2012 county & metro area estimates released today

[I spent the winter writing songs about getting better]

You could end up with a 500 people per square mile density in an urban area if it's a mostly commercial district. There's a suburb in Minnesota with around that density that consists of a couple townhomes and a few car dealerships (the total population by the way is less than 500, minuscule place in both population and area.) Minneapolis' Downtown East neighborhood by the way has a density of 237 people per square mile.

[I spent the winter writing songs about getting better]

FWIW, I suspect the reason 500 sq/mile is used as a cutoff is because you have lots of counties that have a small city that has borders that extend far beyond the actual populated area. Lots of places in Alaska like that for example. Similar to how the county I grew up in has a population density of only 35 people per square mile but the city is more urban than anywhere in many counties with 20 times the density, it's just the city and LOTS of empty space.