Tag Archives: diversity

Intersection density & centrality

What is the best method to quantify what makes a place walkable?  The Journal of the American Planning Association recently published some powerful documentation from Robert Cervero and Reid Ewing on the value of pedestrian-oriented design (following up on yesterday’s links).  Grist has the article (hat tip to Planetizen), citing Laurence Aurbach’s PedShed blog – again, the “Three D’s” or urbanism emerge front and center – density, diversity, and design:

Their findings? Of all the built environment measurements, intersection density has the largest effect on walking — more than population density, distance to a store, distance to a transit stop, or jobs within one mile. Intersection density also has large effects on transit use and the amount of driving. The authors comment,

This is surprising, given the emphasis in the qualitative literature on density and diversity, and the relatively limited attention paid to design.

In other words, intersection density is the most important factor for walking and one of the most important factors for increasing transit use and reducing miles driven, but gets relatively little attention in research and in public policy.

In other words, the other two D’s (density and diversity) get more play than design.  Perhaps that’s because density and diversity (of land use, of people, of incomes, etc) were easier to quantify than something as seemingly subjective as design.

Intersection_Density

Kaid Benfield ties these principles to those angry about the ever expanding oil slick in the Gulf of Mexico, hitting on another theme of the research – location:

The study’s key conclusion is that destination accessibility is by far the most important land use factor in determining a household or person’s amount of driving.  To explain, ‘destination accessibility’ is a technical term that describes a given location’s distance from common trip destinations (and origins).  It almost always favors central locations within a region; the closer a house, neighborhood or office is to downtown, the better its accessibility and the lower its rate of driving.  The authors found that such locations can be almost as significant in reducing driving rates as other significant factors (e.g., neighborhood density, mixed land use, street design) combined.

The clear implication is that, to enable lifestyles with reduced driving, oil consumption and associated emissions, environmentalists should continue to stress opportunities for revitalization and redevelopment in centrally located neighborhoods.  As Ewing and Cervero put it:  ‘Almost any development in a central location is likely to generate less automobile travel than the best-designed, compact, mixed-use development in a remote location.’

Aurbach is quick to note the limitations of the study, but even with those this is an exciting quantification and potential metric for walkable and sustainable design.  It builds off the Jacobs legacy of ‘short blocks’ and adds some science behind recent GGW posts from Erik Bootsma and Daniel Narin on the variety and histories of street grids.  This kind of research lends weight to the anecdotal accounts of Portland’s small blocks resulting from the belief that corner lots were more valuable, as well as ideas of better utilization of alley space – such as this recent post from Richard Layman.

Changing suburbia

Some suburban items to share today:

Design: Infrastructurist takes a look at the problem of culs-de-sac (which I believe is the proper plural of cul de sac).

cul-de-sacs

Commenters take note of some serious issues with this particular study, but the general point still stands – culs de sac remove key links from the street network, requiring longer and more circuitous routes to get to the same destinations.  Developments of these kind of street patterns are no small part of America’s long history of vehicle miles traveled increasing far faster than the rate of population growth.

Diversity: The Washington Post has an article on the changing face of suburbia – more socially and economically diverse, and dealing with new sets of problems that many of these communities have never had to deal with before:

Demographers at Brookings say suburbs are developing many of the same problems and attractions that are more typically associated with cities. And cities, in turn, have been drawing more residents who are young and affluent, so the traditional income gap between wealthier suburbs and more diverse cities narrowed slightly.

“The decade brought many cities and suburbs still closer together along a series of social, demographic and economic dimensions,” said the report, titled “State of Metropolitan America.”

The other substantive point is about how Americans perceive their surroundings (urban, suburban, rural) compared to how their city and their urban economy actually functions:

The report outlines a decade in which several demographic milestones were passed as the nation’s population topped 300 million midway through. About two-thirds of Americans live in the nation’s 100 largest metropolitan areas, virtually all regions with populations of 500,000 or more.

“We think we’re a small-town nation,” Berube said. “But small towns exist because they’re connected to something bigger, which allows residents to make a living.”

Density: Ryan Avent has long marked the economic benefits of density and the nature of urban agglomerations, but he has an interesting point on the marginal benefits of added density, noting that modest increases in the less dense suburbs could have a troubling impact, while modest increases in the already dense core, already designed at a walkable scale, would have serious benefits for local retail.

So let’s think about the effects of doubling density in Fairfax and the District. Now on the one hand, the benefits to doubling density in Fairfax are likely to be larger than those in Washington for reasons of scale alone — in the Fairfax example, more people are added. That makes for a deeper labour pool, a larger skills base, and so on. On the other hand, Fairfax density is likely to be less effective density. Fairfax is built in a fairly standard, suburban way. It’s not built at a walkable scale, the road system is arterial rather than gridded, transit options are limited, and so on. Doubling density, absent major infrastructure improvements, might actually reduce the metropolitan access of Fairfax residents.

Not so in the District. Yes, with more people roads, buses, and the Metro would be more heavily taxed. At the same time, every neighborhood would become individually more convenient. Brookland is fairly low density for a District neighborhood, but it’s basically built to be walkable. Were density in Brookland to double, the retail and commercial options within easy walking distance of Brookland residents would more than double.

The problem with doubling the density in a place like Fairfax County, aside from the infrastructure issues that Ryan highlights, is that you’d end up with a place that’s stuck in the no-man’s land of density – too dense for the auto-oriented infrastructure to function smoothly, but not dense enough to really tap into the critical mass and benefits of walkable urban places.

Perceptions of density often miss the mark

Photo from cacophony76.

Photo from cacophony76.

Density is one of the most important elements of any city, but also one of the most misunderstood.

However, the density of a site is often not what it initially seems – people will key on things like height, design, maintenance, and context rather than actually looking at what density means to them.  It’s a natural, emotional reaction – but often misses the underpinning reality.  Educating people on what density looks like is vitally important, as density is a crucial element of sustainable, urban places.

In Washington, DC, like many other places, people often have a visceral reaction against density.  They assume more density means taller buildings in a low-rise city, but that need not be the case.  These fears of density are not unfounded, however.  Complaints about density often reveal other concerns, such as traffic congestion or design.

Dan Zack is a planner for Redwood City, CA.  He recently gave a presentation out in California which included the following ‘quiz,’ asking attendees to quickly assess how dense a building or development is based on a passing glance at a photograph of the site.  The clip is just shy of 12 minutes long.  Take a look and see how accurate your perceptions of density are:

Density often gives rise to fears from neighbors about traffic congestion, crime, environmental quality, and many other factors.  Outside the immediate community, people scream about social engineering and forcing people to live in dense environments, despite the fact that increased density is a product of market forces and substantial pent-up demand.  Mr. Zack’s quiz shows how density is often not what it seems.

Height, for example, is only one factor in density.  Paris is almost uniformly low-rise in nature, yet has extremely high densities.  For DC, the takeaway message is that the city can continue to grow and add density without fundamentally altering the low-rise nature of the city.  As DC continues to grow, adding more housing supply will be of vital importance.  More households can also help certain areas of the city reach a critical mass of retail buying power, enabling stores and restaurants to survive and thrive.

Just as height is only a factor in density, density itself is only a factor in the overall health of a city.  Put in simple terms, a city needs the Three D’s – Density, Diversity, and Design – to thrive.  As Mr. Zack’s quiz shows, diversity (of housing sizes, price points, neighborhoods) and design all factor in to how we perceive density.  Each of the Three D’s is deeply interwoven with the others, and touch on all urban issues, from transportation to affordable housing.

Emphasizing the need for density at this juncture is important, as well.  Cities are not static environments.  They change a great deal over time.  In the next 25 years, approximately 75% of the American built environment will either be renovated or built anew. Even accounting for a lull in demand from the Great Recession, American cities are in for a great deal of change.

The entirety of Mr. Zack’s presentation is well worth watching, and can be found below.  His presentation is about 50 minutes long, and includes the ‘quiz’ clip above.  In the remainder, he discusses at length all of the companion issues that need to be dealt with in addition to adding density, such as design, parking, transit, and walkability.

Cross-posted at Greater Greater Washington