Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

I suspect that most people could come up with a fairly accurate estimate to get them fairly close to a sensible adjustment in behaviour

You suspect that, but it turns out that your suspicions are wrong; people make wildly inaccurate guesses, both about what their absolute risk is and about the effectiveness of any mitigation they employ. Staying on topic, the benefits of helmet use in everyday road cycling are so marginal that any change in behaviour between using and not using one is a massive over-reaction.