What a saturation ceiling is
Given a drive time, a population threshold and a road network, there is a finite number of non-redundant catchments in the United States. That count is the geographic ceiling. It is not a forecast of demand. It is the point past which additional units necessarily overlap ones you already operate.
Why targets and ceilings disagree
A unit target is usually a statement about trade-area size that nobody has said out loud. Halve the drive time you are willing to underwrite and the ceiling rises sharply, because you are accepting a much smaller catchment per store. That is a legitimate strategy. It is also a different business than the one the current footprint implies, and it changes what a defensible site looks like.
The right way to use it
Do not ask whether a target is achievable. Ask what drive time makes it achievable, then ask whether the unit economics survive at that catchment size.
Narrative Geo does not sample. Every trade area is routed on the actual road network with OSRM, against full Census/ACS population and a maintained POI set. The drive-time matrix is precomputed at 1.08 billion rows, so a national query across a 20,000-store chain returns in seconds rather than being metered per location.