Enter a center ZIP code, choose a radius, and optionally constrain to the I-5 corridor.
Any area: no corridor filter.
I-5 / I-10 / etc.: after radius filtering, keep only ZIPs whose centroids are within about 20 miles of that interstate route.
This is a corridor proximity filter, not a county or state boundary filter.
Strict centroid only: include ZIP when centroid distance is less than or equal to the radius.
Approximate overlap: include ZIP when centroid distance is less than or equal to radius + an estimated ZIP radius from area (`sqrt(area/pi)`).
Approximate overlap is not true polygon intersection against ZIP boundaries.