RedheadDane said:
Where else would you be riding?
It depends on the situation.
Roadway positioning is probably the least understood and least appreciated skill among cyclists, including among very experienced bicyclists.
The first mistake almost everyone makes is even thinking about positioning relative to the road edge. There might be some disagreement about how far from the edge you should be, but few realize that where you are relative to the road edge should often not even be a consideration when deciding where to ride. There are much more important considerations. If you follow a course relative to the road edge, you are often following a path that is weaving relative to other traffic. So, I always think of where I am relative to the roadway center, or relative to the nearest traffic lane stripe, which runs parallel to road center, unlike shoulder and bike lane stripes, which run parallel to the road edge. By the way, per the MUTCD, bike lane stripes are
supposed to be painted parallel to traffic lane stripes, but they almost always follow the edge of the road, like shoulder stripes. Following a shoulder stripe or road edge is not problematic when there is only one line of traffic in your direction, as it is for car/truck drivers on most rural roads, but it's basically never the case for bicyclists.
Anyway, I could write a chapter on the topic, or maybe even a whole book, but others have already done that. I again strongly recommend Cyclecraft by John Franklin. The original version is for the UK, but there is a new edition for North America.
Riding near the edge, even two feet from the edge as recommended above, should be avoided as much as possible, because your safety is greatly compromised riding there as compared to being well out in the traffic lane. Not only is the road edge where all the rubble and debris collects, but it's also where you're closest to roadside hazards (like a pedestrian suddenly stepping off of the sidewalk, or a kid or dog suddenly running out from between parked cars), you're in the door zone if there are parked cars, sight lines to you and from you are compromised as compared to being out in the traffic lane left of center, and you're much less conspicuous and so easier to overlook. That's because you're not riding where most people are looking for traffic most of the time. Bicyclists complain about not being noticed, but they usually don't realize how that's not just because of their relatively small size, but because of where they are riding. Motorcyclists have trouble being seen too, but that's partly because they are typically going much faster than bicyclists (so further away and even smaller so harder to see when someone might look), but still not nearly as often as typical bicyclists, because they tend to ride out in the traffic lane where others are looking.
Have you ever been passed by someone who then turned right in front of you? If you talk to them, they'll say, "sorry, I didn't see you", and no, they're not lying. The thing is what people "see" depends a lot on relevance (google for "inattentional blindness"). A cyclist riding along near the road edge is not relevant to motorists - this is obvious by how they drive by without slowing down or adjusting laterally,
as if you're not even there. So you're not really noticed... plus they're on auto-pilot, not really thinking. On the other hand, if you're out in the traffic lane, you're forcing them to notice you and deal with you. Use a mirror just in case, but you'll be shocked at how much better you're treated. Without exception everyone I know who has changed their riding like this has reported vast improvements in how well he or she was noticed and treated.
Here's a good article and video on the topic:
http://commuteorlando.com/wordpress/2010/11/29/helping-motorists-with-lane-positioning/