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    On 'Travel and the Built Environment'

    continued

    Since cities overwhelmingly show both these patterns — decreasing density toward the edge, and increasing VMT toward the edge — making the sample size larger by including more cities, each measured according to its own aggregated data set, would not reveal the general correlation between density and VMT as non-causal.

    Ewing and Cervero have surmounted this traditional error by carefully limiting their meta-analysis to include studies only with disaggregated data, from which location-based measures of VMT can be calculated. "Travel and the Built Environment" may be the first meta-study to apply this level of discipline, and it pays off handsomely in the clarity of the findings.

    As they say:

    "Equally strongly related to VMT [as location accessibility more generally] is the inverse of the distance to downtown. This variable is a proxy for many Ds, as living in the city core typically means higher densities in mixed-use settings with good regional accessibility."

    The first of those two sentences seems now to be a bedrock finding: "Equally strongly related to VMT is the inverse of the distance to downtown."

    The second sentence — "This variable is a proxy for many Ds, as living in the city core typically means higher densities in mixed-use settings with good regional accessibility" — I find less convincing.

    Of course it represents a natural viewpoint, coming from the established perspective that density is something primary to these issues. Arguably, however, things could be the other way around.

    We at ArchitectureWeek suspect that the "distance to downtown" really is the primary factor, and the other Ds represent the supporting cast and proxies.

    This alternative hypothesis seems to be something the field of planning research overall has hardly begun to consider — perhaps it seems much more obvious from the geometric mindset of the environmental designer than from the statistical mindset of the sociologist.

    Moving from aggregated data to data that can be analyzed by location was a big step in the right direction. These improvements can be taken farther.

    To explore this, we'd like to see research, building on "Travel and the Built Environment," that considers the possibility that the simple effect of actual trip lengths is the most potent underlying factor in the geography of VMT.

    Does Geometry Rule?

    Reid Ewing may not remember this, but at a conference in Oregon two or three years ago, I challenged him to "do the math" on the proportional contribution of central location versus mixed-use or smart-growth development style. He promised that he would.

    Ewing and Cervero have done the math. The clear results of "Travel in the Built Environment" have powerful implications for environmental design and planning.

    And there's always a little more math to be done!

    Kevin Matthews is Editor in Chief of ArchitectureWeek.   More by Kevin Matthews

    Reference

    Ewing, Reid, and Robert Cervero. "Travel and the Built Environment: A Meta-Analysis." Journal of the American Planning Association, Volume 76, Issue 3, June 2010, pp. 265-294. DOI: 10.1080/01944361003766766.

     

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    Job accessibility by transit is a subvariable of "destination accessibility" as defined by Ewing and Cervero.
    Photo: Charbel Akhras Extra Large Image

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    Job density, a subvariable of "density," has greater bearing on walking than on driving.
    Photo: Xiang Chen Extra Large Image

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    Ewing and Cervero note that demand management, including parking supply and cost, is a sixth "D" variable, included in a few studies.
    Photo: David Owen/ Artifice Images Extra Large Image

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    Carbon dioxide emissions and VMT per household per year follow a consistent radial gradient pattern, from low in the urban core to high at the urban fringe, across a comprehensive range of U.S. cities. Mapped here are Chicago, Illinois (shown in thumbnail), Houston, Texas; Portland, Oregon; and Atlanta, Georgia.
    Image: © 2010 Center for Neighborhood Technology Extra Large Image

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    Although cities tend to produce high concentrations of carbon dioxide per land area, per-capita CO2 production from automobile use tends to be lower in urban areas than elsewhere.
    Image: © 2010 Center for Neighborhood Technology Extra Large Image

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    Graph of VMT per household per day by regional location, for Seattle, Washington, and Atlanta, Georgia, showing the gradient of increasing VMT from urban core to exurban areas.
    Image: Courtesy Larry Frank

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    Weighted average elasticities of walking use with respect to built-environment variables.
    Image: Reid Ewing and Robert Cervero/ ArchitectureWeek Extra Large Image

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    Weighted average elasticities of transit use with respect to built-environment variables.
    Image: Ewing and Cervero/ ArchitectureWeek Extra Large Image

     

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