So I was doing some research on the ways in and out of Las Vegas and I stumbled upon Route 95 which not only goes by where Shady Sands should be but it's actually a few days walk from the two cities, so the question is..why isn't it explained that the NCR doesn't use Route 95 as a supply line? It shouldn't have been effected by the Divide since the Divide is to the west of Vegas, not the north which Route 95 goes.

https://www.google.com/maps/place/Las+Vegas,+NV/@36.4849541,-115.7668884,9z/data=http://forums.bethsoft.com/topic/1503179-from-shady-sands-to-new-vegas/!4m2!3m1!1s0x80beb782a4f57dd1:0x3accd5e6d5b379a3

http://fallout.gamepedia.com/File:FOC_Region_Map.png