It does feel, to me, as if a few inches this way or that make a difference, not so much on dual carriageways, but on roads in general.
I started a regular commute a few years ago and had unpleasant close passes often. As time passed they reduced and reduced to a much lower level. Partly down to route selection, partly down to cycling at a speed nearer to that of the traffic, but mainly, I think, down to positioning at key points, basically moving further out from the kerb at the right moment. It's far more subtle than switching from secondary to primary; I feel as if I have a dozen degrees of primary in my repertoire, and timing is as important as position, as is listening to engine note and judging how close the car behind is - usually without looking.
And I'm still learning. My cycling is still developing and I'm tending to cycle further out more often and earlier, and to make myself more visible and, by positioning and indicating, to make sure I'm seen.
A fast dual carriageway would be beyond my skill set. Primary or secondary, I would just have to choose one and trust in the other drivers, and that doesn't appeal. I would try very hard to avoid it, perhaps set off earlier and have a longer, more pleasant ride.
