See the book Cyclecraft, by John Franklin. In short, it's to be in drivers' primary zones of attention, to be more conspicuous, and to improve sight lines and safety buffers with respect to potential crossing movement hazards in front of you.

I think the problem here is that I don't believe tunnel vision is a prevalent as you do.

Clearly define "safe" as it applies in this context and then I will happily answer.

I didn't think the question was that hard. Get a dictionary for any words you don't understand.

"Safe" is a very general, subjective and ambiguous term, so much so that it arguably borders on being meaningless.

Is rock climbing "safe"?
Is skydiving "safe"?
Is riding a bike without a helmet, "safe"?
Is riding a bike in a bike lane "safe"?
Is regularly rolling stop signs "safe"?
Is taking a lane "safe"?
Is it "safe" to run with scissors?
Is it "safe" for a 10 year old to walk to school?

If someone answers "yes" or "no" to any of these questions, can it mean anything to anyone other than whoever is answering the question?

i think allister is asking - i've asked the same thing of you before -

if you think a lane position to the side, sharing a lane is 'safe' (to you personally) when other traffic is present, what makes that same lane position unsafe when there isn't traffic present.

how does a position to the side of a wide lane deemed 'safe' when traffic is present become 'unsafe' in the absense of traffic?

i think allister is asking - i've asked the same thing of you before -

if you think a lane position to the side, sharing a lane is 'safe' (to you personally) when other traffic is present, what makes that same lane position unsafe when there isn't traffic present.

how does a position to the side of a wide lane deemed 'safe' when traffic is present become 'unsafe' in the absense of traffic?
I'm pretty sure I've successfully avoided making qualitative statements about the safety of various positions in terms of meaningless words like "safe" and "unsafe".

What I've talked about is relative safety - the relative safety of position A as compared to B, or the relative safety of a given lateral position in one situation as compared to the safety of the same position in another situation.

The reason a given position in the margin can be more safe (note: a relative term) when fsdt is present than when fsdt is absent is because while fsdt is present it effectively runs interference with respect to potential crossing hazards in front of you.

To illustrate, consider the attached diagram. There are two streets that run left to right. There is a yellow bike on the upper street, sharing his wide lane with two same direction purple cars. On the lower street is a green bike with no other same direction traffic.

Both the yellow and green cyclists are riding in the same positions in the margin and theoretically face the following potential crossing movement hazards at the upcoming intersection:
A left cross from the red car, which is planning to go straight across the intersection and is "late on red".
A left cross from the blue car which is planning to turn left across the intersection.
A right cross from the gray car which is planning a right on red.The practical differences are that:
The driver of the red car has stopped in the upper situation because he noticed the purple cars; the driver of the red car in the lower drawing did not notice the lone cyclist riding in the margin, and is gunning it to cross the intersection even though the light turned red.
The driver of the blue car in the upper situation is yielding because he saw the purple cars; in the lower street he's proceeding to make his left turn across the path of the cyclist.
In the lower situation the driver of the gray has looked left, not noticed the cyclist in the margin, and is about to turn right, right in front of the cyclist. In the upper situation he's waiting because he sees the purple cars coming.That's why the position in the upper situation for the yellow cyclist with same direction traffic present is safer than the same position in the lower situation with same direction traffic not present.

56245 Click on the thumbnail image to see the large image.

Yes, I realize that the yellow cyclist is potentially vulnerable to a right hook and of course the green cyclist is not. So, of course the yellow cyclist should be looking back, judging relative speeds, and, if it's possible that either of the purple driver can and might turn right, negotiate/merge left accordingly. But the right hook threat is all besides the point, which is that the threat posed by the red, blue and gray is less for the yellow cyclist than for the green cyclist because of the presence of the purple for the yellow, and their absence for the green.