In other words, is it safe to assume that for entity in a zip code is less than x distance away from the closest entity in the same zip code?
zip codes don't even need to be contiguous. It's a mail delivery route, not a polygon.
There are 5 cases where the assumption is violated:
- Non-contiguous areas
- Zip codes that are a single point (some big companies get their own zip with a single mailbox, e.g. GE in Schenectady, NY is zip 12345)
- Zip codes that are a single line (highway-based delivery routes)
- Overlapping boundaries (since mail routes are linear, choosing a polygon representation is arbitrary and often not unique in space)
- Residents of some zip codes are not stationary (e.g. houseboats)
In short, asking questions about the area of a zip code is a category error - zip codes do not have a uniform representation in space. And we should be highly skeptical of any geospatial analysis that assumes polygons.
Please see: https://opencagedata.com/guides/how-to-think-about-postcodes...
I write this as someone who grew up in the ZIP code 09180
What they do not have is any sort of spatial consistency, they are a convenience for mail sorting. So if you start analyzing patterns across zip codes, you are pulling in information that is likely useless for or harmful to answering your question.