Why are there ZCTAs for some dedicated PO Box ZIP Codes and not for others?
Dedicated PO Box ZIP Codes present a special challenge for ZCTA delineation. While addresses with these ZIP Codes generally cluster around a post office, they may also be widely scattered. Additionally, there may be the option of regular/street delivery present in the areas around these post offices, resulting in a mix of valid USPS ZIP Code options for residents. For ZCTA delineation, the Census Bureau attempted to isolate those PO Box Only ZIP Codes for which there was a relatively compact, well-defined cluster of census blocks where the majority of residents chose or were required to receive their mail via PO Box delivery. For areas where the households receiving their mail via PO Box delivery are more widely scattered and intermixed with households receiving mail via regular/street delivery, the delineation process gives preference to the street delivery ZIP Code.
