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.

2 comments to Intersection density & centrality

  • I wonder what exactly Diversity means… When I think diversity I also think of design. Both in variety of building types, variety of architectural styles and varieties of scale. For instance in DC, Georgetown is immensely more walkable with it’s varying forms than some of the monolithic areas like Petworth or downtown. Or even those horrid new historical areas of giant buildings in NOMA or Arlington.

  • Alex Block

    Christopher, I think the definition in this circumstance is left in broad terms on purpose. I take it to mean economic diversity (of housing stock, price points, etc), social diversity, ethnic and racial diversity, etc.

    I suppose that design diversity could fit into that category as well, but I think it would better be described within the ‘design’ category.

    The larger point isn’t to define these concepts in exact terms, however – just to keep them in mind as some of the core ideas for what makes cities work.

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