This topic is closed.
After twisting my brain in attempts to pinpoint the locations of Big Empty and The Divide -- unsuccessfully -- I decided to brute-force the issue, placing these locations as plot and logic dictate.
1. Big Empty straddles I-15, close enough to it to threaten traffic (as it is mentioned that its existence prevented caravan routes from forming between the Boneyard and New Vegas). I thus assume that trafic from Mojave outpost doesn't go along I-15 but turns north along 127, making the area around and to the east of Necropolis (which I assume to be Barstow) a sort of blank spot where people disappear too often to travel through. So the route from Hub to Vegas would go north through NCR capital, then south through the Death valley, then North again. That would rationalize why Vegas wasn't explored during FO2 even if it seems close by when you look atthe map.
2. The Divide is placed in such a way that when it forms, it blocks 160, possibly making the usage using 127 difficult. What is most vexing - and mysterious - is that 95 must be impassable (as it is mentioned that the NCR scouts first reached the Hoover Dam, *then* the Vegas ruins.)
Well, I I can use the ass-pull of "landscape being saturated with faults" for the 95. Kinda, the war had triggered some nasty tectonic effects :\
P.S. Uh, maybe the area being inhabited by Deathclaws is a factor? Even these beasts are nothing against a regular army... Except when they have a *reputation*, and the said army shits bricks at the first mention of them. *Especially* if they breed there in numbers.
yet at the same time useful due to being able to use it without worrying about connections or power.
As I noticed, the internet map have a nasty problem of the small details disappearing too soon as you zoom out. So the paper maps win by simply being packed more densely with information.