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 sent the Crasche folks an email asking for data documenting the performance of their product. I received the following results as part of a brochure:
LACROSSE CROSS-IMPACT TEST RESULTS (these appear to be stick-to-head):
Sample # Impact Location Cross Velocity (MPH) Severity Index Peak G
Bare Headform Side 38.19 1051 188
Size Small Side 39.94 68 49
Size Large Side 38.64 8 24
LACROSSE BALL IMPACT TEST RESULTS (these appear to be ball-to-head):
Sample # Impact Location Ball Velocity (MPH) Severity Index Peak G
Crosse BXX 1 68.34 278 179
Crosse BXX 2 69.3 208 146
Crosse BXX 3 70.75 294 186
The following sentence was also included in the reply: "Note - these tests are for impact coming onto the head. In a drop test, which simulates a head on collision, a 9 mph event showed an impact reduction of about 25 %."
For reference, I remember reading somewhere that football helmets tend to increase impact duration from about 3 ms to about 8 ms. Assuming uniform force distribution over the duration of the impact, this amounts to ~60% reduction in peak acceleration (over whatever force domain that acceleration is correct for.) Crasche seems to guard effectively against forces similar to that of a lacrosse stick being swung at the head. Unfortunately, the results of the second test don't really tell much about the usefulness of the hat, as the Crasche folks seemed to be satisfied with concluding that it's effective based on seeing a Severity Index (I assume this is based on the head injury criterion?) <300 when taking a lacrosse ball to the head, despite the huge accelerations.
Regarding the drop test - 9 mph gives around 137-82g of acceleration against a hard surface, assuming impact duration of 3-5ms. So we're looking at 6700-4000N (human head weighs around 5kg). In this domain, we can compare the 25% advertised reduction against the ~60% ballparked estimate for force reduction due to a football helmet.
In summary, the performance appears to be inferior (in terms of pure linear acceleration reduction) to a sports helmet, as one would expect. The hat appears to be more effective against object-to-head impacts (involving smaller forces) relative to impacts that result in the head being brought to a halt from motion (involving larger forces) - perhaps the crushable elements providing the resistance are crushed by the larger forces? My gut says that the performance in a vehicle collision will probably bring the head to a halt against a relatively immobile object, so the hat won't do much of anything as the crushable bits are crushed too fast to be effective.
I don't see how the latter clause follows from the former. You said that in the drop test, the impact reduction was roughly 25%. This isn't huge, but I can't say it "won't do much of anything." Were you thinking of something else to support your claim?