18th Annual Congress of the New Urbanism “New Urbanism: Rx for Healthy Places”

CNU 18: Atlanta, Georgia / May 2010

“New Urbanism: R_x for Healthy Places”

Measuring Connectivity as Implementation of Sustainable Communities

Background: Planning for connectivity of transportation systems at many different levels is becoming a critical component of broader smart growth, growth management, sustainability and climate change policies and programs.  Nationally, many agencies and state legislatures are considering policies aimed at sustainable community initiatives similar to the partnership between HUD, EPA and USDOT while Congress is debating strengthened performance metrics as part of transportation reauthorization.  Over the past decade several municipalities have adopted connectivity requirements as a means of assessing new development and the connectivity of its transportation system that reflect the goals of this sustainability effort.  Additionally, fundamental urban design principles emphasize quality transportation network development by minimizing block lengths and dead-ends to achieve multiple, multi-modal connections.

Purpose: This study compares the most often-applied measures of street and non-motorized system connectivity and summarizes their ability to address emerging policy for sustainable communities.

Methods: Several metrics for street and non-motorized system connectivity have already been adopted by many agencies for various purposes, but these rely on proxies (e.g. intersection density, percent route completeness) or singular route performance measures while overlooking the most meaningful factor–systems connectivity. Previous research has analyzed and compared several metrics of system connectivity, including link-node ratio, intersection density and route directness index (RDI); concluding RDI as having the strongest corollary measure for connectivity.  Research also notes the difficulty of applying RDI to consistently measure multiple land use origins and destinations offered within the transportation system.  Using new software, varied neighborhood street networks (Abu Dhabi, UAE), transit station (Sound Transit – Seattle, WA) and fire station (Charlotte, N.C.) access examples are used to compare and contrast the subject connectivity measures for purposes of this research.

Results: Findings of the study show that the direct measurement of connectivity by means of RDI can be effectively and consistently applied for neighborhood, transit station, and fire station analyses.   RDI is found to be the most comprehensive yet practical and direct measure of connectivity for a variety of sustainable community issues:  quality of neighborhood networks and improving transit station and fire station access. Correspondingly, tools were developed to make these measurements more robust and useful to practicing professionals by allowing point-to-point, parcel-to-parcel and point-to-parcel analysis through GIS.

Tips for Practice:    Study examples and findings are used to illustrate how RDI can be effectively applied as a performance-based measure.  also suggest that current policies and programs that employ proxy measures of connectivity (e.g. link-node ratio or intersection density), while perhaps helpful in raising awareness of the importance of quality network connections or stopping poor forms of land use/transportation system development, are insufficient as performance-based analytical tools.  Study findings indicate how RDI is a better, applied measurement in the analysis of transportation system connectivity as one important element of emerging sustainable community policy.