General Discussion

Even if every car on the road was fully equipped with the modern range of airbags there would still be fatalities. And many (most?) of those fatalities would be from head trauma.

A secondary collision happens after the airbags have gone off... you hit someone and then a truck hits from behind.

Being hit from the side... the side curtain airbags slow your head down some but there are limits.

And so on.

Of whatever number of head trauma deaths occurred, would helmets prevent a chunk of them? Sure.

Lots and lots of cars, so the numbers of those saved would be large, in absolute terms.

But the rise in net inconvenience would be much greater than the relative gain. Helmets save a much greater percentage of cyclists than they would auto drivers.

It kind of like looking at traffic fatalities at different speeds. To save the most people the national speed limit should be zero, so we know going into it that speed limits are a social balancing act between convenience and safety.