Friday, October 28, 2011

Friday Read: Fogelson on Restrictive Covenants

Although Robert Fogelson's Downtown: Its Rise and Fall is his best known work, the 2005 follow-up Bourgeois Nightmares, the title a play on Robert Fishman's 1989 book is a quick read which provides, so far as I know, the only book-length history of restrictive covenants in the United States.

Although race restrictions seem to be the focus of most studies in this area, Fogelson looks at other aspects of this history as well, examining how courts reacted to the imposition of use-based restrictions in the late 1800s, and how the public's conception of property rights came to be transformed over this period in ways which have remained more or less unchanged down to the present day. 

One of the more interesting points I thought was Fogelson's explanation as I've argued here before in the context of zoning that the main purpose of the restrictions was not to increase property values, but ultimately to constrain them:
"The suburbanites were confident that most people would refuse to sell to an African- or Asian-American or other "undesirable" person even if offered a very high price. ... But the suburbanites were far from confident that most people would turn down a good offer from a desirable person, a well-to-do white Christian like themselves, who intended to use the property for an undesirable purpose. They were afraid that most people who owned a corner lot would sell to a builder who wanted to erect an apartment house (or, even worse, open a store, saloon or gas station) and was ready to offer five or ten times what the lot was worth as the site for a single family home. ... They were afraid that was what most people would do because they knew that if they found themselves in the same position that was what they would do." p. 143.
The early 20th century restricted suburb was, as Fogelson shows, the result of a process of elimination as much as it was a positive vision: an elimination of "undesirables," a group which could comprise over 95% of the population, an elimination of commercial uses, of attached houses, of multi-family residential buildings, of houses too close the front property line, or too close to the sides, of houses that were too big, or too small, or too ugly, or too cheap, an elimination of fences, of farm animals, and even of household pets.  Above all, there was a fear of market forces, and an inability to conceive of change and urbanization in anything other than apocalyptic terms. 

From the residents' perspective, the scenario that Fogelson describes was essentially perceived as a market failure. Although the seller might reap a windfall, the thinking went, the remaining residents believed that the arrival of a saloon heralded the beginning of the end of their way of life, and would send homeowners stampeding for the exits.  That this was a self-fulfilling prophecy didn't render its effects any less real.

Although Fogelson observes that it was often fear that gave rise to these restrictive covenants, it's worth noting that their existence was not necessarily incompatible with a dense residential environment.  Boston's Back Bay, as Fogelson describes, was deliberately planned as an urban domain for the well-to-do, and was subject to numerous use-based restrictions from the outset. Giving up a degree of urban flexibility in order to retain, at fairly high density, high-income residents who might otherwise have fled to the suburbs apparently seemed a reasonable trade to Boston's Public Land Commissioners. 

In spite of this success, the Back Bay, as a literal infill project, was unusual in that it provided a blank slate close to the city center.  In general, late 19th and early 20th century America produced very few examples of compelling urban environments for the wealthy.  The Back Bay was one.  New York's Upper East Side was another.  Apart from those, and a handful of others, there was little to curb the flight of large numbers moneyed citizens out of cities and into restricted suburban communities.  A belated realization of the harm done to cities by this emigration occurred decades later and played into urban renewal schemes for the demolition of downtown working class neighborhoods, but that part of the story is beyond the scope of Fogelson's book.

At some point I'll have some additional thoughts on Fogelson's magnum opus, Downtown, but if you've been through that one and haven't yet discovered Bourgeois Nightmares, it’s a quick read that's worth your time.

Monday, October 24, 2011

A Hope VI Gallery

In the comments to last Monday's post, several commenters suggested that the example I'd chosen, J. Henry Hale Homes in Nashville, was not representative of the design quality of many Hope VI projects.  For the sake of balance, I've put together a series of aerial and street-level images of each of several projects chosen from a variety of locations.  Rather than editorializing, I present them accompanied by a short statement from their planners or from other relevant sources to let you judge for yourself the merits or shortcomings of each of the projects (click on the images to enlarge):

Bedford Hills Apartments, Pittsburgh

"Through the community-driven design process, the planners learned of the residents’ interest in maintaining the look of the existing housing stock. It was important that the new units fit in the context of the neighborhood. Consequently, the design team spent much of its time in the surrounding neighborhood documenting existing housing types and architectural styles. ... Each rowhouse, including the rental townhouse flat units, has a front door that opens up onto the street, providing residents a sense of ownership and a direct connection to the community. ... Resident surface parking lots, accessible by driveways, were situated in the rear of the housing units. Small setbacks of the houses enabled the planners to incorporate landscaping into the plan." -Project Summary
Broadway Overlook, Baltimore

"The final master plan for the Broadway Overlook was more human in scale than the former Broadway Homes and oriented to pedestrians instead of automobiles. Parking is either on-street or in the rear of the units. Not all of the units have dedicated parking, but the steep topography of the site made it possible to place some of the resident parking beneath the new units, to be accessed from the rear. Although the master plan worked with the existing street grid, it incorporated two new streets to facilitate greater density.  These streets were designed to be one-way with on-street parking to help buffer pedestrians from direct traffic flow, therefore making it a more pleasant and safe streetcrossing experience. Like the surrounding community, the townhomes borrow from the Federal and Italianate architectural styles and consist of single-family and two-family dwellings that range in height from two to four stories." -Project Summary

Centennial Place, Atlanta

"Over 900 families live in new garden apartments and townhomes. Some families make a few thousand dollars a year and some make more than $150,000. But you won’t know which by looking at their housing units. They live side by side in an attractive neighborhood of tree-lined streets. Two swimming pools and a fitness center in the development, and a new YMCA nearby, provide recreational opportunities. New commercial development is underway – being built with private investment." -Development Description

Flag House Courts, Baltimore

"Torti Gallas and Partners responded to the special challenges of this revitalization project with sensitive solutions. Rental, homeownership, and live-work units are designed with a consistent esthetic treatment that dissolves the stereotypes surrounding public housing. Housing density is reduced, allowing the introduction of planned open spaces. Blighted structures are demolished to create a rejuvenated environment. Safety and security are intrinsically planned into the neighborhood, and units are designed so that living areas face streets and public spaces. Creative plans fit comfortable units within the existing narrow lot widths." -Project Summary

High Point, Seattle

"By the end of the decade, High Point will have nearly 1,700 new affordable and market-rate units across its 120 acres. Most homes will have private yards and porches. They will sit on safe streets with controlled traffic, and will show great variety in architecture, with character and styles on each block. ... To maintain the green, garden-like feel, the plan designated over 20 acres of land for parks, open spaces, and playgrounds. A four-acre, central park will be at the heart of the community. Another park features a large pond and a jogging trail, and several other community and pocket parks are scattered throughout High Point.  Even planting strips along streets will be greener and wider than elsewhere in Seattle. The plan triples the number of previously existing trees." -Redevelopment Plan

Martin Luther King Plaza, Philadelphia

"Significantly, new housing is equally divided between on-site reconstruction and off-site renovation and infill. Long term sustainability of the reconstructed project will be assured by eliminating blight and creating stability in the broader community.

One key to the on-site master plan is the creation of a new residential square founded in the tradition of neighborhood-making established by William Penn, founder of Philadelphia. This new open space will be a focus for the broader Hawthorne neighborhood and will marry new and renovated housing, retail with residential above, and a host of local institutions, including a new daycare and community center, a local church and an existing elementary school. The architecture of the new units includes a diverse set of types modeled on traditional patterns of Philadelphia neighborhoods." -Project Summary

Mechanicsville Commons, Knoxville

Existing houses in the neighborhood reflect the architectural styles that were popular in that era, primarily Victorian cottages and Craftsman bungalows. The house designs created for Mechanicsville Commons and for infill in the adjacent neighborhood respect these architectural traditions, incorporating elements from them – deep front porches, Victorian fretwork, and Craftsman brackets – to enrich the facades. The houses are painted in rich colors that give distinct personality to each house and create a lively streetscape. -Architect's Summary                      "As a minimum requirement, all new residential structures should have seeded lawns around the entire house and foundation plantings along their front facades. Landscape plantings and treatment help to define the mood for the residence as well as that of the entire neighborhood. ... In addition to the general landscape guidelines, one small ornamental tree shall be planted in the front yard; a minimum of one large ornamental or canopy tree shall be planted elsewhere on the lot." -Design Guidelines

New Columbia, Portland
The vision for New Columbia was to create a vibrant new neighborhood with a mix of housing types affordable to people at all income levels. New Columbia includes the following features: A mix of residents, representing a variety of cultures, age groups, and income levels; A community-friendly design, with front porches, parks and public spaces; A new street grid that provides easy circulation within New Columbia and connects the community to the rest of the Portsmouth neighborhood; A Main Street that offers a variety of recreational, cultural and educational opportunities both for New Columbia residents and the surrounding neighborhood. -New Columbia website

"A sustainable stormwater management system retains 98 percent of stormwater onsite, treating and infiltrating water into the ground, avoiding piping overflows into local waterways. The system includes 101 pocket swales (or as residents call them, “rain gardens”), 31 flow-through planter boxes, and 40 public infiltration dry wells." -New Columbia website

"Portland doesn't really have food deserts ... . But it has patches such as New Columbia, a community of 3,000 mostly low-income residents where people must travel long distances to get to groceries." -Portland Tribune, March 10, 2011

North Beach Place, San Francisco

"North Beach Place is one of the largest mixed income, mixed-use complexes in California, comprising a 341-unit development; 20,000 square feet of commercial space, including 3,000 square feet of incubator space for resident entrepreneurs; and a childcare/community center. Located in San Francisco’s popular North Beach area, with proximity to both transportation and employment, this complex is a model for transit-oriented urban infill developments.  ..." -Builder's Summary                     

"PGA’s green roof concept allows for underground parking, street-level shops and 341 apartments interwoven with inviting outdoor spaces, including courtyards, gardens, seating areas and six childrens’ play areas, all designed to enhance urban life. PGA was also responsible for the design of surrounding urban streetscapes." -Landscape Architect Summary

Renaissance Place at Grand, St. Louis

The developer's website does not describe the design approach.  However, this project recently won LEED-ND certification:

"Renaissance Place at Grand neighborhood was recently Certified in the U.S. Green Building Council LEED for Neighborhood Development (LEED-ND) Pilot program.  Renaissance Place received outstanding LEED-ND scores in the areas of “Smart Location & Linkages” and “Neighborhood Pattern & Design” rising from its connections to amenities in the surrounding community—a variety of retail stores, cultural institutions, schools and recreational options—all accessible within a ½ mile walking distance or convenient Metro transit lines. Renaissance Place also met its “Green Construction and Technology” targets by using environmentally sensitive construction techniques and by ensuring that brownfield contamination, storm water runoff and pollution from construction activities were significantly reduced or completely abated. The site also avoided environmental disruptions by using pre-existing water and wastewater infrastructure, restoring the historic street grid and using a dense design that helps to minimize unnecessary land use."

Thursday, October 20, 2011

Debating Height in Washington Heights

The New York Times recently ran a story on a developer's ongoing efforts to construct several high-rise apartment buildings in the characteristically mid-rise Washington Heights neighborhood.  While much of the article focuses on concerns about gentrification, the zoning angle is equally noteworthy:

"Local residents call the neighborhood the Heights because of its steep terrain and riverside bluffs, not because it has vertigo-inducing buildings.

So a developer’s plan to build four apartment towers ranging from 23 to 39 stories tall has set off alarms in Washington Heights, where buildings typically run 6 to 10 stories. Some residents have protested that the proposed towers, with a total of more than 800 apartments, would darken the sky and introduce more residents than the area’s schools and subways could accommodate.

Last month, the local community board unanimously rejected a proposal for the four towers, which would have required a zoning change by the city, and it urged the developer, Quadriad Realty Partners, to return with a scaled-down blueprint for shorter buildings at the site, at Broadway and 190th Street. ... Despite the community board’s opposition, the developer said it planned to apply to the city’s Planning Commission for a zoning modification in the next few weeks. But if the change is not granted, the firm says it will then build two stouter, market-rate buildings, 28 and 24 stories tall, which it has the legal right to do under existing zoning."
Below is an image of a typical area of this neighborhood, showing an abundance of "new law" apartment buildings with an approximate ground coverage of only around 55 percent, accounting for streets (40 percent) and space between buildings (5 percent).  Even so, the neighborhood is very dense: approximately 130,000 people per square mile, according to the 2000 census.

My question is: what exactly is the purpose of New York's zoning law here?  Do height limits (or FAR limits, as they may be) in the neighborhood reflect a judgment as to adequate light and air, and to population density?  If so, there can be no justification for bargaining with the developer to raise them, any more than it would have made sense to bargain over the setbacks in the 1916 Zoning Resolution.  If the city is open to negotiation, on the other hand, is the limit simply an arbitrary cap designed to trigger review by the planning commission?  And how does the possibility of negotiation affect land values and development?

In an article from 18 years ago, but still relevant today, Peter Salins writes:
"All these changes [to New York's zoning code] have moved the city increasingly toward a regime of discretionary review, in which city planners must review and approve the way a developer uses the options and exceptions that have been added to the original code. Much of this discretion is used to grant developers zoning concessions in exchange for specific public amenities: not just plazas and arcades, but expensive benefits such as subway improvements, theaters, and low-cost housing. This “zoning for sale” is a game only a handful of well-funded developers, flanked by costly lobbyists, lawyers, and expediters, can play. The benefits to the city are questionable: There is no economic test of the costs or benefits of such trade-offs, and the practice distorts the price of land by encouraging speculation based on the possibility of negotiating zoning concessions at some later date. Moreover, for the city to make such trades suggests that the supposedly critical goals of zoning are not really important, since they can be waived for a price.

The quest to custom-tailor land-use regulations is an endless one. Even as the zoning ordinance has grown increasingly complex, it can never quite custom-tailor enough, so planners must give themselves discretion to approve or reject particular projects on a case-by-case basis. Zoning, which was meant to foster orderly and predictable development, has instead become a chaotic, capricious process that deters the development and renewal of the city."
The point being made here is that if investment is desired, consistency and predictability are virtues.  One of the potential purposes of height limits, in an age where skyscrapers are a technological possibility, is as a check against speculation in land.  For the limits to serve that purpose, they must be absolute, and not subject to waiver or variance, or else they risk becoming a cause of speculation themselves.  Endowing community boards with discretionary veto power adds further complexity and uncertainty to the process.

If the neighborhood believes that ten stories is appropriate, then every proposal for a tall building is doomed to follow the same convoluted process of community hearings, disapprovals, appeals, further hearings, etc., all to end back at the "as of right" allowance, and all squandering a great deal of time, money and goodwill.  Those who wish to avoid controversy by constructing a ten-story building, on the other hand, will find many holdout buyers pricing their parcels for much taller buildings, and development will stall.

On his blog, Alon Levy has recently been advocating a consensus-based approach to planning.  If the Washington Heights neighborhood collectively supports new mid-rise development, could a straightforward height restriction, along the lines supported by the community, be a preferable, and achievable, substitute for the current system?  Libertarian-leaning urbanists are fond of criticizing height limits, but such a limit, in this case, could hardly be more unfriendly to new development than the system that the Times and Salins describe.

Related posts: Can New York Build Its Way Up to Affordability?

Monday, October 17, 2011

Hope VI and Public Housing: The More Things Change...

Stephen Smith has posted a perceptive article critiquing the design of contemporary public housing projects.  The story of public housing over the last 20 years is largely the story of the Hope VI program, which was adopted by Congress in the early 1990s to provide a source of federal funding for rehabilitating, and even more so demolishing and rebuilding, distressed public housing.  Accompanying the new funding initiative was a new design philosophy: HUD secretary Henry Cisneros embraced the principles of the Charter of the New Urbanism the mid 1990s, reflecting a change in course from the tower-in-the-park design principles which had prevailed for many decades. 

This was not an empty gesture, as a vaguely New Urbanist design approach came to be reflected in dozens of Hope VI projects built over the ensuing fifteen years.  But just how new was this approach, really?  A close examination of justifications given for the projects themselves reveals certain overriding concerns, which for those familiar with the history of urban planning in the 20th century may seem familiar, but which have little do with New Urbanist principles.

A concrete example might help illustrate the point.  The project I'm most familiar with is the recent Hope VI redevelopment of Nashville's J. Henry Hale Homes, a project that was typical of the low-rise public housing constructed in Nashville during the 1950s.  Although the original project was demolished around 2006, Google earth shows it as it existed before:

The buildings shown are all two-story structures.  The planning principles at work in creating such a development were perhaps not as careless and insensitive as they're sometimes made out to be:
"...[W]ith houses and gardens in compact groups ... there can be playgrounds for different age-groups and parks and perhaps even a community center, instead of dead chasms between houses and acres of unnecessary pavement in streets, sidewalks and alleys. The whole neighborhood may be just one super-block, which means complete play safety for the children and clean, quiet green surroundings and outlook for all the houses. For access to the houses there may be small dead-end streets. Garages and parking spaces are conveniently located at the nearby street-front. The house itself, shallow and never more than two rooms deep, has sun in every room and far more real privacy than an 'individual' house on a narrow lot."    -Catherine Bauer, A Citizen's Guide to Public Housing (1940).
This is hardly traditional urbanism, but there is a clear acknowledgement here of the problems of welcoming the automobile into the heart of a residential environment, as well as a backlash against the repetitive, amenity-poor process of speculative lot subdivision that had characterized the seven decades or so preceding 1940.  The development relegates the handful of parking lots in groups to the fringes of the development, or to dead-end access ways.  The few auto streets are quite narrow and naturally control speed with T-intersections and sharp turns.  Far more numerous are the narrow pedestrian paths linking the housing units to the streets and to the two large park areas within the project.  Overall, it is a pedestrian-focused environment, with a balance of built space, streets and park space reminiscent of a college campus. 

For reasons that have been hotly debated, this project, like many others, quickly degenerated into a focal point for crime and poverty. Was it the fault of its design?  Nashville's director of the Metro Development and Housing Agency, Phil Ryan, thinks so:

“Part of the problem with public housing in the U.S. is overly dense housing that’s unappealing and unattractive,” Ryan explained. “The extreme density has created a bad environment for people. ... We think a beautiful mixed-income neighborhood is a net gain for low-income people versus an overly dense, traditional 60-year-old housing development,” Ryan said.
Does this sound at all familiar to the claims of the early 20th century Progressive housing reformers?  It's a position Jacob Riis would have applauded, and which Lewis Mumford might have endorsed.  The key elements might be summarized as environmental determinism, or belief that one's built surroundings influence social behavior, and out of that thinking a belief that decongestion, or a dispersal of population, is the best means of positively influencing behavior -- among a certain segment of the population anyways.  (At least, the Nashville planners, to my knowledge, have never criticized nearby high-end condominium developments for being "overly dense.")   These are not mere idle words, on this project (see below) or others, as a net loss of housing units is characteristic of the entire Hope VI program.

In any event, here is the new project, developed with $20 million in federal Hope VI funding:

A comparison of the projects reveals the following changes:

Is $20 million dollars for a loss of 255 housing units a good use of taxpayer money, one could ask, even if the new units are of higher quality?   Long-term maintenance liabilities have presumably increased as well, with the total paved surface increasing from 7% to 30% to serve less than half the number of units.  And of the new street grid, that presumably New Urbanist-inspired addition?  Does this automobile connectivity serve any purpose in the absence of the mixed uses that New Urbanism advocates but which are absent here, or have Catherine Bauer's "acres of unnecessary pavement" simply reappeared?

The streets have been dramatically widened, flattened and straightened, as well, which has necessitated the placement of speed bumps throughout the project:

And although an article touting the project refers to it as "a neighborhood with real front and backyards, green space and duplex homes," not only has green space been reduced and identifiable parks nearly eliminated, the homes do not, in fact, have backyards:

Front yards do exist, though, with thoughtfully-located utility boxes.  Brick is reserved only for the facades, or rather parts of the facades:

I'll let the reader compare the design and aesthetics of these homes to those of the nearby Andrew Jackson Courts, public housing built some 15 years before the original J. Henry Hale homes:

With this design-focused analysis I won't delve into the city's motives, why it might have wanted to dramatically reduce density in existing public housing projects, or whether the city planners truthfully believed their own pronouncements on density.  The project did "succeed" at largely replacing the population in the project, and there is evidence that tenant management, one the key elements of the relative success of New York's public housing, is now given greater attention. 

From a planning perspective, though, is it possible to argue that this new design approach is always an improvement over the public housing of the 1950s?  Even some of the most sensitively-designed developments replacing notorious projects, such as the redevelopment of the Robert Taylor Homes in Chicago, show a similar emphasis on wide streets, low building footprints, large parking areas and residual "green space."  Perhaps the design philosophy underlying these redevelopments has not changed quite as much as one might think.

Friday, October 14, 2011

Measuring Connectivity, part II

In the previous post, I mentioned the concept of metric reach which is being developed by John Peponis and his students (including commenter Patrick!) in Atlanta.  What I didn't say was why I thought this particular measurement so useful for measuring connectivity.  Again, consider the reinforcing virtues of density, mixed uses and accessibility:
"Well functioning cities can ... be thought of as 'movement economies'. By this it is meant that the reciprocal effects of space and movement on each other ... and the multiplier effects on both that arise from patterns of land use and building densities, which are themselves influenced by the space-movement relation, that give cities their characteristic structures, and give rise to the sense that everything is working together to create the special kinds of well-being and excitement that we associate with cities at their best." Bill Hillier, Space is the Machine, p. 113-14.
Or, as put more bluntly by the early 20th century economist Paul Nystrom:
"The greater the number of people, other things being equal, who live near, who come to, or who pass by a certain location, the more valuable that location is." Paul Nystrom, The Economics of Retailing, p. 189.
As Hillier alludes to, the overall effectiveness and efficiency of urbanism can be expressed as a multiple of density, land use and movement.*  For a person either living or visiting a city, is there a higher purpose of its streets than providing access to the greatest possible number of attractions over the shortest possible distance?  This is essentially the basis behind the Walkscore website, and it is what metric reach simply and elegantly tries to show us: a city's overall potential for providing the greatest range of nearby options from each point in the urban street network. 

One study examining this measure in the context of walking to transit stations in Chicago, Dallas and Atlanta has shown its usefulness in predicting pedestrian behavior:

"When multivariate regressions are run ... street connectivity [based on metric reach] is found to be a rather significant predictor of ridership levels in all three catchment areas when controlling for population density and transit station measures. However, the best results are obtained for the 0.5 mile range. This supports the findings of various studies which suggest that within short distances people will walk to transit regardless of local street connectivity ... . In other words, people residing within 0.25 mile distance from a station are inclined to use transit irrespective of the street connectivity levels of the station area. Higher correlation coefficients within the 0.5 mile buffer suggest that the decision to walk a slightly longer but still very manageable distance is strongly affected by the density of street connections."  The Effects of Street Configuration on Transit Ridership, Ozbil, Peponis and Bafna (2009).
Since the area between .25 and .5 miles from a given point contains three times the total area as the space within .25 miles, this is an important insight for TOD at the very least, and for thinking about walkability in general.

Even reach, as great a tool as it is, does not capture the whole picture.  Steve Stofka on his blog draws attention to what he has termed "traversal amenities," which represents the ease and comfort in walking a given distance.  This is not a subjective point: a line drawn down the midpoint of a street cannot tell us its width, or its traffic conditions, or any of the other factors which may affect as simple a decision as crossing from one side to another.  Disincentives to crossing a street, however, may reduce a pedestrian's range of movement options, effectively limiting metric reach (Walkscore, which Steve notes does not take into account these factors, has acknowledged this limitation). 

Ultimately, measuring pedestrian connectivity, and putting a number to it, is a difficult task given the  fine-grained and spontaneous nature of pedestrian accessibility and movement, and the ease with which it can be disrupted by vehicular traffic or other obstacles.  As it is, the most intricate feats of "pedestrian engineering" currently take place in indoor, self-contained or controlled environments, such as malls, stadiums and cruise ships rather than in urban planning, but Peponis and his students, through their efforts, are developing one way of thinking that may help change that.

Next and last in the series: a quick look at navigability, or, finding your way around.

*See also Modeling street connectivity, pedestrian movement and land-use according to standard GIS street network representations: A Comparative Study, p. 1.

Monday, October 10, 2011

Measuring Connectivity, part I

The subject of street connectivity is one which gets to the heart of what is and is not good urbanism.  It formed the basis for one of Jane Jacobs' most memorable discussions in The Death and Life of Great American Cities when she singled out New York's 800-foot blocks for particular criticism.  Two decades later, Professor Bill Hillier developed the analytical method for spatial patterns known as space syntax, an approach which promised an objective means of ascertaining the intelligibility and effectiveness of  street networks.  The area has become a fruitful field of study, while an abundance of free software tools have put spatial network analysis within the reach of the humble blogger.

What is connectivity, then, and why is it so important?  It is not a secret that the purpose served by streets is, among other things, to provide a public means of accessing the built environment.  It is just as evident that a public way adds value to the adjacent land.  A city therefore exists in tension between the value of land for buildings, which encourages a maximization of the built footprint, and the value added by streets, which by their nature diminish that same footprint.

The value conferred by the streets themselves, however, depends upon their accessibility to the public, not merely in a legal sense, but in terms of proximity and navigability.  A shop at the end of a long, dead-end street benefits from that street far less than the shop at an intersection benefits from its adjacent streets.  In general, then, a city as a whole will have an economic incentive to maximize continuous streets and minimize cul-de-sacs, except in rare cases.  Connectivity, in this sense, describes to the extent to which various points within a given area are linked by the street network, just as density can describe the intensity of the built environment.

Academics and planners, in the field of space syntax and outside it, have developed numerous different approaches aimed at measuring this elusive quality.  Among these are:
  • Intersection density (the classic measure often used to contrast between gridded patterns and late 20th century suburbia).
  • Link-node ratio.
  • Connected node ratio.
  • Distance between origins and destinations.
  • Average block sizes.
  • Block density.
  • Block face lengths.
  • Street density (street area).
  • Metric and directional "reach."
(For a summary of many of these, see here).

The newest of these, metric reach, is a concept introduced by Georgia Tech's John Peponis just a few years ago.  The concept is defined as:
"[T]he aggregate street length that is accessible from the mid-point of each road segment within a metric radius of actual movement.  ... Distances are measured along street center lines ... Reach, therefore, is a measure of street density. Implicitly, it is a measure of urban potential: the greater the average [reach value] of an area the greater the interface between the public streets and private properties, the greater the the likely number of properties that are within range, the greater the likely number of potential destinations or land uses." (Peponis et al. 2007).
To illustrate the method, consider the central areas of two cities of roughly equal population: Omaha, Nebraska (at left, of course), and Malaga, Spain.  A single point is selected from which are drawn all possible routes that could be covered in a 900 foot walk:

As it turns out, a person in Omaha can, within 900 feet, access 13,800 linear feet of block fronts (several of which happen to consist of parking lots), while the pedestrian in Malaga, in the same walking range, can access just under 25,000 feet.  And despite this, Malaga has a considerably higher ratio of built area to street area, thanks to streets which are, overall, much narrower than in Omaha.  Malaga obtains both higher density and connectivity, at least by this single data point and using this particular method. 

What predictive power does this measure have for pedestrian and economic activity? I'll discuss that in the next post.

Related reading: Laurence Aurbach has a fantastic series on connectivity which I highly recommend.

Monday, October 3, 2011

Note on Gentrification and Preservation

There have been several great pieces on the dynamics of gentrification over the past several days. Market Urbanism's Stephen Smith, who is now writing at Forbes, observes how permitting more development in desirable areas will help "soften the blow of gentrification." Alon Levy theorizes about the causes of central city revitalization. Emily Washington, in response, considers the time preferences of the upper and middle-classes. At the City Paper, Lydia DePillis discusses the relationship between historic preservation and gentrification, among other things.

In DePillis' article, in particular, there is this observation linking preservation and neighborhood revitalization:

"The thing is, preserving distinguished architecture and well-constructed neighborhoods can be one of the best ways for real estate to hold its value. Places that look like they were once prosperous send a message that they could be again."
However, one can argue that it is well-constructed buildings and distinguished architecture which attracts gentrifiers long before historic districts are contemplated.  The following chart was put together some years ago for a project looking at four southern cities (Nashville, Atlanta, Memphis and Savannah) in which all census tracts in 1940 were grouped into quartiles based on housing value in that year.  The tracts were then revisited in the 1970 and 2000 census years, keeping the groupings the same (the numbers show quartile housing value divided by the median housing value for the city in a given census year):

The pattern over time distinctly shows the operation of residential "filtering" from 1940 to 1970, as homes in wealthier neighborhoods came to be occuped by those from poorer neighborhoods, and an almost mirror-image reversal of this trend from 1970 to 2000 (intervening census years show that the turning point is in fact around 1970).  The middle class was simply returning to the same homes that their parents' generation had abandoned some twenty to thirty years before.  In 1970, moreover, one could purchase a home built by, say, an upper middle-class family of the 1920s, for what was essentially a 50 percent discount a house that went for four times the price of those in the poorest quartile in 1940 was, in 1970, selling for only twice that, even though these were in most cases the very same houses. 

This is different result than some other studies (see e.g. Old Homes, Externalities, and Poor Neighborhoods: A Model of Urban Decline and Renewal) which find that neighborhoods should cycle over time as housing stock ages, deteriorates and ultimately becomes ripe for redevelopment.  Yet the poorest two quartiles have not cycled at all in 60 years, while the housing in the first quartile was not actually redeveloped, but only renovated by new buyers (that is, its age was an amenity, perhaps the primary amenity, rather than a liability).  The hidden factor is the quality of the housing stock, which overwhelms the effects of aging and deterioration.

This can help explain the perceived attitudes of preservationists toward neighborhood change that DePillis discusses in her article.  It also suggests an explanation for the suburbanization of those with low incomes: as the better urban housing is gentrified, the former occupants may have two choices: move back into impoverished central city neighborhoods or move out into the aging suburban housing vacated by the gentrifiers or those moving to newer suburban houses. If costs are roughly comparable, the decision is easy. 

This type of gentrification is different from the the arrival of residents back into a formely commercial or industrial urban core, where the pattern is much more analagous to greenfield development (as Alon mentions in his post).  This is occurring simultaneously with gentrification of existing neighborhoods in Nashville and other cities, but takes a very different form.