[cigarpowder]

Fascinating ideas... I was astounded to hear that drivers pass closer to bicyclers wearing a helmet. I'd be prone to do so myself, as they do induce a false sense of security for both parties. I also agree that perhaps drivers and pedestrians should wear helmets as well.

[SoftwareMaven]

If it's true (there are others here who say it's disputed), I would bet it is more because the driver trusts the cyclist more. The more the cyclist looks like she is a competent, experienced cyclist, the less you worry she will swerve toward you suddenly, fall over, or do some other random thing. The less you worry about that, the closer you feel you can get.

[femto]

It could be that bare headed cyclists get noticed more? Where I live has mandatory helmet laws, so a helmeted cyclist is normal. If I see a bare headed cyclist, it strikes me as an unusual thing to see, and my attention is automatically drawn to him, and I will then carefully pass. If all cyclists were bare headed, maybe they wouldn't get noticed as much and the passing distance might reduce?