Introduction

In terms of the public realm, no element is more important than streets. This is where active travel to work, shop, eat out, and engage in other daily activities takes place, and where walking for exercise mostly occurs. Parks, plazas, trails, and other public places also have an important role in physical activity, but given the critical role and ubiquity of streets, this book focuses on the qualities that make one street more inviting and walkable than another. Think of your last trip to a great European city and what, other than the historic structures and the food, was memorable. You walked its streets for hours and did not tire. It is the magic of a great street environment.

Until recently, the measures used to characterize the built environment have been mostly gross qualities such as neighborhood density and street connectivity (see reviews by Ewing and Cervero 2010; Handy 2005; and Ewing 2005). The urban design literature points to subtler qualities that may influence choices about active travel and active leisure time. Referred to as perceptual qualities of the urban environment, or urban design qualities, such qualities are presumed to intervene between physical features and behavior, encouraging people to walk (see figure 1.1). Testing this presumption requires reliable methods of measuring urban design qualities, allowing comparison of these qualities to walking behavior.

Many tools for measuring the quality of the walking environment have emerged in the past few years. Generically called walking audit instruments, these are now used across the United States by researchers, local governments, and community groups. Robert Wood Johnson's Active Living Research (ALR) website alone hosts sixteen walking audit instruments. They involve the measurement of such physical features as building height, block length, and street and sidewalk width.

Urban design qualities are more than the individual physical features that they comprise, as they have a cumulative effect that is greater than the sum of the parts. Physical features individually may not tell us much about the experience of walking down a particular street. Specifically, they do not capture people's overall perceptions of the street environment, perceptions that may have complex or subtle relationships to physical features.

Perceptual qualities are also different from such qualities as sense of comfort, sense of safety, and level of interest, which reflect how an individual reacts to a place—how a person assesses the conditions there, given his or her own attitudes and preferences. Perceptions are just that—perceptions. They may elicit different reactions in different people. They can be assessed objectively by outside observers; individual reactions cannot.

Our challenge in creating a tool to measure urban design qualities was to move from highly subjective definitions to operational definitions that capture the essence of each quality and can be measured reliably across raters, including those without training in urban design.

Why You Should Read This Book

Measuring Urban Design provides operational definitions and measurement protocols for five intangible qualities of urban design: imageability, visual enclosure, human scale, transparency, and complexity. To help disseminate these measures, this book also provides a field survey instrument that has been tested and refined for use by lay observers.

This instrument has several strengths. First, it is grounded conceptually in constructs from architecture, urban design, and planning. Second, it has been carefully tested and validated. Third, it comes with detailed instructions for assessing the five urban design qualities. For these reasons, the instrument offers researchers a "gold standard" for the systematic measurement of urban design. A test in New York City showed that the instrument can be implemented in large-scale studies relating the built environment to social, psychological, and health outcomes.

Initial Screening of Qualities

Key perceptual qualities of the urban environment were identified based on a review of the classic urban design literature. Without much empirical evidence, these qualities are presumed to influence people's decisions to walk rather than drive to a destination, stroll in their leisure time, or just hang out and socialize on a street. Perceptual qualities figure prominently in such classics as those listed in box 1.1.

The research team also reviewed the visual preference and assessment literatures, which attempt to measure how individuals perceive their environments and to better understand what individuals value in their environments. Partial listing of this voluminous empirical literature is provided in Ewing (2000) and updated in Ewing et al. (2005). These literature reviews go beyond the boundaries of urban design to the fields of architecture, landscape architecture, park planning, and environmental psychology, as perceptual qualities of the environment figure prominently in these fields as well.

Our review yielded a list of fifty-one perceptual qualities of the urban environment (box 1.2). Of these fifty-one qualities, eight were selected for further study based on the importance assigned to them in the literature: imageability, enclosure, human scale, transparency, complexity, coherence, legibility, and linkage. Of the eight, the first five were successfully measured in a manner that passed tests of validity and reliability.

Imageability

Imageability is the quality of a place that makes it distinct, recognizable, and memorable. A place has high imageability when specific physical elements and their arrangement capture attention, evoke feelings, and create a lasting impression. It is probably not one element by itself that makes a street imageable but rather the combination of many.

According to Kevin Lynch (1960), a highly imageable city is well formed, contains distinct parts, and is instantly recognizable to anyone who has visited or lived there. It plays to the innate human ability to see and remember patterns. It is a city whose elements are easily identifiable and grouped into an overall pattern.

Landmarks are a component of imageability. The term landmark does not necessarily denote a grandiose civic structure or even a large object. In the words of Lynch, it can be "a doorknob or a dome." What is essential is its singularity and location, in relationship to its context and the city at large. Landmarks are a principle of urban design because they act as visual termination points, orientation points, and points of contrast in an urban setting. Tunnard and Pushkarev (1963, p. 140) attribute even greater importance to landmarks, saying, "A landmark lifts a considerable area around itself out of anonymity, giving it identity and visual structure."

Imageability is related to "sense of place." Gorden Cullen (1961, p. 152) asserts that a characteristic visual theme will contribute to a cohesive sense of place and will inspire people to enter and rest in the space. Jan Gehl (1987, p. 183) explains this phenomena using the example of famous Italian city squares, where "life in the space, the climate, and the architectural quality support and complement each other to create an unforgettable total impression." When all factors manage to work together to such pleasing ends, a feeling of physical and psychological well-being results: the feeling that a space is a thoroughly pleasant place in which to be.

Imageability is influenced by many other urban design qualities—enclosure, human scale, transparency, complexity, coherence, legibility, and linkage—and is in some sense the net effect of these qualities. Places that rate high on these qualities are likely to rate high on imageability as well—the neighborhoods of Paris or San Francisco, for example. However, places that rate low on these qualities may also evoke strong images, though ones that people may prefer to forget. Urban designers focus on the strength of positive images in discussing imageability and sense of place.

A panel of experts we assembled most often mentioned vernacular architecture as a contributor to imageability (Ewing and Handy 2009). Other influences mentioned were landmarks, striking views, unusual topography, and marquee signage. Beyond Kevin Lynch's (1960) detailed qualitative characterizations, and two quantitative studies of building recall, we could find no attempts to operationalize imageability in either visual assessment studies or design guidelines.

Enclosure

Enclosure refers to the degree to which streets and other public spaces are visually defined by buildings, walls, trees, and other vertical elements. Spaces where the height of vertical elements is proportionally related to the width of the space between them have a room-like quality.

Outdoor spaces are defined and shaped by vertical elements, which interrupt viewers' lines of sight. A sense of enclosure results when lines of sight are so decisively blocked as to make outdoor spaces seem room-like. Cullen (1961, p. 29) states: "Enclosure, or the outdoor room, is, perhaps, the most powerful, the most obvious, of all the devices to instill a sense of position, of identity with the surroundings.... It embodies the idea of hereness." Alexander, Ishikawa, and Silverstein (1977, p. 106) say that "an outdoor space is positive when it has a distinct and definite shape, as definite as the shape of a room, and when its shape is as important as the shapes of the buildings which surround it." Likewise, Jacobs and Appleyard (1987, p. 118) speak of the need for buildings to "define or even enclose space—rather than sit in space." Richard Hedman (1984) refers to certain arrangements of buildings as creating intensely three-dimensional spaces.

In an urban setting, enclosure is formed by lining the street or plaza with unbroken building fronts of roughly equal height. The buildings become the "walls" of the outdoor room, the street and sidewalks become the "floor," and if the buildings are roughly equal height, the sky projects as an invisible ceiling. Buildings lined up that way are often referred to as street walls. Alexander et al. (1977, pp. 489–91) state that the total width of the street, building to building, should not exceed the building heights in order to maintain a comfortable feeling of enclosure. Allan Jacobs (1993) is more liberal in this regard, suggesting that the proportion of building heights to street width should be at least 1:2. Other designers have recommended proportions as high as 3:2 and as low as 1:6 for a sense of enclosure.

At low suburban densities, building masses become less important in defining space, and street trees assume the dominant role. Rows of trees on both sides of a street can humanize the height-to-width ratio. Henry Arnold (1993) explains that trees define space both horizontally and vertically. Horizontally, they do so by visually enclosing or completing an area of open space. Vertically, they define space by creating an airy ceiling of branches and leaves. Unlike the solid enclosure of buildings, tree lines depend on visual suggestion and illusion. Street space will seem enclosed only if trees are closely spaced. Properly scaled, walls and fences can also provide spatial definition in urban and suburban settings. Kevin Lynch (1962) recommended walls and fences that are either low or over six feet tall.

Visual termination points may also contribute to a sense of enclosure. Andres Duany and other new urbanists advocate closing vistas at street ends with prominent buildings, monuments, fountains, or other architectural elements as a way of achieving enclosure in all directions (Duany and Plater-Zyberk 1992). When a street is not strongly defined by buildings, focal points at its ends can maintain the visual linearity of the arrangement. Similarly, the layout of the street network can influence the sense of enclosure. A rectilinear grid with continuous streets creates long sight lines that may undermine the sense of enclosure created by the buildings and trees that line the street. Irregular grids may create visual termination points that help to enclose a space; cul-de-sacs, for example, tend to create more sense of enclosure than through streets.

Enclosure is eroded by breaks in the continuity of the street wall, that is, breaks in the vertical elements, such as buildings or tree rows, that line the street. Breaks in continuity that are occupied by inactive uses create dead spaces that further erode enclosure; vacant lots, parking lots, driveways, and other uses that do not generate human activity and presence are all considered dead spaces. Large building setbacks are another source of dead space. Alexander et al. (1997, p. 593) say that "building setbacks from the street, originally invented to protect the public welfare by giving every building light and air, have actually helped greatly to destroy the street as social space."

Our expert panel suggested that on-street parking, planted medians, and even traffic itself contribute to visual enclosure. They opined that the required building height to enclose street space varies with context, specifically, between a big city and a small town.

The visual assessment literature suggests that enclosure is an important factor in human responses to environments, and that solid surfaces are the important variable in impressions of enclosure. Using photographs of Paris, Stamps and Smith (2002) found that the perception of enclosure is positively related to the proportion of a scene covered by walls, and negatively related to the proportion of a scene consisting of ground, the depth of view, and the number of sides open at the front. These results were confirmed in later visual simulations (Stamps 2005).

Enclosure is defined both qualitatively and quantitatively in many urban design guidelines and several land development codes. The qualitative definitions sometimes capture the multifaceted nature of the concept, for example, in Denver, Colorado's design manual: "Building facades should closely align and create a continuous facade, punctuated by store entrances and windows. This produces a comfortable sense of enclosure for the pedestrian and a continuous storefront that attracts and encourages the pedestrian to continue along the street" (City of Denver 1993).

Human Scale

Human scale refers to a size, texture, and articulation of physical elements that match the size and proportions of humans and, equally important, correspond to the speed at which humans walk. Building details, pavement texture, street trees, and street furniture are all physical elements contributing to human scale.

The urban design glossary for the City of Seattle (2004) defines human scale as "the quality of a building that includes structural or architectural components of size and proportions that relate to the human form and/or that exhibits through its structural or architectural components the human functions contained within" (par. 57). Moderate-sized buildings, narrow streets, and small spaces create an intimate environment, and the opposite for tall buildings, wide streets, and large spaces.

Alexander et al. (1977) state that any buildings over four stories tall are out of human scale. Lennard and Lennard (1987) set the limit at six stories. Hans Blumenfeld (1953) sets it at three stories. In taller buildings, Roger Trancik (1986) says that lower floors should spread out and upper floors step back before they ascend, giving human-scale definition to streets and plazas. Richard Hedman (1984) emphasizes the importance of articulated architecture and belt courses and cornices on large buildings to help define street space and scale.

Several authors suggest that the width of buildings, not just the height, defines human scale. For human scale, building widths should not be out of proportion with building heights, as are so many buildings in the suburbs.

Human scale can also be defined by human speed. Jane Holtz Kay (1997) argues that today, far too many things are built to accommodate the bulk and rapid speed of the automobile; we are "designing for 60 mph." When approached by foot, these things overwhelm the senses, creating disorientation. For example, large signs with large lettering are designed to be read by high-speed motorists. For pedestrians, small signs with small lettering are much more comfortable.

According to Alexander et al. (1977), a person's face is just recognizable at seventy feet, a loud voice can just be heard at seventy feet, and a person's face is recognizable in portrait-like detail up to about forty-eight feet. These lengths set the limits of human scale for social interaction.