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)?
I did my undergrad engineering capstone project at the beginning of this year creating a linear accelerator to subject networks of mouse brain cells to repeated 50g acceleration loads, based specifically off of football helmet impact data.
I was only assisting the PI running the research so I hadn't read all of the literature, but from what I know the jury is out on a good model of risk from repeated head impacts. We can tell you pretty well what the risk is for single impact events, but expect a few years for the first characterization of repeated trauma to be published. This is based on my lab's timing of course - I'm not sure how far along other labs are with this.
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.