The Fuse

How Urban Design Affects VMT, Part I

by Hart Schwartz | March 12, 2018

This article is the first in a series looking at how urban design affects vehicle miles traveled.

  • A simple VMT equation highlights key transportation inputs linking urban design to VMT.
  • Mixed-use design is seen by urban planners as a key element for reducing fuel consumption.
  • Data on population density, a proxy for mixed-use, can assess long-term impact.

Clear recognition of the link between urban design and vehicle miles traveled (VMT)—crucial to understanding the relevance of urban planning movements pursuing “compact development” in reducing fuel consumption and oil dependence—often goes unnoticed. This article and subsequent ones in this series will directly address this gap. With respect to the compact design programs known as new urbanism and transit-oriented development, thousands of pages have been written—government reports, brochures, pamphlets, manifestos, design books—and many developments constructed. But in touring these developments or reading these extensive publications, it’s easy to lose sight of the bottom line: How do their design features affect transportation outcomes?

This article will provide an overview of a logical framework to show what needs to be done in order to affect change in VMT and fuel consumption outcomes. What data really counts and why? A simple VMT equation can provide a consistent, unified framework which spotlights mode choice, trips, and trip distance as inputs into fuel consumption, and shows why these variables are directly targeted by compact development movements such as new urbanism and transit-oriented development. In addition to identifying the relevance to fuel consumption of these input variables, the VMT equation renders far more comprehensible the core function of the movements’ common design element of mixed-use (which combines housing, commercial, retail, and cultural land uses in a compressed space).

Comparing population density statistics over the past century to transportation variables such as transit ridership and VMT can help assess potential impact on fuel consumption.

Overall, untangling the overlapping chains of causation between urban design elements, transportation inputs, and fuel consumption outcomes can help to understand whether the recent revitalization of many American urban cores—often dotted by the design features of compact development movements—can have meaningful long-term impact or is only of passing significance. Comparing population density statistics over the past century to transportation variables such as transit ridership and VMT can help assess potential impact on fuel consumption. Because the key design element of mixed-use is hard to measure, population density can be used as a proxy for whether cities are becoming more mixed-use and attaining the intended goals of the compact development movements, or if not, how far these movements have to go before attaining their goals.

Unraveling the chain of causation: Urban Design, VMT & Fuel Consumption

The following VMT equation serves as the crucial pivot between urban design and fuel consumption:

  • VMT = (number of vehicle trips) (length of vehicle trips)

If vehicles either travel more often, or longer distances per trip, then VMT will increase. Therefore, if urban design can impact these inputs, then it can impact fuel consumption:

  • Fuel consumption = vehicle-miles-traveled / fuel-efficiency

If VMT increases in the numerator, then fuel consumption increases (assuming fuel-efficiency is held constant). Therefore, either more trips or longer trips will tend to increase fuel consumption.

Here is where urban design steps in. Compact development movements such as new urbanism or transit-oriented development hope to reduce both number and length of vehicle trips. Note that a third variable, mode choice, is implied. Choice of travel mode—personal vehicle, ride-sharing, transit, walking, biking, etc.—affects number of vehicle trips, and any trip transferred away from a motor vehicle reduces VMT and reduces fuel consumption.

Compact development movements such as new urbanism or transit-oriented development hope to reduce both number and length of vehicle trips.

All of these variables are drawn together in an overarching desire for mixed-use development, a cornerstone, systemic pattern of design. Mixed-use is believed to impact all the travel input variables at once. Before proceeding further, it is important to briefly profile the compact development movements, to place their emphasis on mixed-use in clearer context.

New urbanism

New Urbanism is an urban design movement for which the “new” is misleading, because in fact the movement’s goal is to replicate styles of houses, streets, and neighborhoods that were common in the 1920s and earlier. Founded by Miami architect Andres Duany, new urbanist theory declares that homes placed close together, with little setback from the curb and little distance between the houses, on narrow interconnecting streets that intersect at right angles and have sidewalks, will promote walkability and community togetherness, increase population density, and reduce the need for vehicle travel.

Suburban Sprawl Street Pattern                               New Urbanist Street Pattern

urbandesign1

Source: Seattle Transit Blog, & Congress for New Urbanism

New urbanist design stands in stark contrast to contemporary suburban design trends. Modern sprawling suburbs tend to have winding roads, large houses set on large lots with ample space between lots, and cul-de-sacs feeding into connector roads that lead to the Interstate. Vehicle travel in sprawling suburbs tends to be a necessity because most developments are residential-only, meaning that driving is often the only time-practical way to arrive at daily destinations. New urbanism seeks to completely reverse this trend by placing disparate uses close enough so that walking can re-emerge as a viable travel mode.

urbandesign2

Source: The New Urban, “Walkable Neighborhoods: Street Block vs. Cul-de-Sac

Transit-Oriented Development (TOD)

Often used in conjunction with new urbanism, transit-oriented development aims to stimulate walking and public transit use, by placing a dense mixture of land uses, such as retail, residential, and office, in clusters near transit stations. TOD theory holds that vehicle travel can be substantially reduced if people can either ride transit to their job, or live within walking distance of work, as well as other common daily destinations. The TOD movement has attempted to either redevelop existing transit stations to include a greater mix of uses, or else construct new transit stations which anchor mixed-use, walkable districts. An excellent example of TOD would be Lindbergh City Center, a downtown Atlanta subway station, where the district around the station has been redeveloped with mid-rise office buildings, street-level retail, and medium-density apartment buildings all within a few blocks of the station. 

Mixed-Use: Key Common Element      

Both new urbanism and transit-oriented development seek the construction of mixed-use, walkable, compact city districts. The importance of mixed-use in reducing vehicle travel can be seen from statistics on the distribution of trip purposes, as highlighted in the following chart containing data from the National Household Transportation Survey (NHTS).

urbandesign3

If different types of trip destinations are closer together in a “mixed-use” district, people can theoretically travel less often, take shorter trips, or dispense with driving altogether.   

These statistics show that no particular type of trip accounts for more than 28 percent of all miles driven. This strongly suggests that mixing land uses in close proximity may indeed have a non-trivial impact upon VMT. The VMT equation outlined above says that if different types of trip destinations are closer together in a “mixed-use” district, people can theoretically travel less often, take shorter trips in personal motor vehicles, or dispense with driving altogether. If shopping malls, work spaces, social activities, and other destinations (school, church, etc.) sit in closer proximity, it stands to reason that there could be fewer trips, shorter trips, or more walking trips. In other words, the inputs to the VMT equation would each be reduced by mixed-use design, and thus VMT and fuel consumption would be reduced.

Real-Life Impact? Mixed-Use Hard to Measure; Density Used as Proxy      

The question naturally arises whether the prevalence of mixed-use design has been systematically measured, and whether this has correlated with transportation outcomes. Population density has sometimes served as a proxy to estimate mixed-use prevalence, because it is hard to measure the presence of mixed-use design and because districts with land uses placed in closer proximity logically tend to have higher population density. Using density as a proxy, the next article in this series will examine density statistics for central cities of metropolitan areas to see if mixed-use prevalence has changed over time, and to see whether such change correlates with transportation outcomes in terms of VMT, transit ridership, or vehicle ownership.

Change in short-term more likely at the margin

The statistical look in Part II, the next article, will identify dramatic long-term changes throughout the 20th century, and will underscore just how far new urbanism would have to go, to truly reach its goal of recapturing the urban design of the 1920s. As a previous article on The Fuse demonstrated, the relevant place to look, going forward, for change in vehicle ownership and travel statistics is at the margin. Individual decisions are made at the margin, so any long-term change will likely begin with a small change by a few individuals which eventually grows into a large, sweeping change. But when dealing with prebuilt patterns of urban infrastructure, etched into the landscape in concrete and steel, how much can truly change at the margin, when individual behavior is so conditioned by the physical environment in which it occurs? Stay tuned for Part II.

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