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

TL;DR

This study analyzes nationwide trends in American street network design, revealing a decline in grid patterns until the 1990s, followed by a recent rebound, with implications for reducing car dependence and emissions.

Contribution

It develops a comprehensive measurement framework for street network griddedness and empirically tracks its evolution across US neighborhoods from 1940 to the present.

Findings

Griddedness declined from 1940s to 1990s and rebounded after 2000.

Higher griddedness correlates with lower car ownership and emissions.

Street network patterns influence urban sustainability and planning strategies.

Abstract

This morphological study identifies and measures 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 alongside declines in urban form compactness, density, and connectivity as urbanization sprawled around automobile dependence. But less is known about comprehensive empirical trends across US neighborhoods, especially in recent years. This study uses public and open data to examine tract-level street networks across the entire US. It develops theoretical and measurement frameworks for a quality of street networks defined here as griddedness. It measures 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. Yet, despite this rebound, when controlling for topography and built environment factors all decades post-1939 are associated with lower griddedness than pre-1940. 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. 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.