Originally Posted by Mark in Idaho

You cannot suffer an head injury in the car unless the car has hit a solid object. This also was not a sub-concussive impact.

The only way there is risk of injury without hitting a solid object (another vehicle) is when the road is violently rough and you drive down that rough road for a few minutes of continuous shaking. This is unreasonable because the driver could just slow down to get past the rough area after the first few bumps.