I did a quick count in my old helmet cam video from LART and I came up with ~75% riders wearing helmets. I think the percentage is even higher in San Diego. I don't know if it says anything about effectiveness of helmets, or it just underscores that the set of recreational roadies and the set of cyclists who get killed have relatively little in common. (I'm also pretty sure that very few spandex-wearing types I encounter are legally drunk.)
So more than a quarter of all bicycle-related fatalities were only on their bikes because they were too buzzed to drive home from the bar.
It's more likely that a quarter of bicycle-related fatalities were on their bikes either because they had no money for a car, or because they had no license on account of a past DUI or two.