Off the Grid…and Back Again?

Abstract

Problem, research strategy, and findings

In this morphological study I identify and measure recent nationwide trends in American street network design. Historically, orthogonal street grids provided the interconnectivity and density that researchers identify as important factors for reducing vehicular travel and emissions and increasing road safety and physical activity. During the 20th century, griddedness declined in planning practice along with declines in urban form compactness, density, and connectivity as urbanization sprawled around automobile dependence. But less is known about comprehensive empirical trends across U.S. neighborhoods, especially in recent years. Here I use public and open data to examine tract-level street networks across the entire United States. I develop theoretical and measurement frameworks for a quality of street networks defined here as griddedness. I measure how griddedness, orientation order, straightness, 4-way intersections, and intersection density declined from 1940 through the 1990s, while dead-ends and block lengths increased. However, since 2000, these trends have rebounded, shifting back toward historical design patterns. Despite this rebound, when controlling for topography and built environment factors, all decades after 1939 are associated with lower griddedness than pre-1940 decades. Higher griddedness is associated with less car ownership—which itself has a well-established relationship with vehicle kilometers traveled and greenhouse gas emissions—while controlling for density, home and household size, income, jobs proximity, street network grain, and local topography.

Takeaway for practice

Interconnected grid-like street networks offer practitioners an important tool for curbing car dependence and emissions. Once established, street patterns determine urban spatial structure for centuries, so proactive planning is essential.

Keywords:

• street grid
• street networks
• transportation
• urban form
• urban morphology

ACKNOWLEDGMENTS

Various pieces of this work in progress were presented over the years at the Association of Collegiate Schools of Planning annual conference, the Urban Affairs Association annual conference, and the Transportation Research Board annual meeting. I thank those conference session participants for their feedback and especially thank Adam Millard-Ball, Lisa Schweitzer, Sara Jensen Carr, Jana Cephas, and Jake Wegmann for their invaluable comments on early drafts.

RESEARCH SUPPORT

This study was funded in part by a grant from The Public Good Projects.

SUPPLEMENTAL MATERIAL

Supplemental data for this article can be found on the publisher’s website.

Notes

1 See, for instance, ITE’s 1993 Guidelines for Residential Subdivision Street Design, ITE’s 1994 Traffic Engineering for Neo-Traditional Neighborhood Design, Oregon’s 2001 Neighborhood Street Design Guidelines, and the 2009 LEED-ND Neighborhood Pattern and Design certification criteria.

2 Because these data are from the 2018 ACS, the 2010s decade does not cover the entire decade and thus includes a smaller set of tracts (see Technical Appendix Table A3).

3 See the Technical Appendix for details on interpreting parameter estimates in a spatial-lag model.

Additional information

Notes on contributors

Geoff Boeing

GEOFF BOEING ([email protected]) is an assistant professor in the Department of Urban Planning and Spatial Analysis at the University of Southern California’s (USC) Sol Price School of Public Policy and the director of USC’s Urban Data Lab.