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urban density mystery

For those who haven't seen it, I'd like to shill last year's urban density post. Data and Fermi estimates, tasty!

A highlight of it is that you don't need that much height to get high density. Manhattan has 26,000 people per km2, but Paris has 22,000, with basically nothing over 8 stories. Brooklyn is at 14,000, twice as dense as San Francisco or Somerville, and my personal impression of it was that it wasn't that high, maybe wall to wall four story buildings.

Well, I finally went looking at random spots of Brooklyn in Google Streetview, and now I'm confused: I seem to have greatly overestimated the average form. Brooklyn Heights, right across from Manhattan, has a bunch of 12 story buildings (possibly office). I do see some wall to wall 4-5 elsewhere, also wall to wall 2-3. But also a lot of detached homes, even 1-2 story. It's not really obviously different from SF or Somerville (at least the parts I'm familiar with), yet has 2x the density.

Maybe it's a change in distribution? SF does have the Sunset, a district of 1 story homes on top of garages. Or in how many people are living per unit.

Or, hmm, back yards. I just switched to the Google Earth view of the last spot I'd checked, and there are none, just lots of smaller buildings in the back. Though I'm not sure if they're housing or garages. By contrast, in SF I lived in a 3 story Victorian, wall to wall, but half or even 2/3 of the lots were back yards, not that you could tell from the sidewalk.

Well, that was one spot; in a second, I do see back yards, some with swimming pools even, but they're 1/3 of the lot length.

OTOH I'm looking at Somerville now, and it doesn't look more generous with back space, though there's maybe more space between the buildings (mostly driveways.)

OTOH again, I just checked San Francisco, and it's what I remember. There's variation, but backyards in the Richmond are commonly half the lot, sometimes less, sometimes more. Ditto for the Sunset, something I never appreciated. Both have a layout where if you walk around the block you'll see nothing but building, but half the block is a contiguous (but property-divided, not communal) greenish-interior.

So compared to SF, I can see why Brooklyn is twice as dense: similar buildings but less yard space. Now I'm wondering why Somerville isn't denser, though... it doesn't even have major parks! But looking again, I think the spaces between buildings, plus greater yard space I now see elsewhere, may explain that.

As for Paris, I was partly wrong: there is a lot of green space, but largely enclosed by buildings like Brooklyn or San Francisco -- often enclosed by what looks like *one* building, in a private courtyard. I was basically right about the height, I haven't seen anything shorter than 4 stories, usually 5-8, though I did see something as high as 12.

I still wonder about my Fermi models; they generally predict more people than we find. I might be overestimating land use, or underestimating non-residential use, or how much area is taken up by walls vs. the internal usable area. E.g., consider a "Main Street" model: 30% streets, 70% lots, half of a lot built up, ground story businesses, two residential stories on top. 700,000 m2 of residential area (700,000 * 1/2 lot use * 2 stories); at a rather generous 100 m2 per person, that's 7000 people per km2. Hmm, that's not far off from SF or Somerville, though my impression is that Somerville is short on local businesses and jobs, and neither has ubiquitous businesses like that. And 100 m2 is high... at 50, say, that'd be 14,000 people, more like Brooklyn, with entirely 3 story buildings and 50% open space (not counting streets.)

See the comment count unavailable DW comments at http://mindstalk.dreamwidth.org/455530.html#comments



( 2 comments — Leave a comment )
Oct. 9th, 2016 01:10 am (UTC)
A relevant for you, about Parisian building heights: for generations, you could skip including an elevator in your residential Paris apartment building as long as it was 5 stories or less. (I believe an elevator is now required for all, but I could be wrong). As a result, many 5 story apartment buildings were built.
Oct. 9th, 2016 01:16 am (UTC)
Interesting, I hadn't heard of that legal threshold.

Relatedly: https://clubnimbyblog.wordpress.com/2016/02/15/econ-101-and-the-missing-middle/ on the US missing much building of multi-unit wood frame, that can house a lot of people cheaply; instead we jump from single family to 4-7 story mid-rise, with elevators in a concrete core, jacking up costs (along with the cost of building that high.)
( 2 comments — Leave a comment )


Damien Sullivan

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