17 de agosto de 2012

Measuring Urban Form

Is Portland Winning the War on Sprawl?

Yan Song and Gerrit-Jan Knaap

Many articles on urban sprawl start with the disclaimer that sprawl is ill defined, but we know it when we see it. In this article, we present several measures of urban form and use these to evaluate development patterns and trends of single-family residential neighborhoods in the Portland, Oregon, metropolitan area. In doing so, we seek not only to increase precision in the debate on urban sprawl, but to evaluate whether the Portland metropolitan area - internationally known for its leadership in growth management - is winning its war on urban sprawl.

Research Context
Quantitative Research on Urban Sprawl
Quantitative Measures of Urban Form
- Street Design and Circulation Systems
- Density
- Land Use Mix
- Accessibility
- Pedestrian Access
Definition of Neighborhoods
Measures of Urban Form for Two Different Neighborhoods
Comparing the Urban Form of Neighborhoods Built at Different Times
- Connectivity
- Development Density
- Land Use Mix
- Accessibility
- Pedestrian Access
Study Limitations
Summary & Conclusions

In this article, we presented several measures of urban form and computed them for neighborhoods in Washing- ton County, Oregon. We first tested these measures in two select neighborhoods and found that they appear to meas- ure meaningful differences between a typical neighborhood and a highly touted New Urbanist neighborhood. We then examined whether our measures of urban form varied systematically with the age of neighborhoods.

We found systematic changes over time in most meas- ures of urban form, excluding measures of land use mix and distance to parks. Further, we found that many of these measures improved in the early 1990s. Specifically, we found increases in measures of connectivity, pedestrian access, and density—especially after 1990. The results suggest that neighborhoods in Washington County are becoming better internally connected, more pedestrian friendly, and denser but remain relatively homogeneous in land uses. Taken at face value, the results suggest that either market forces or the many growth management policies in the Portland metropolitan area are causing sig- nificant changes in urban form. As mentioned before, we emphasize that our intent in this article was not to conduct a policy analysis of the impacts of a particular plan or regu- lations. Because of the complex nature of growth manage- ment strategies, we do not attempt to isolate the impacts of any one instrument nor to answer the question of which specific instruments of the region’s growth management program are connected to the identified urban form changes noted here. Nevertheless, we speculate that re- cently developed neighborhoods have relatively higher internal street connectivity and better pedestrian access, and this might be attributed in part to street connectivity policies adopted in 1996 by Metro as part of the Urban Growth Management Functional Plan (Daisa et al., 2001). External connectivity is decreasing, and this might be due to the Access Management Program (AMP) in state and county road departments. The AMP has limited local street connections to the arterial network over the past 20 years. Another factor is that since the late 1980s, most single-family developments have been sited on parcels abutting existing development. In many cases, these abut- ting developments lacked future street connections.

Link para o artigo:

Journal of the American Planning Association, Sprig 2004, Vol. 70, Nº 2

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