A few links: Cycling faster than driving. Lively downtowns. Density.

New study shows urban cycling faster than driving:
http://www.treehugger.com/files/2010/12/urban-cycling-faster-than-driving.ph
From Lyon. The same is probably true for short trips (under one or two miles) in small cities, when you factor in parking.

Insights into a Lively Downtown (video)
http://www.youtube.com/watch?v=VsrqBHEOT0k
Case study of Ann Arbor. A vibrant downtown is "a beautiful mess". University towns are a special, advantaged breed, and a lot of the points may seem obvious, but an interesting watch.


At NYC density, entire planet's population would fit into texas.
http://urbanomnibus.net/2010/07/this-land-is-our-land/
Interesting thought experiment, though doesn't really address agriculture and other space consuming human activities. At Manhattan's density, the world would almost fit into New Mexico.