In Depth: Zip Codes & Carrier Routes
Key Points:
ZIP Codes and carrier routes are technically not areas.
ZIP Codes and carrier routes are simply collections of addresses.
Since they are not areas, official ZIP Code or carrier route boundaries do not exist.
Nevertheless, ZIP Codes and carrier routes are often treated as geographical areas.
Over time, ZIP Codes and carrier routes become fragmented and intermingled.
As a result of the fragmentation, ZIP Code boundaries are fuzzy.
Zip Code coverage areas (and thus directory scoping) are changing all the time.
The fragmentation of carrier routes hinders delivery quality.

By David Parks

Related Articles: Compare Grid Mapping with Postal Carrier Routes

ZIPS (Zone Improvement Plan) are Not Zones and Carrier Routes are Not Routes

Neither ZIP Codes nor postal carrier routes are area or line geographies. Both ZIP Codes and mail carrier routes are collections of addresses. Historically, a ZIP Code is the collection of addresses served by a particular Post Office branch while a carrier route is merely the collection of addresses served each day by the same mail carrier.

But Nearly Everyone Treats Them As Areas

When ZIP Codes and carrier routes were originally designed, the delivery points they served were typically well organized and bunched together - thus many people treated them as areas. The US Postal Service also unwittingly encouraged the concept of ZIP Codes and carrier routes as geographies by their naming. ZIP is short for Zone Improvement Plan yet ZIP Codes are not zones. A zone is a two-dimensional area yet addresses are points and points have no area.

The usage of ZIP Codes as areas has a lot to do with the fact that there are few alternatives. Counties are too large for most purposes. Census tracts and block groups would be perfect but Census naming schemes are too cumbersome. More importantly, everyone knows what their ZIP Code is because ZIP Codes are a critical component of their mailing addresses. Hardly anyone can name their own Census tract and block group.

The concept of ZIP Codes as points rather than areas is dramatized by the fact that some ZIP Codes consist of a single delivery point. A large business may have its own ZIP Code. A large office building may include more than one ZIP Code. Naval ships often have their own ZIP Code.

From Order to Chaos: How Carrier Routes Become Fragmented

When new homes and businesses are built along a mail carrier's route, the time required to deliver that route obviously increases. To minimize overtime labor costs, the US Postal Service must reallocate parts of carrier routes to other mail carriers.

Imagine starting with a hundred mail carriers for a hypothetical community. Within a few years, population growth could cause the time required to deliver each route to increase significantly. To minimize overtime, parts of each of those routes are then re-assigned to new carriers. Thus what started as 100 relatively congruent routes then become 100 routes plus 100 additional pieces of routes. What once started as rather simple, efficient routes becomes, over time, much more complex and inefficient

For US Postal Service mail carriers, the fragmentation of carrier routes isn't as problematic since carriers deliver the same route day after day after day. No matter how complex and fragmented their routes become, postal workers quickly adapt from sheer repetition. For a directory delivery worker that delivers the route once per year instead of every day of the year, however, the fragmented route is a much bigger obstacle.

The lesson: The fragmentation of postal carrier routes is problematic for directory distribution.

Postal carrier routes are notorious for being fragmented.

 In the carrier route map on the left, notice how the carrier route shown in light green is in 7 pieces.

In contrast, grid-based delivery zones - like those shown on the right - are usually in one piece. One piece means less errors.

See: Potential Directory Delivery Problems Common to Carrier Routes

Fuzzy Boundaries

Since ZIP Codes are collections of carrier routes, ZIP Code boundaries also become increasingly complex and less defined over time. Remember, the Post Office is only concerned with addresses which are points. A critical ramification of ZIP Codes as points rather than areas is that the Postal Service does not bother to publish ZIP Code boundaries. Nevertheless, since lots of people consider ZIP Codes as convenient forms of area geography, there are mapping companies that attempt to define suitable boundaries. Different mapping companies, however, may define very different boundaries for the same ZIP code.


The ZIP Code on the left is the SAME ZIP Code on the right. The difference illustrates how two mapping companies may define different boundaries for the same ZIP Code.

In this particular case, the variance in boundaries is of little or no significance because the population (shown as red dots on the right) is concentrated in central areas common to both boundary versions.

Real world application for publishers:

When discussing delivery areas with your distribution vendors, be aware that ZIP Codes are constantly being changed by the Post Office. Their service areas change. ZIP Codes are often split and new ZIP Codes created. The ZIP Code boundaries shown on a map printed three years ago may vary significantly with the current boundaries. Furthermore, different mapping companies may show radically different boundaries for the exact same ZIP Code. For confirming delivery areas, we can provide current ZIP Code, ZCTA, and other boundary maps.

The USPS sometimes create new ZIP Codes by splitting existing ZIP Codes.

Entire ZIP Codes have been known not to receive directories because a publisher's directory scoping did not reflect current ZIP Code boundaries.

Over time, ZIP Code and carrier route delivery points have a tendency to become scattered and intermingled as specific addresses are allocated and re-allocated to different mail carriers. The "boundaries" become increasingly obscured. Other factors also contribute to the fragmentation of carrier routes. Streets sometimes become one-way roads, for example, and new growth requires the building of additional post offices with their own carrier routes.

The points on the right are the locations of real addresses. The colors of the dots indicate their current ZIP Codes.

To create ZIP Code boundaries, a mapping company would prefer to draw boundaries such that all the addresses are segregated by ZIP Code. To do so, however, would often produce boundaries that are impractical - especially given the layout of the area's road network and the resolution at which the map will be printed or displayed.
To avoid overly complex boundaries - and to conform to local road locations - the mapping company will have to make compromises. Invariably, some addresses will be located in the wrong ZIP Code area.

These fuzzy boundaries often explain why some homes and residences in a ZIP Code may get books while next door neighbor do not receive books.