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.
Most of the studies that I've seen either seem to be looking at high-acceleration impacts or are trying to quantify impacts received in "daily life". I have repeated impacts flagged in my head as "result unknown" - I haven't come across anything that I can remember that would give legit thresholds for how hard a repeated impact has to be before it would cause damage (other than the 10g figure noted above). People seem to agree that repeated impacts have the capacity for great harm, and I remember seeing that people with a certain gene appeared to be more prone to symptoms if subjected to repeated impacts, but that's pretty much all I remember. Let me know if you find anything.