LTB is an acronym for "low traffic bathroom". This is usually used in reference to bathrooms in public places, such as airports or convention centers.
LTB's are a desirous commodity, in that they provide one with a stench free place for defication, contemplation, urination, and mediation.
All the stalls in the first floor bathroom are full, and there are hobos performing handjobs in the second floor bathroom. The bathroom behind the kitchen next to the janitor's closet on the fifth floor is an LTB.