It's a good question, and unfortunately, the answer is that zipcode does not map well to other kinds of geographical regions, or to day-to-day-reality for that

It's a good question, and unfortunately, the answer is that zipcode does not map well to other kinds of geographical regions, or to day-to-day-reality for that matter. To directly answer your question, it is not uncommon for zipcodes to span county boundaries. And there are even a handful that span more than one state: https://gis.stackexchange.com/questions/53918/determining-which-us-zipcodes-map-to-more-than-one-state-or-more-than-one-city https://gis.stackexchange.com/questions/53918/determining-wh... To the broader question of why zip codes can be lacking for geospatial analysis, here's a real-world situation: https://theconversation.com/how-zip-codes-nearly-masked-the-lead-problem-in-flint-65626 https://theconversation.com/how-zip-codes-nearly-masked-the-... Here's a more broader essay from CartoDB: https://carto.com/blog/zip-codes-spatial-analysis/ https://carto.com/blog/zip-codes-spatial-analysis/