UK researcher, Dr. Mayer Hilman, studied all(?) the fatal cyclist head injuries for one year. He found that close to 92% of them would have died, even without the head injuries, as the impact with a motor vehicle had done things like crushing the rib cage, cause massive internal chest/stomach cavity bleeding, etc., etc.
Which isn't to say that a helmet cannot, ever, save one's life, or prevent/reduce serious injury, simply that the pro-helment case has been overstated as a means of making serious inroads into the numbers of head injuries experienced by cyclists.
Unfortunately, Dr. Hilman's studies have also shown that, since a hgher proportion of vhicle and pedestrian fatalities are caused by head injuries, governments should clearly pass laws which require them to wear them, with the consequent reduction in health care and other costs related to head injuries and fatalities.
Naturally, the US and UK publics will wholeheartedly endorse such policies. Won't they?