how about MSAs with >4000/sq mile tracts? this IMO best corresponds to the urban/suburban vs exurban/rural split.
4000/sq mile is the cutoff between city+suburbs and everything else, which is always a bit challenging in the US with the exurban fringe surrounding many cities.
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Joined the bus on the 33rd seat
By the doo-doo room with the reek replete
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