statistics don't mean anything".

One more thing to consider about statistics is this.  The way statistics are gathered, we need to use actual accidents, and not all non-fatal accidents are reported.  We generally figure out what percent of people ride with helmets, and then compare motorcycle fatalities due to head injury with and without helmets.  So if 50% of people ride with helmets in a given state, and 75% of fatal motorcycle head injuries are not wearing helmets, then I think it works out to a 50% better survival rate for wearing a helmet.  (These are made up numbers, just to illustrate the math.)  If everybody wears a helmet all the time, then you can't calculate what the survival rate is, because everybody who dies would also be wearing a helmet.

But there is another thing to consider.  Are people wearing helmets better or worse drivers on the average?  You might think they would be more careful, but I'm not sure we have any data to back that up.  Also, they may be worse drivers because they feel invulnerable, and also the quietness of the helmet may fool them into thinking they are going slower than they actually are.  All these things may affect the survival rate, at least the way I think we measure it.

Now what kind of helmet?  To know that answer, we need to consider that most of the time, when you are riding a motorcycle, your head is moving at a good speed.  When your head is moving at speed, so is your delicate brain.  Now if the skull contacts a solid, non-moving object, at a speed of say 30 kph, you brain has to also come to a sudden stop inside your skull.  When it does, things will break inside your brain, and you may die as a result, or suffer permanent brain damage.  How much damage to your brain depends on G forces, or how quickly your brain has to decelerate from 30 kph to 0 kph.  That force depends on two things, how fast you are moving when contact is first made, and how many centimeters your brain travels before stopping.  Inside your skull, the brain can move a little bit before coming to a stop.  The helmet adds a little bit more stopping distance by providing a crushable inner Styrofoam lining.  But that crushable lining must not be too soft, or too hard.  It must be just the right density to crush at the same rate all the way from 30 kph to 0 kph.  That crushable liner may be about 2 cm thick, so even when working perfectly, it only gives you an extra 2 cm to stop from 30 kph.  In other words, it cuts the G forces inside your head by about a half.

If you wanted a helmet to cut the G forces by about 90%, you could wear a helmet with a (softer) crushable liner 10 cm. thick.  But the fact is nobody wants to wear a helmet that big.  So everybody opts for smaller helmets, and we accept a 33% survival rate instead of a 90% survival rate.  Furthermore, some people opt for no helmet at all, and go with 0% survival rate.  OOOPS- am I misusing statistics?  What I mean by 0% survival is compared to chances of surviving an accident that would have killed you if you were bare headed. So if you ARE bare headed, and happen to have the type of accident that will kill you if you are bare headed, then mathematically your chance of survival should be 0.  We calculate the survival rate as improving (or getting worse) by wearing a particular helmet.  And just to be scientific, yes it is possible to design a helmet that would kill you sooner than not wearing a helmet ( a negative survival rate).  It is also possible to design a helmet nobody would want to wear, that would be ten times safer than our current helmets.

There are other types of head injury that have little to do with G forces.  Here is an example.  A rock from a passing gravel truck hits your head with a difference of speed of 30 kph.  Because the rock is small compared to your head (hopefully), the rock will not be able to change the speed of your head very much, and actually, it will be the rock that changes its speed, not so much your brain.  If the rock is about the same weight at your head, then the G forces are still only about 50% (or divided equally between the rock and the head)  So small rocks cannot impart enough G forces to damage your brain.  Of course, this logic is the same with any object you hit.  The more that object gives way, the more the energy is shared, the better your chance of survival.

In the end, what you are doing by wearing a helmet, is carrying with you at all times a slightly softer surface that will share about 50% of the impact force no matter what your head hits.  That's assuming the helmet has been tested for structural integrity and crushability of the liner.  And that you're wearing it on your head, not hanging from the rear turn signal because you are in Florida, where there is no helmet law.

Picture: from

1 comment:

  1. Well, even a $800 helmet cannot repeal the laws of physics (and physiology).

    Although your point about risk homeostasis ('they may be worse drivers because they feel invulnerable' is well taken, the preponderance of evidence is that helmets do reduce fatality and injury levels.

    A study released this past summer by the CDC in their Morbidity and Mortality Weekly Report indicates a 400% differential between states with mandatory helmet laws and those without ...