Seth Herd comments on Wear a Helmet While Driving a Car

Seth Herd 25 Mar 2026 21:48 UTC
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Impact duration up to 8ms can’t be important. The brain is sloshing in fluid for this reason. It’s not stopping in anything like 8ms anyway.

I concur with the conclusion that it’s worse than a helmet. Not sure at all why you think it wouldn’t help in an auto collision at all. Less than a football helmet let alone a motorcycle helmet (built for high vs low speed collisions).
- Elliot Callender 23 Jun 2026 12:24 UTC
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  Impact duration up to 8ms can’t be important. The brain is sloshing in fluid for this reason. It’s not stopping in anything like 8ms anyway.
  The subarachnoid space, where cerebrospinal fluid sloshes, is 3-6mm in adults. Along with dura, that gives ~1cm.
  9mph 4m/s, which is 400x the 1cm deceleration distance per second; loosely approximating as s deceleration time, 5ms seems like an upper bound.
  - Seth Herd 23 Jun 2026 13:17 UTC
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    What about the rest of the brain deformation?
    The brain deforming is basically the problem, but it does deform and that is going to change how it is affected by impact.
    But you’re right that 8ms is a relevant impact reduction.