Tuesday, March 27, 2012

Density in Los Angeles: a Matter of Tolerance?

Why LA can build densely. Source: Google Streetview
In the previous post, I claimed that building coverage for residential development, based on North American block usage norms, likes to stay under 70% building coverage since building layouts need to grant adequate daylight exposure to dwelling units. As I began to think more about this, I realized that there are notable exceptions, local norms were high building coverage is possible and even tolerated. Some of these exceptional places are the compact subdivisions of Western/Southwestern U.S. and great swaths of urban Los Angeles.  I have actually never heard others remark on the unique qualities that make LA well-suited for high building coverage, but its set of complimentary conditions - including climate, building typologies, cultural history and block morphology - should be appreciated more about LA.  The remarkable fact that a bountiful portion of LA's blocks incorporate relatively high building coverages with multifamily development may be one of the primary reasons why the city has such a high overall population density (average people per square mile) in comparison with other cities. (By the way, over at Discovering Urbanism, Daniel Nairn has just posted a really handy synopsis of the city density trends revealed by the 2010 Census.)

One way to achieve high coverage density is through block morphology.  You can conceivably create a block pattern that accommodates 100% building coverage for multifamily lots. 100% lot coverage is actually possible on the right kind of block: a very, very narrow block of 130-feet wide at the most (measured street centerline to street/alley centerline) so that you can daylight the units on both sides of a building without providing any private open space at all. You just use the rights-of-way to daylight units. (Such a block structure would actually allow Ed Glaeser to claim we can build 150 1600 sq. ft. dwelling units per net acre without having to go higher than six stories.)  But in the prewar urban fabrics, block widths rarely consistently go under 330 feet in most cities.  Of all the North American city grids that I know, only historic Savannah has a block width as narrow as 130 feet as a normative feature of its grid (these are the "Trust Lot" blocks that face the east and west sides of each historic district square). But a 200-foot wide block, as in Manhattan and Portland, does allow these cities to get high coverages, because relatively little open space can be provided in the interior of the blocks when you have 100-foot parcel depths.

A subdivision in Las Vegas, Nevada, demonstrating the 100-foot deep lot morphology that is pervasive there. (Source: Google Maps
In the West and Southwest, subdivision blocks are also commonly this narrow, allowing for some of the highest building coverages you can find in single-family neighborhoods.  These lots grant homeowners some of the tiniest backyards in America.  In time, as in Savannah (whose town lots are also this deep), these 100-foot deep parcels can conceivably redevelop into denser Savannah-style townhouses, apartments, offices and mixed use buildings. One might argue that it may take longer than the history of Savannah to achieve Savannah's urbanism in these subdivisions, perhaps, since their patterns are conceived mainly to keep strangers out, but the hope to realize a modicum of urbanism is there theoretically.  In these cases, one may achieve Jane Jacobs's lower threshold for urban vitality (in the ballpark of 100 dwelling units per net acre) without, perhaps, needing to build above 6 stories. The narrow lot just makes the overall utilization of land more efficient. That assumes however that you transition from single-family development to attached brownstone-style walk-ups or mid-rise development created by assembling rows of single-family lots.  We'll call this strategy for intensification of land usage the "Savannah Strategy": the strategy of assembling and building higher on shallow single-family lots. The morphology of the block pattern is important to this strategy: how narrow the blocks are and how well the streets can be connected in time.

A block interior in Boston's Back Bay. (Source: Google Streetview)
But placing homes tightly together without actually going all the way and attaching them grants an important advantage with respect to average density.  Actually two important advantages (the second of which we will get to later).  We mentioned already exactly what it is: granting access to daylight.  If buildings have to share walls, then they only have the street-facing side and the back-side for daylight access.  That means the building will rarely get deeper than 70 feet.  Boston's Back Bay is an excellent illustration of this fact. See how much open space in the interior of the block can't be used for building coverage...
A typical block in the Back Bay with building perimeters highlighted for clarity. (Source: Google Maps)
However, note also that for Boston's climate this is very good. You do appreciate the adequate daylight that falls into the space, and notice the pleasant balance between shadow and sunlight at midday in the Streetview image above. Note that the open space to building height ratio is 1:1, around what New Urbanists recommend for alleys and pedestrian rights-of-way. At the ends of the blocks, taller buildings at the corners of the block seal up the sides of the block, creating an intimate semi-private environment in the block interior. You can maybe claim that the Back Bay has the perfect block typology for Boston's climate and the building coverage seems appropriate.  

Now compare the Back Bay with this plan view of multifamily buildings in LA...

Both maps are at the same scale.  If Jane Jacobs observed that building coverage over 70% is "intolerable" in Boston's North End, LA doesn't seem to care. As with the shade-loving Western subdivision, this is where LA's sunshine-soaked climate grants LA one dramatic advantage. In LA, people like shady courts. Snug closeness between neighboring buildings (instead of party walls) is tolerated because direct sunlight is just not as coveted and the fact that there is just an accrued cultural tolerance for detached nearness. This means the buildings can be very deep indeed from front to back. The entire building perimeter serves that all-important function of daylighting units and, therefore, you can go deep into the block with many units at level, with units between the front side and back side units.

In sum, high density in LA is achieved with lot coverage, not building height. To make the comparison visually clear, here's a block to block comparison between the Back Bay and Central LA blocks at the same scale...

A typical block in the Back Bay with building perimeters highlighted for clarity. (Source: Google Maps)

A typical block in Central LA. Note the abundant perimeter available for daylighting units. (Source: Google Maps).
The interesting thing about LA is that LA's building typologies, a product of its cultural legacy, free it from the need to achieve high lot coverages with the "Savannah Strategy", i.e. using lots and lots of right-of-way land to daylight units.  In fact, in LA, the fatter the block the better. Part of the reason the courtyard apartment typology suits it so well is that the half-blocks are just deep enough (in LA, the 330-foot wide streetcar suburb block pattern predominates). This is remarkable for one simple reason: it cuts down on the amount of right-of-way land needed overall.  LA needs fewer streets! And so, it bumps up average density thus across the city. Of course, from the standpoint of Jane Jacobs, this quality of LA poses liabilities.  But it does suggest interesting ways to begin to work in LA (and other sun-blessed cities in California and the Southwest) to achieve urbanist goals with qualities other cities simply don't possess in great abundance.  We can intensify land uses by encouraging more mixture of primary uses in the multifamily fabric, allowing some parcels to go high, and connecting the city better with high frequency transit and bike and pedestrian supporting urban design to lessen dependence on automobile storage. We'll call this the "LA Tolerance Strategy": a strategy where block morphology is just not as important as a cultural tolerance for alternative means.


Daniel Nairn said...

Eric, your observations about urban form never fail to be instructive. This post reminds me of a point a professor of mine made at one time that's stuck with me: commercial buildings cycle through every couple of decades, residential buildings every several decades, but infrastructure will continue to form the city for centuries or even longer. I guess you could add climate as one more layer on top of this. As cities continue to regenerate themselves, I think these insights into the more permanent constraints/opportunities are very helpful.

One question that I've wondered about but have no answer to. How does the orientation of the street grid effect daylighting possibilities? I wonder if some orientations are better than others, if the goal is to maximize winter sun while minimizing summer sun. I guess it might also depend on your latitude, longitude, topography. Any ideas about this?

Eric Orozco said...

For building energy efficiency, an East-West building orientation is recommended. In fact, the Block examples above are nearly optimal in this way for each of their climates. So for detached buildings you want your blocks oriented N-S, and for attached buildings you want your blocks oriented E-W. The Back Bay's would be better if it inclined the other way from true E-W.

But for strictly daylighting reasons, that is a good question! I would guess that a block pattern that is oriented 45deg from the cardinal would be optimal because it would give all street walls access to sunlight at some part of the day. Downtown LA is in fact oriented this way. So is Center City Charlotte. Can't say I've noticed a huge impact, but I do like the fact that all windows on the different walls of my unit bring in nice sustained sunlight at various parts of the day (I share a party wall on the northeast, which I'm actually happy about). Depending on the hour, one room will always have a warm glow. My bedroom is on the southeast and living room on the northwest. Perfect.