As anyone who has read The Black Swan knows, averages are dangerous. Outside of natural phenomena, nothing follows the Bell Curve. But it's a frequently used metrix, especially in sustainability comparisons.
Pleasingly, someone has taken the time to actually depict the distribution of city density.
It's an impressive comparison, and it helps show why NYC does so much better on transit ridership than anywhere else--there is a LOT of high density there. If we were to cut the graph off at 10^4, and integrate the area under the curve past that point, NYC would beat out all the other cities combined.
Once the new census data is available in shapefile form, it's going to enable all sorts of new analysis. One of which might be doing a nation-wide analysis of the relationship between 'Transit Density' and actual transit networks.