British psychologist, cyclist, and traffic researcher Ian Walker has done the most comprehensive studies to date on how motorists and cyclers interact, and he says motorists give more space to helmetless riders, women, and riders in 'street' clothing.My anecdotal evidence seems to confirm this. I've been riding in street clothes exclusively since I started riding again. I also avoid wearing a helmet, usually. Now, if you always wear a helmet, or you haven't established a habit for yourself yet, please understand: I am not advocating that you or anyone ride without a helmet. You should decide that for yourself, based on laws that prevail in your area, or whatever else might be important to your decision making process.
But I will say that riding in street clothes (regular-looking shirt or sweater & casual shorts of some kind, or even Levis), seems to get me a little more passing room; riding with a helmet (even with my 'street clothes') seems to get me just a little less.