Expanding the boundaries yields about 1 sq. mile, so 30,000 per sq. mi. That's like the densest part of Charlotte.
Anyway, I didn't mean "really high" like in a bad way. A huge downtown would be sick.
I meant that seemed like a super high figure to be the critical mass to generate sustainable retail. Like, if you told me you needed a downtown to look like the densest part of America's, what, 7th largest metro to sustain retail, I would tell you you're saying there's almost no viable urban retail in the country. I suppose you could define "viable place for retail" in such a way that it becomes true--if you mean "elite, dense shopping corridor," well, there aren't Magnificent Miles around--but I would say Buckhead, University Park, and Carytown are all viable places for retail in their own, different ways and none of them have densities that high. That being said, I don't know what research your professors had access to, so I'm not saying you or them are necessarily wrong. That 30,000 figure just sort of jumped off the screen.