A 2006 study showed that “280,000 people in the U.S. receive a motor vehicle induced traumatic brain injury every year” so you would think that wearing a helmet while driving would be commonplace. Race car drivers wear helmets. But since almost no one wears a helmet while driving a regular car, you probably fear that if you wore one you would look silly, attract the notice of the police for driving while weird, or the attention of another driver who took your safety attire as a challenge. (Car drivers are more likely to hit bicyclists who wear helmets.)
The $30+shipping Crasche hat is designed for people who should wear a helmet but don’t. It looks like a ski cap, but contains concealed lightweight protective material. People who have signed up for cryonics, such as myself, would get an especially high expected benefit from using a driving helmet because we very much want our brains to “survive” even a “fatal” crash. I have been using a Crasche hat for about a week.
I've looked up a few studies over the past few months in an attempt to understand direct causes of TBI and what kind of accelerations are "safe" - those under which a TBI (or diffuse axonal injury, DAI) is unlikely to occur, and to get a sense of what types of activities different accelerations correspond to. I'm dumping some info below without many citations as it's much lower effort than the alternative. If there's anything in particular that one is interested in, I may be able to find the relevant article. if you see anything that is wrong, please let me know.
TBIs appear to be caused by a combination of linear and rotational acceleration; these appear to cause injury via different mechanisms. Linear force/acceleration appears to create a high-pressure volume on the side of impact and a low-pressure volume on the opposite side of the head, with injuries possibly resulting from either. Rotational acceleration can, broadly speaking, cause brain tissues to stretch and twist in a way that they're not resilient to. However, this only appears to cause direct damage in the most severe of cases. In most cases, the majority of damage appears to be caused by a chemical cascade initially triggered by the damaged tissues. Bleeding of the brain is also possible, though I am not very clear on the causes of this.Regarding the chemical cascade, calcium ions spring to mind as one of the damaging species - curiously, repeatedly recovering from chronic alcohol abuse seems to be contraindicated also due cumulative damage from increased release of calcium ions, and they also may be rather indirectly linked to Alzheimers. More detail on the impacts and causes of calcium ion release in the brain is on my to-learn list.
In severe cases, it appears that hospitals sometimes administer treatment to control intracranial pressure or manage the chemical cascade to reduce damage. (I am not sure whether the focus is on inhibiting the cellular mechanism perpetuating the cascade or neutralizing the offending species themselves.)
Concussions seem to start happening around linear accelerations of ~50g (though one article places this at around 20-30g if I remember correctly, and expresses concern regarding repeated impacts as low as 10g) and/or a rotational acceleration of ~2000 rad/sec^2. Impact durations appear to range from ~3 milliseconds (against a hard surface) to 8 milliseconds (wearing a football helmet). As expected, lateral impacts (from sides, or front/back of head) produce greater rotational acceleration compared to impacts to the top of head. I would personally expect that injuries involving cessation of motion of the head would be hazardous at much lower peak velocities than those involving an object striking the head, as the 1) head is (usually) more massive than the object striking it, resulting in greater net force and 2) neck strength may reduce peak accelerations in the latter case, but perhaps not the former.
These links give examples of accelerations that appear to be produced during assorted daily activities. An experiment was done in which people of various ages shook their heads up/down in an "unconstrained manner", resulting in peak linear accelerations of ~4-5g and peak rotational accelerations of ~250-500 rad/sec^2. This appears to be somewhat difficult to reconcile with reports of injuries from headbanging, as these accelerations are very much below typically expressed hazardous levels.This article seems to refer to a similar motion, while reaching radically different results regarding accelerations (though, as far as I tell, it only does modeling, no measurement of actual head movement.)
Weren't there a lot of studies about frequent low-intensity shocks to the head -- specifically in the context of American football (and boxing before that)?