Sorry for being unclear. By saying "better off" I did intend to take into account the cost of wearing a helmet. Currently, this cost would be high because others would think less of the wearer. When I was young, no one wore a bicycle helmet, now not forcing your kid to wear one would, in my neighborhood at least, be considered bad parenting so it's more costly for a parent to not make his kid wear a bike helmet than to force him to use one.
I don't think that the (dis)approval of neighbours is the largest cost component, but there is a bigger issue.
In doing the cost-benefit analysis that involves uncertainty, a major factor is risk aversion. When you say "we would be better off" you imply that a particular level of risk aversion -- similar to yours, in this case -- would be optimal in some way. Why do you think so?
For example, my risk aversion seems to be noticeably lower than yours. On the basis of which criteria do you think that your level of risk aversion is more suitable as a social norm than mine?
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.