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

Some evidence from practical trials suggests that motorists pass closer to cyclists wearing helmets, I think the CTC probably has the results on their site. Also, looking like a female gets you a wider berth, so go figure.

I would be amazed if the drivers I encounter would even be observant enough to notice a helmet or not (seriously). I actually think a hi viz top would have more of an effect but have no evidence!