Off the Grid horizontal ellipsis and Back Again? The Recent Evolution of American Street Network Planning and Design

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.