Density on the Ground: Cities and Building Coverage

Buenos Aires

Alon Levy has a new post up about height and density where he argues that to move beyond a certain density, a city will ultimately have no choice but to build upwards.  Density is not simply a function of the height of buildings, however, but also depends upon the area that they occupy.  A city in which buildings occupy a relatively small proportion of the total city area will have room to grow either vertically or horizontally (the latter of these represented by the process of street narrowing, or other forms of infill).  

Allan Jacobs in his book Great Streets features a section in which he presents painstakingly drawn figure-ground maps of several dozen of the world's best-known cities.  Although he calculates the intersections and number of blocks per square mile, both indicators of connectivity and pedestrian accessibility (see Laurence Aurbach's site for more on this topic), no figures are given for the proportion of city land occupied by buildings and rights-of-way (and other public spaces) for each city. 

Portland

Although performing these calculations for all of his chosen cities would be a major undertaking, I've put together some approximate figures, with abundant help from Google maps, for a selected range of cities, both American and not.  Shown are the proportions of each city occupied by 1) buildings or buildable land; 2) rights-of-way, including streets and sidewalks, and 3) other public spaces, including parks and plazas.  Unless otherwise stated, these figures refer to the general downtown areas of each city:

A few quick notes: St. Nicholas Houses is a public housing project in central Harlem composed of cruciform towers on a superblock.  New York's figures include Central Park.  Measuring park and plaza spaces in organic cities like Paris or Vienna is a bit of a challenge: depending on the area examined, for instance, Paris' figure for parks might be somewhat higher.  Fes is just an estimate, as the narrow footpaths of the city's old town are barely visible from above, but in any event it appears to be about the maximum possible built coverage possible without moving to an entirely streetless city.

Hopefully these numbers can provide one additional angle on the density question.  I have some ideas how they might be paired with Jacobs' figures and some other measures to provide an even fuller picture of both density and connectivity, but I'll save that for another post.

Related posts: Charting the Grid

Glazer v. Glaeser on Density

I've been reading through Nathan Glazer's essay compilation "From a Cause to a Style: Modernist Architecture's Encounter with the American City," and want to excerpt the following couple paragraphs in light of Ed Glaeser's recent book, Triumph of the City:

"Most significant, designers failed to explore just what it is people find attractive in areas and buildings for whose design characteristics not much, if anything, can be said.  How often have we seen those pictures of endless Levittowns, little houses with scarcely grown trees spread out over the landscape, as horrible lessons to avoid?  And how often have we contrasted them – to their disadvantage – with gleaming visions of great towers?  Let us put aside for a moment the detailed economic arguments that justify density, and let us recognize that very often the overall cost of single-family homes packed closely, or apartments in low-rise buildings, would compare favorably with twenty-story apartment towers. ...

"Wherever social scientists examine these issues, they find a taste that architects on the whole do not find it interesting to satisfy, a taste for the low-rise, the small scale, the unit that gives some privacy, some access to the ground, a small piece of land wholly under one's control.  I am not, of course, describing a universal taste,  But for people raising children – and indeed many others – it is a near universal taste, if people have a choice.  Nor is there any reason to think that is is necessary or desirable that people be educated against that taste and develop a taste for a larger or gargantuan scale…"

This particular perspective represents, I think, one side of an ongoing split of opinion among "pro-urbanist" writers and thinkers.  On the one hand (at the risk of oversimplification, apologies if I have misrepresented), there is Ed Glaeser and others such as Matt Yglesias, Ryan Avent and Stephen Smith, who overall take an economically libertarian approach to urban density, opposing height limits and touting the benefits of increasing concentrations of jobs and residences in urban areas.  The advantages of an increase in density are always, or nearly always, understood to exceed those of restricting that density by regulation; or, put another way, that any artificial distortion in the market for the ownership and free development of land, however well intended, is likely to lead to economically inefficient outcomes.

On the other side are what I might call the "quality-of-life" urbanists, such as Christopher Alexander, who advocated a four-story height limit, and others including Glazer, Michael Mehaffy, Nikos Salingaros, Leon Krier and evidently David Sucher, who promote a view which, while considering economic arguments, gives equal attention to those which cannot be easily quantified.  The desire of families to live in single family or low-rise homes is accepted, the general dislike of living in tall apartments, where another choice is available, is acknowledged, and the task instead becomes one of finding the urban form which maximizes density while producing an environment pleasant enough that it does not send families fleeing for "Levittown" when the first child arrives.  If this task requires establishing height limits, then these limits will be considered, as will certain other forms of regulation.  

This of course an oversimplification of these two perspectives, but they have been contrasted recently (some of the finer distinctions are brought out in this article by Kaid Benfield, for example, and the recurring debates over the DC Height Act).  At the same time, both positions share a common dislike of other forms of regulation: excessively segregated zoning, mandatory low density development and so forth.  There may actually be more in common than not, with quibbles over certain issues interfering with agreement on other certain fundamental concerns. 

Related posts:  

Mehaffy on Skyscrapers

Can New York Build Its Way Up To Affordability?

More On Density

The New York That Wasn't (But Might Have Been), part II

Before showing an illustration of a Manhattan as it might have grown in the absence of a single central plan, it's worth considering for a moment how it was that the plan that the Commissioners drew up actually came to be, in spite of the extensive network of roads that already covered Manhattan in 1807.  Answers are difficult to come by, as the Commissioners' very brief account of their decision-making process raises nearly as many questions as it answers.

The Commissioners did, however, claim that they had attempted to follow existing streets and property lines:

"Having determined, therefore, that the work in general should be rectangular, a second, and, in their opinion, an important consideration was so to amalgamate it with the plans already adopted by individuals as not to make any important changes in their dispositions.  ... This, if it could have been effected consistently with the public interest, was desirable, not only as it might render the work more generally acceptable, but also as it might be the means of avoiding expense. It was therefore a favorite object with the Commissioners, and pursued until after various unsuccessful attempts had proved the extreme difficulty, nor was it abandoned at last but from necessity. To show the obstacles which frustrated every effort can be of no use. It will perhaps be more satisfactory to each person who may feel aggrieved to ask himself whether his sensations would not have been still more unpleasant had his favorite plans been sacrificed to preserve those of a more fortunate neighbor."

1782 British Headquarters map with existing streets
and roads highlighted

Leaving aside the absence of any explanation on this fundamental point, and the debatable conclusion that it is preferable to frustrate everyone's interests than to frustrate only some, another point worth noting is that the Commissioners attempted to accommodate existing interests to the plan only after "having determined ... that the work ... should be rectangular."  Since the existing roads of Manhattan largely followed an organic pattern, as can be seen at right, any attempt to conform them to an inflexible rectilinear grid was doomed to failure from the start – a fact which would have been immediately obvious to an experienced surveyor such as Commissioner Simeon De Witt.

This order of consideration could only be justified if the Commissioners believed that the public advantages of the grid plan outweighed any public benefits to be gained from following existing streets and property lines.  Such a comparison of costs and benefits is not made in the Commissioners' report, however.  Instead, the Commissioners compare the unadorned grid to a caricature of Baroque city planning ("those supposed improvements by circles, ovals, and stars which certainly embellish a plan") an oversimplistic and unserious exercise which seems designed to set up the choice of the grid without having to explain its supposed benefits. 

The same map with farmland boundaries
drawn in as new streets and roads

In fact, only one benefit of the grid is ever stated in the report: that "a city is to be composed principally of the habitations of men, and that straight-sided and right-angled houses are the most cheap to build and the most convenient to live in."  Even assuming for the moment that this statement is true, it is irrelevant, since virtually all cities, from L'Enfant's Washington D.C. to the medieval city centers of Europe are composed overwhelmingly of straight-sided and right-angled houses -- including 17th century New York itself, where every house appears to be straight-sided and right-angled, just as most of the houses of old Amsterdam were. 

By contrast there are two clear advantages of following the existing roads within the Commissioners' own report: 1) it had more public support and 2) it was less expensive -- not surprising, since the roads were already present, and did not need to be surveyed, marked or excavated.  Had the existing farmland boundaries formed new streets and lanes leading from the existing roads, in a typical pattern of incremental organic growth, the pattern would have grown still more complete, as shown at left.

So what was actually underlying the Commissioners' decisionmaking?  Peter Marcuse in his 1987 essay concludes that "the grid in 1811 in New York signalled the triumph of the interests of larger landowners and speculators over all competing concerns," indirectly suggesting that the Commissioners' stated concern for the public good was an empty gesture.  In this view, the "convenience" of the plan mentioned in the report reflected only that the grid was easy to survey and created lots that were easy to track and sell.  Perhaps the Commissioners, having been entrusted to "plan" the city, also could not imagine a form of "planning" that did not involve drawing straight lines and right angles.  Whatever the reason (and ease of surveying can't be ruled out), this planning idea caught on, and as Marcuse observes the remainder of the century saw thousands of American towns and cities laid on on regular grids.  

Still, with use of the 1782 map shown here, it is possible to consider what might have happened if the Commission's task had been limited by the New York legislature to only surveying the roads in existence, and marking out new lanes along existing paths and boundaries -- or if no Commission had come into existence at all (while further taking the license of omitting some of the speculative grids which were laid out between 1782 and 1811).  With many additional streets filled in according to topography and property lines, I'll leave you with one version of how that lower Manhattan might have eventually looked:

The New York That Wasn't (But Might Have Been), part I

The Commissioners' plan.

An attempt at a more reflective piece today as a short break from garages and alleys. (Updated 6.18.11 with Urbanphoto link).

March 2011 marked the bicentennial of the Commissioners' Plan of New York, the document which laid out Manhattan's familiar street grid from the vicinity of Houston Street north to Harlem.  In recognition of this milestone, there's been a modest amount of reflection on the plan both in the press and in the blogosphere, with some praising the plan, and others thoughtfully ambivalent.

In the past, however, the critical voices have generally been the loudest. Peter Marcuse in a 1987 essay referred to it as "one of the worst city plans of any major city of the developed world;" Frederick Law Olmsted criticized it for its "rigid uniformity;" John Reps noted its scarcity of sites for public buildings, lack of enough north-south streets, and frequent intersections creating traffic congestion; others have faulted it for obliterating Manhattan's natural landscape, while Recivilization has perhaps the most memorable turn of phrase, calling the Commissioners' plan "breathtaking in its monotony."

In spite of all these critiques, there's been relatively little thought devoted to what an alternative to the grid might have looked like.  To the extent that they are discussed at all, alternatives generally take the shape of the Baroque plans of the late 18th century in the manner of L'Enfant's plan for Washington D.C., with diagonal boulevards and grand circles, as one Columbia design class recently imagined.*  More realistically, Urbanphoto has an outstanding post on an earlier plan drawn up by city surveyors John Mangin and Casimir Goerck which was somewhat more more accommodating of the street networks already emerging north of the city.

*(The Commissioners had, in fact, considered such features and rejected them for ostensibly utilitarian reasons, although little more than a century later the planning departments of checkerboard cities such as Portland would note a lack of diagonal routes as a major utilitarian shortcoming of the grid).

There is another possibility, however, one that requires acknowledgment that the Commissioners' plan did not cover virgin ground, but rather overlaid and blotted out a complex network of roads, streets and paths that already covered Manhattan as early as the 1770s.  In the British Headquarters map of 1782, featured prominently in Eric Sanderson's recent book Mannahatta, not only is the former Indian trail of Broadway clearly discernible running the length of the island, but dozens or hundreds of other roads also, large and small, serving the many farmhouses and budding villages north of the city limits.  These roads were not planned by a single mind, but worn into the terrain by countless feet following a subtler intelligence: the intuitive search for the smoothest, most level and most direct paths to the most important destinations, perfected by repetition over years, decades and centuries. 

Oddly, it is this process of road and street formation -- which Spiro Kostof refers to as the organic typology -- which earns the least respect from writers on urbanism.  On New York's organic street network in lower Manhattan, dating to the days of the Dutch settlers, it's easy to find statements like these:

"Until the grid most New Yorkers were clustered below Houston Street in a confused maze of streets and lanes."

"Prior to the Commissioner’s grid plan, the city’s streets had been laid out rather willy-nilly, as befitting a city that was founded in essentially late mediaeval times..."

There seems to be an assumption that if a street network does not have obvious regularity or other indisputable evidence of direct human design, then it must be simply random and therefore, impliedly, inefficient or unsuited to the needs of the people living along it.  But was this street network really "confused" or "willy-nilly"?  Here is an adaptation of the Castello Map of New York, dating from 1660 and showing the streets and homes of the city at that time:

The Dutch origins of the city, with its canals, are obvious, but the functional logic of the streets, though evidently not formally planned, are anything but confused or irrational.  Broad streets carry commerce to and from the waterfront and gently angle northwards, reflecting the natural course of trade unencumbered by sharp right-angled turns.  Single-family homes are being built along the quieter narrow lanes running perpendicular to the thoroughfares, with the process of block subdivision reflecting the higher value of land close to the docks. 

There is an elegant mathematical order to the street network as well: each curve of the broad streets defines a shape which mimics the shape of the island itself.  This is a typically fractal pattern, defined as "a rough or fragmented geometric shape that can be split into parts, each of which is (at least approximately) a reduced-size copy of the whole."  (See Mathieu Helie's extensive writings about this idea in the context of architecture and urbanism.)  This particular pattern is presumably the result of accommodating a grid-like network of streets, with blocks of roughly equal size, to the constrained geography of Manhattan. 

In the following century, this initial plan was organically extended north of the wall, guided by pre-existing paths and roads.  In the following post I'll show a rendering of what might have resulted had the city of the 1790s simply decided to let this process, ongoing at that point for almost 175 years, continue to follow its natural course.

New World Economics on SFDR in the Traditional Style

Nathan Lewis's latest urbanism post is up at his website, this time dealing with the topic of whether it is possible to create a neighborhood of single-family detached residential (SFDR) with room for cars which nonetheless is dense, walkable and pedestrian-centered on narrow streets:


Seijo neighborhood, Tokyo



"I am rather hesitant to take up this topic, because it is not my goal at all to build some sort of slightly improved auto-dependent suburb in Suburban Hell, but to create Traditional Cities instead where a car is not at all necessary. However, there is a place for this I think. First, the SFDR pattern is also a valid one for the pedestrian Traditional City. European examples of Traditional Cities tend to have a lot of apartments, but Tokyo in fact had very few apartments until the 20th century. The normal pattern was for very small SFD houses on very small plots. In fact, the first Western-style apartment building in Tokyo wasn't built until the 1920s. It was a Western idea they borrowed. ...

Second, although I promote a Traditional City in which cars are entirely unnecessary, and that you can walk everywhere or take a train to another place where you can walk everywhere, nevertheless there is perhaps a need for a kind of transition format in the U.S. For example, let's say you lived in the Washington DC area within walking distance of a train station, that you can take to work in downtown DC. So, you don't need a car for commuting, but still you need a car to interact with the rest of the U.S., which is still car dependent."

Thus, can we think of a pattern which is compatible with BOTH today's need for two to three cars per household, the desire for the SFDR format, and ALSO compatible with the Traditional City pedestrian-centric design including of course Really Narrow Streets? This is a rather touchy design goal, as you can see, since we want to have one foot in two worlds which have a fundamental incompatibility.

Let's formulate some ideas along these lines..."

Read the whole thing, with tons of illustrative maps and photos, here, and feel free to leave any feedback below as Nathan's website doesn't allow comments.

Garages and Alleys 3: Reworking the Block

In response to the previous post, commenter Cambias perceptively noted that the example I'd posted of Washington D.C.'s Capitol Hill neighborhood has considerably larger individual lots than those on the new blocks being built up in Capitol Quarter.  This of course makes it easier for the Capitol Hill lots to accommodate both private backyards and detached garages. 

To show that this is an issue of design, not simply acreage, I've taken the same block from the previous post (below, at left) and proposed an alternative configuration with the goal of providing 1) some backyard space for each house with 2) at least one parking space per unit.

By eliminating front setbacks and narrowing the streets by about five feet on the east and west sides -- still enough to allow for parking on both sides of the street -- this goal can be achieved.  On street parking has increased to 65 from 52, thanks to the new center street, serving a total of 64 rowhouses, compared to 54 previously.  Additionally, each house now has a small, courtyard-like outdoor space.  It's just one possible layout of many, and I'm sure people can suggest changes and improvements, but my intent is only to show that this very same area can actually be redesigned to accommodate those features I'd criticized other plans for lacking.

The only complication here is the 1950s era DC Zoning Code, which appears to still require 1 off-street space per every 2 units in this particular residential zone (R-5-B).  This can result in the absurd scenario of interior garages and curb cuts which take away as many on-street spaces as they add, although it appears off-site requirements may now have been waived for houses without alley access.

Update: In response to Alex's comments, here's another arrangement that allows 1/3 of the townhomes space on which to construct garages fronting on the center street.  As to how many people would purchase a garage if offered one as an optional feature, or what the price differential is between homes with the potential of a garage and without, I have no idea (although it's worth mentioning that DC does have the second lowest proportion of car commuters of any major US city).

More Garages and Alleys

I received so many thoughtful comments on the last alleyway post that I thought I might contribute a few more examples from modern American suburbia.  In the comments, several people noted that the Orlando example I chose was poorly designed (I can't disagree), and that better examples of street-and-alley configurations exist.  After doing a few Google maps "fly-bys" of a number of cities, however, if better examples do exist they are not abundant or easy to find.  More typical is this example from Raleigh, NC at right: in this neo-traditionalist neighborhood, homes are painstaking replicas of Craftsman bungalows and foursquares on small lots.  Unlike the Orlando example, however, the developer has incorporated green space in the form of generous median strips, resulting in the streetscape shown at right (the central circle contains a white gazebo which, as far as I can tell, is purely decorative, as it appears to have no entrances).

When it comes to the rear alley, though, a familiar sight is seen: the attached garage and impervious paved driveway occupying the entire backyard.  In the aerial shot it appears that every single property conforms to this design, even where there is clearly sufficient space for a backyard between garage and house (even accounting for mandatory rear setback).






And here's a New Urbanist example.  There are some efforts to create small backyard spaces behind detached houses and rowhouses, but overall the vast majority of the land behind homes is occupied by garages and pavement (this is Kentlands, MD Lakelands, MD).

Here is Kentlands, showing considerably more attention to backyard spaces (thanks for catching my mistake, Laurence):

This is a New Urbanist-inspired development not far from Kentlands, showing clearly the presence of a decorative fringe of front and side yards, with rear spaces reserved almost exclusively for vehicle access.  The ratio of pavement to inhabitable built footprint appears to exceed 1:1.

Not only suburban greenfield development but also urban infill  follows this format.  This is a block of new townhouses in southeast Washington DC, built in a traditional architectural style.  The planner has provided a small common area within the block, but again there is no private outdoor space except for a few rooftop patios.

By contrast, here are a couple blocks in the Capitol Hill neighborhood, just north of the infill townhouses in the previous image, featuring both fronting streets, backyards and alleys: 

If anyone is aware of better examples of street-and-alley neighborhoods in modern developments, please let me know!

Urban Reads

If you're looking for some new reading for yourself or a person you know who's interested in history, cities and urban planning, a reader has kindly sent along to me a list of 50 of the best books on urban history.  The classics are on there, including Jacobs, Mumford and Reps, but there are many lesser known titles as well covering everything from Chinese urbanism to American infrastructure.

Along the same lines, last month Austin Contrarian compiled a list of recommended readings on urban economics, including the Death and Life follow up The Economy Of Cities, which is worth reading alone for the provocative theorizing about the urban origins of agriculture, but which goes far beyond that to illuminate the economic basis for many of the conclusions in Death and Life.

I could add many others to these lists which I've enjoyed over the past few years, but would invite commenters to share their own recommendations.

Suburban Follies: The Rear Alleyway

For a few years running James Kunstler has had on his website an "Eyesore of the Month" feature, which catalogs various failures in architecture and urban design.  In a similar vein, every now and then I'd like to focus on some more mundane poor design decisions, beginning with one that's been mentioned here before: rear access alleys, with reference to a new subdivision in Orlando that's typical of the type:

Unlike the typical late 20th century suburb of detached houses on large lots with front-facing garages (the much-derided "snout house" typology), this suburb clumps houses together in batches of six, and, in a presumably New Urbanist gesture, places the garages in the rear of the homes along ten-foot lanes, as can be seen above.  From the fronting street, the result looks like this:

The street is only about 20-25 feet wide, but ponder in this case: what is this street actually for?  On-street parking is unneeded due to the ample garage and driveway parking in the rear, and from the street layout it appears that the most of the fronting streets are duplicative and unnecessary in providing access to the rear alleys.  If the intention were to create a more pedestrian-friendly street, it matters little since, in this large single-use neighborhood ringed by wide arterials, there is nowhere to walk to.  And as for private outdoor space:

The garages and driveways in the rear have destroyed that greatest of suburban attractions: the grassy private backyard.  And this in mild Florida, where a backyard is a year-round amenity.  The house is now an island surrounded by pavement on all sides, except for the ornamental front and side yards.  Remove the garage, driveway and alley, bring the property line to the midpoint of the alley, and a spacious backyard is created at the sole cost of having to park one's car on the street.

This design idea appears to have caught on in recent years.  While visiting a friend in a very similar development in Birmingham, I was told that the local homeowners association actually prohibited parking on fronting streets altogether!  This officially reduces fronting streets themselves to expensive ornamentation, as symbolic access roads to the functional access roads in the rear.

I'm not entirely sure how to account for this.  Did the ugliness of garage-fronted streets finally become so intolerable that buyers became willing to sacrifice even their backyards for the illusion of a car-free environment in the front?  I don't know and would be interested to hear anyone's thoughts, but for now, one more shot from Orlando:

This is the village of Hogsmeade from the new attraction The Wizarding World of Harry Potter in Orlando's Universal Studios, a replica of the movie version of a fictional medieval town from J.K. Rowling's series.  Apparently this one traditional city street and its associated rides have revived the theme park's fortunes, sending attendance soaring by 68% and turning a $35 million quarterly loss into a $45 million profit.  One could argue this success is purely due to the books and movies, but why were those such a runaway success?  Nathan Lewis can tell you.

Zoning: The Other Reasons

Nathan Lewis and Benjamin Hemric have written some great commentary on last week's zoning post on the origins of zoning and the concerns that may still drive its popularity today.  If zoning does not preserve property values, as research suggests and even zoning's defenders admit, what are some of these other concerns?  As they suggest:

1. Protection against the effects of "congestion," which in practice means automobile traffic and parking spillover.  This partially accounts for opposition to higher-density projects which are clearly indicative of increasing property values: for example, new luxury apartment towers in Queens' Long Island City neighborhood, where existing residents demanded large quantities of garage parking even in the absence of city parking minimums.  Traffic associated with "apartment houses" also made in appearance in the Euclid v. Ambler decision as a justification for density limitations. 
2. Exclusion of the poor, for both quality of life and economic reasons.  The latter point relates to property taxes: even if upzoning increases land values, it may be that net property tax revenue, per resident, will fall, resulting in a decline in the quality of city services and the necessity of a tax hike.  Exclusionary zoning could then be seen as a way to keep property taxes low -- at least for so long as the mounting externalized costs of a very low-density development pattern can be postponed.
3. The "neighborhood character" argument, which Benjamin Hemric alludes to and which is captured in this 1994 paper and in the old Disney animated film Benjamin mentions (The Little House).  This I think gets at the heart of what zoning means when it states an intention to "conserve" the neighborhood.  In essence, the neighborhood as it exists meets the density preference of the existing residents, and that any further densification would result in a perceived decline in quality of life, regardless of any increase in property values.  Cashing out by selling and moving away is not always a desirable solution if one has a long-term investment in unique neighborhood institutions and amenities.  To some extent this is the same argument advanced by the foes of gentrification.
4. Finally, protection from harmful or noxious uses.  Although this reason is far less significant than it was 100 years ago, when promoted as a pre-emptive measure against a select group of activities it has a certain logic as an efficient alternative to costly and drawn-out nuisance lawsuits.

As to each of these justifications, several questions could be posed, such as: Is this goal a legitimate end of the police power?  If it's a legitimate end, does Euclidean zoning actually help achieve it?  And if so, is it the best or most narrowly tailored way of achieving it?  Are there unintended consequences or other negative side effects associated with the regulation?  Could the same concerns be addressed more simply and less intrusively by some other approach?  And so forth. 

In answer to the last of those, Benjamin's suggestion of reviving a variant of the original, 35-page 1916 New York zoning resolution is an interesting one.  He's not the only one to have this thought: a 1993 City Journal article on New York's modern zoning code, still very relevant today, had positive things to say about its simplicity, predictability and clarity.  I do have some thoughts of my own on all this that I'll get around to in a post or two.