It’s really contentious and complicated.
The ASTM standard isn’t intended to protect against concussions, though there exist designs that are.
No helmet companies make their helmets to specifically protect against vehicles, though it can reduce head injuries depending on the type of collision.
The types of automobiles on the road now with more weight, ground clearance, and higher noses (that’s probably not the technical term) appear to produce less survivable impacts though that doesn’t mean everyone struck goes under.
The data is clear that motorists pass helmeted cyclists closer though whether they pass them close enough to cause a danger is still hotly debated.
In countries with very safe cycle infrastructure like the Netherlands the injury rate is low because cyclists are protected from automobile traffic yet helmet use is incredibly low, at about half a percent in NL.
There’s been a few studies that I could find on whether people wearing helmets are more liable to behave more recklessly negating the protection provided by the helmets. And they seem to suggest that yes people wearing helmets do take more risks.
Latest fatality stats I could find is that 54% weren’t wearing helmets. And it’s hovered around 50% for the last few years. Which sounds bad when it comes to helmets preventing fatalities. But for 17% helmet use was unknown. So 29% of fatalities were cyclists who were confirmed to have been wearing helmets vs 54% who were confirmed to not be wearing them.
Now when mandatory helmet laws are looked into things get weird. They do reduce the overall amount of injuries, but not the rate, because the laws serve to significantly discourage people from using a bicycle.
Less people on bikes means less bike related injuries.
However the hospitalization rate per trip doesn’t decrease.
Mandatory helmet laws also create more excuses for the police to harass minorities. Here’s an article from someone opposing helmet laws that touches on a little bit of everything, including that.
And as mentioned a little in the article above, bike share users just aren’t going to be wearing helmets in any appreciable amount. It’s not gonna happen. Even if there’s a setup to allow for a helmet share along with the bike, people aren’t gonna be putting shared headgear on their head. It’s not happening. Yet widespread bike share usage, helmets or not, actually increases cyclist safety and reduces accidents per mile, by increasing the amount of cyclists on the road and acclimating motorists to their presence.
In my opinion helmets are indeed helpful and should be (voluntarily) worn but they’re currently treated like they’re far more helpful than they really are, as they’re, falsely, treated as a substitute for proper infrastructure.
Which both makes helmet wearers act less safe since they feel like they have a fictional forcefield around them, and also allows the onus of safety to be pushed entirely onto whether or not a killed or injured cyclist was wearing one and away from whether or not they were on a path sufficiently protected from automobiles.
The largest threat to someone riding a bicycle is far and away automobiles and cyclists getting hit at all should be rare.
