BBCnews is carrying this report which summarizes the findings of a study which claims wearing a helmet actually increases the chances of a bicyclist getting into an accident. While a whacky idea on the face of it, the scientist who performed the experiments seems to have used rigorous and technical methods. So what gives? It seems that drivers give less space as they pass to someone wearing a helmet than they do to someone who is not. Stranger still, they gave more space to women than men.
From my own experience, most drivers actually do a good job of giving me enough space on the road. City bus drivers are especially nice, doing what they can to move as far away as possible. Which is good, because there's nothing quite as unnerving as a solid wall of roaring diesel passing a foot away from your shoulder.
Of course, it only takes one inattentive driver to completely ruin a cyclist's day, so I'm really careful, use trails whenever possible and avoid narrow or shoulder-less roads wherever I can. It's worked so far.