Put a Lid on It

from Road Rights By Bob Mionske
No state requires adults to wear a helmet when they ride (although some municipalities do). But you could still face legal consequences if you fail to strap one on. Consider this scenario: A negligent driver hits you, and you sue him for the cost of your medical expenses. However, his insurance company claims that you were negligent for not wearing a helmet. The outcome of the case will depend on whether the jury decides it was reasonable for you to ride without one. They might not: Many people believe that helmets prevent all head injuries, although it is still possible to suffer one while wearing head protection.
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https://bicycling.com/blogs/roadrights/2011/08/16/put-a-lid-on-it/oldId.20110816210942168