Australia

I genuinely appreciate the replies and detail.

I must confess I hadn't read the articles you cited. I know the issue is complicated, so my goal was to make a very simple and transparent model to try and understand what's reasonable. The numbers you quote imply that an assumption in that model is wrong, or something really critical is missing that flips the result. It's hard to see what or understand why. In terms of head injuries, in a given day, riding is more safe than driving maybe ...? Or more people ride than drive ...? Probably not. So I'm left more confused by these numbers.

So then I went back to your references. The source of your numbers (from what I can tell) is here, which you gave above. This is an anti-helmet website, not a study. Noting that anyone can write a blog and claim numbers, I looked for the references to support their claims. There are none. So it's hard to tell if these are real studies (probably), but from when, where, cherry-picked, and so on (these details matter). Not linking to the source is a huge red flag for me and undermines any trust in what's argued.

I'm guessing the Australian study you reference above is the same as the one they mention (thank you for linking directly to it). This is a nearly 30 year old paper from UNE, looking at the numbers one year after helmet laws were introduced in Vic and NSW. Without going into detail about some of the questions I'm left with (e.g. what is the variance in the year-to-year numbers they're comparing the 1-year changes against? - if that variance is larger than the 1-year change then the change is statistically meaningless - and so on), I figured, regardless, this is a poor source to draw such significant conclusions from (old, 1 year of noisy data, limited geography etc). Surely there are more recent studies that track the longer trends with more complete data?

Looking at the papers that cited that paper, I find this, which is a 2018 meta analysis of 21 studies (including the older AU one) from Norwegian researchers. This should be a better source, given it covers a much wider timespan (collectively), populations, geography, methodologies etc. They also spent a lot of time on the uncertainties, biases, etc to aid in the interpretation of their results.

There's a lot of good information in the abstract, but here's what I think is the headline result:

"The summary effect of mandatory bicycle helmet legislation for all cyclists on head injuries is a statistically significant reduction by 20% (95% confidence interval [−27; −13]). Larger effects were found for serious head injury (−55%; 95% confidence interval; [−78; −8])."

There's lots more of interest to this discussion at the link. But I haven't gone through the paper beyond the abstract. I'm happy to take the abstract as written (since I have a job and other things to do today).

So what to make of all this? My experience is that there are two types of people. A small but very passionate minority of riders that are really, really against helmets. And the rest of the population who don't really care that much. They ride and wear their helmet - sometimes forget it and hope they don't get caught - but otherwise don't think about it.

The biggest issue I feel we should be directing our energy towards is improving the cycling infrastructure by separating bikes from cars. Cars are the biggest danger to riders. Until we do a better job at that, and without the benefit of a metal cage around us like cars provide, simple steps to protect our heads - the helmet - make a lot of sense, to me at least.

EDIT: some minor wording clarity.