The answer to this is something like the answer Lincoln gave to the question, “How long should a man’s legs be?” Long enough to reach the ground, Lincoln said. Just so, proper city dwelling densities are a matter of performance. They cannot be based on abstractions about the quantities of land that ideally should be allotted for so-and-so many people (living in some docile, imaginary society).
Densities are too low, or too high, when they frustrate city diversity instead of abetting it. This flaw in performance is why they are too low or too high. We ought to look at densities in much the same way as we look at calories and vitamins. Right amounts are right amounts because of how they perform. And what is right differs in specific instances.
The Death and Life of Great American Cities, Jane Jacobs
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