That's a question that very much has a "it depends" kind of answer.
So, in general the population of the Sixth World tends to be lower than the real world, or at least lower than what real world projections are for the given year (due to the wars, plagues, and general mayhem of the alternate timeline). But that population that does exist is much more concentrated in urban areas than the real world's... not just for all the same reasons that real life urbanization is taking place but also the perverse factor of cities actually being safer than rural areas when it comes to monster attacks and whatnot.
That all means the areas between sprawls are indeed much more depopulated than in the real world. Smaller towns can still exist, perhaps as a small sliver of its former (Fifth world) size. I'm sure quite a few simply become ghost towns and are abandoned entirely- they'd make great havens for gangs or packs of ghouls or who knows what. In some areas, quite a lot of the geography is eaten up by megacorporate agri-businesses, so they'll still have decent infrastructure despite low local populations, but their roads and rail might not be open to the general public. On the other hand, certain sprawls essentially don't have areas between them. FDC to Baltimore to Philly to NYC to Boston is well on its way to becoming one Judge Dredd style Mega City.
Then there's the geo-political angle. North America has balkanized into numerous countries that tend to have more grievances than reasons to maintain open borders. Since the CAS broke ways fairly peacefully with the UCAS, the old USA's interstate system more or less still works east of the Mississippi but when you look at east-west travel, the NAN went to particular effort to raze roads and towns during their internal decolonization projects, which of course ties directly in to your question. You CAN, for example, drive from Seattle to Cheyenne but there's probably only 1, or at most 2, road corridors to possibly choose between.
Due to many of the factors above, also bear in mind that airship commerce is a viable thing in the Sixth World. Roads and Rail are sometimes not as good an answer for shipping goods across North America as airships are, which literally sail above such problems on the ground.
