On Thu, 27 Jan 2005, Russell McMahon wrote:

> AOTBE, do speeding cars have a higher energy at impact and do more 
> damage than NSC. Obviously yes.

I keep reading about this. I once posted to the piclist that the energy 
of the *person* in the car (whether the hitting or the being hit one) 
matters. In those terms, anything going faster than about 20km/h will be 
bad (without protective devices).

The limit of the energy that an unprotected human can take in one hit is 
about 2000 Joules. That's the energy one disperses (at impact) by 
jumping barefoot onto something hard from about 2 meters height (you 
wouldn't do that, right ? - but if you'd jump from a lower height you'd 
dare it), the energy one acquires when traveling in a vehicle with just 
over 6m/sec speed (~22km/h) (you wouldn't run as fast as you can into a 
concrete wall, right ? - but if you'd run a little slower you'd dare 
it), and roughly the muzzle energy of a M16 standard NATO round (info 
from the net, 64 grain weight projectile). I am quite sure that this is 
not a coincidence. It's an unwritten limit of what an average 
unprotected human would 'take'.

So in the car context, >20km/h is 'too fast'. Beyond that, at direct 
impact, you are talking about damage control, i.e. what the doctors can 
patch, not whether you get hurt. The record of impacts with unyielding 
objects (like trees) showed this very clearly. Hitting a tree head on 
without a belt or airbag (or crumpling structures) at 40km/h is 
considered fatal afaik. Skiers often go faster than this.

At the 'safe' speed of 50km/h one would have more than four times the 
energy quoted above. Safe ? Haha. So the 'safe' speed is simply a bar in 
some statistics which says that if the powers that be manage to keep X 
percent of drivers underneath it, then the amount of accidents will be Y 
% less than now.

Peter
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