You cannot build a sign before a corridor connection. It would be nice if there was some sort of space to be able to place the sign on the entrance of a corrdior.
As of now, signs are placed awkwardly to the side of a corridor as displayed in the screenshot.
https://steamcommunity.com/sharedfiles/filedetails/?id=3805022114

you can put screenshots directly in the nolt post using the link button, at least on mobile