How the Sierra Club Will Preserve "Open Spaces"

Betsy Hart in A vast and mostly empty land:

It's no surprise... that in 2001 the powerful and respected environmentalist/conservation group the Sierra Club defined "efficient urban density" as 500 housing units to the acre. Only, shortly after that pronouncement, they were informed by demographers Randal O'Toole and Wendell Cox that such densities were almost three times the highest-density areas of Manhattan, and nearly twice the densest areas of Mumbai (Bombay), India.

So, they revised their definition of efficient urban density down to a still sky-high 100 units per acre - still more than all but the very densest area of Manhattan - and added a "dense urban" category of 400 units per acre - some 1.5 times Mumbai's densest and poorest area.