I think it just past the Horowitz farmstead in the NW of the map, and if you carry on NW to the end of the map you come to a clearing that is very lightly blocked by cars, could this be a DLC starting ace?

It could be a possible road to The Big Empty in Old World Blues, but i'm not sure.

I'd figure that it would lead to New Canaan & Honest Hearts, it being a legitemate city which even the Crimson Caravan mention have strong traders; the shadowy nothern passage will lead to the Big Epty, in my opinion.

I think that that it is nothing just graffiti and the northern passage will lead to honest hearts since Utah is north, and somewhere south is old world blues, but that is what i think.

It could be, but also there is a place in the north [just east of there] that goes straight into the rock that I found. I don't remember it being there in my first playthrough and when you walk up to it it says something about [north passage].