Car Ownership Levels in America Follow Trends of Urban Street Griddedness

As technology, transportation, and design have evolved, so too has the physical composition of American streets or street "griddedness." In "Off the Grid...and Back Again?" in the Journal of the American Planning Association (Vol. 87, No. 1), Geoff Boeing seeks to understand the correlation between geometric spatial ordering in U.S. cities and the ownership and use of automobiles. He hypothesizes that the griddedness, or lack thereof, of American cities is not merely a design choice, but has tangible implications for equity in mobility, public health, and environmental sustainability.

Figure 1: Street grids. Theoretically, a street grid has an internally consistent orientation, is relatively straight, and comprises mostly 4-way intersections. Each of these three characteristics is necessary but alone insufficient: only in unison do they make a street grid.

Return to Gridded Street Layouts

The gridded street design was the primary layout of U.S. cities from the 18th century through the 20th. Griddedness wasn't just a feature of cities; griddedness defined westward expansion as a means of promoting rapid expansion and settlement. As the automobile began to dominate the streets, planners took a car-centric approach to spatial design, and the standards of layout and engineering in American cities and towns evolved to prioritize personal cars. In new urban and suburban developments, streets became more circuitous and then more disconnected, through the 1940s and into the 1990s.

Meanwhile [in the mid-20th century], planning practice shifted away from dense, interconnected, gridded street networks in a bid to simultaneously attenuate the automobile's negative externalities...in residential communities while still empowering the populace to travel by car because it was fast and convenient.

Theorists of the 20th century proclaimed that street network designs promoted particular values. Lewis Mumford disliked gridded designs, arguing in 1961 that griddedness was monotonous and lacked function. More recent academics praise street grids for their legibility, efficiency, and directness. Yet street design is not merely a matter of values and aesthetics. The choices that planners make about network layout pertain to matters of equity, health, and access. More connected streets can promote health by reducing car use and greenhouse gas emissions and by increasing walkability and levels of physical activity.

To determine the extent of a shift toward renewing a gridded typology throughout the United States, and the contemporary implications of street grids in practice, Boeing collected and analyzed data on the street networks of all 74,113 U.S. census tracts. He found that Chicago and New York City were two cities with particularly high orientation order, meaning their streets mostly point in just four directions.

Car Ownership Trends by Era

Nationwide, the Great Plains and the Midwest are the most gridded, while New England and Appalachia are the least grid-like. Boeing also finds a grid-time relationship, noting that the average grid index value in urban tracts built primarily in the 1940s is 84 percent higher than in 1990s urban tracts. Yet, since 2000, the grid index and its components have been on the rise in American cities.

Boeing finds that car ownership follows a similar trend as griddedness, with car ownership today rising with tract vintage from the 1940s to the 1990s but declining in tracts designed after 2000. Households in 1990s-vintage tracts own approximately 50 percent more cars than those in urban tracts designed before the Second World War.

Boeing implores any planner tasked with street layout and design to think carefully about the project, as the implications for such a project are clear and long-lasting. If there is not an imminent street redesign project, planners can update zoning codes and design guidelines to favor four-way intersections and connected grids. Boeing warns that planners should not merely react to mobility trends, but rather plan proactively to disincentivize car usage and advance city climate action plans.

Top image: Getty Images illustration.