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A report in Better! City & Towns on Richard Florida’s presentation to the Congress for the New Urbanism quoted him thus:

One of the false statements is that density and skyscrapers are the key ingredients to urban vitality and innovation. “This rush to density, this idea that density creates economic growth,” is wrong, he said. “It’s the creation of real, walkable urban environments that stir the human spirit. Skyscraper communities are vertical suburbs, where it is lonely at the top. The kind of density we want is a ‘Jane Jacobs density’.”

So what is Jane Jacobs-style density?  I expect most people think it looks like this (Park Slope, Brooklyn):

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What she actually said in Death and Life of Great American Cities (pp.208-212) was this:

What are proper densities for city dwellings?  … Proper city dwelling densities are a matter of performance … Densities are too low, or too high, when they frustrate city diversity instead of abetting it …

Very low densities, six dwellings or fewer to the net acre, can make out well in suburbs …  Between ten and twenty dwellings to the acre yields a kind of semisuburb …

       18 DUA

However densities of this kind ringing a city are a bad long-term bet, designed to become a grey area. …

And so, between the point where semisuburban character and function are lost, and the point at which lively diversity and public life can arise, lies a range of big-city densities that I shall call “in-between” densities.   They are fit neither for suburban life nor for city life.  They are fit, generally, for nothing but trouble …

I should judge that numerically the escape from “in-between” densities probably lies somewhere around the figure of 100 dwellings to an acre, under circumstances most congenial in all other respects to producing diversity.

As a general rule, I think 100 dwellings per acre will be found to be too low.

So what does 100 dwelling units per acre (DUA) really mean?

Well, here’s one place to start – at Better! Cities& Towns: The Dreaded Density Issue by Susan Henderson. But even her comparative images don’t get that high.

36 DUA

Our source is Andy Coupland:

The Olympic Village/Village (right) on False Creek is 1104 units on about 10.6 acreas of land parcels – so 103/acre.

West End (below) is 318 acres of land parcels, and has 32,750 dwellings so 103/acre.
 
You pays your money and you chooses your form!

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 But beware using dwellings as a measure of density – Olympic Village dwelling average size is 1,117 sq ft; West End is only 772. Very different population density is therefore likely from identical DUA.

UPDATE: Urban densification: a panacea or an apocalypse – another point of view from the North Shore.