One of the most significant dimensions of roads and other pathways was not well understood until the early 1980s, when British architectural thinker Bill Hillier developed a theory called space syntax. Hillier argued that the particular spatial arrangement of pathways—whether roads, streets, sidewalks, or building corridors—plays a major role in whether those pathways are well used and animated or empty and lifeless. His work demonstrated convincingly that different pathway configurations can bring users together spatially or keep them apart.
One central concept in Hillier’s theory is axial space, which relates to the one-dimensional qualities of a pathway and has bearing on human movement through a settlement or region as a whole. Axial spaces are illustrated most perfectly by long, narrow streets (see fig. 1). They can be represented geometrically by the longest straight line that can be drawn through a street or other movement space before that line strikes a building, wall, or some other material object. Axial lines are significant for at least two reasons. First, because they indicate the farthest point of sight from where one happens to be, axial lines speak to the lived relationship between “here” and “there” and thus, at the settlement scale, have bearing on environmental orientation and finding one’s way in a place. Second, because they collectively delineate the spatial system through which the various parts of a place are connected by pedestrian and vehicular circulation, a settlement or region’s web of axial lines provides a simplified rendition of the potential movement field of a place. Hillier’s important discovery was that differently configured pathway webs play a major role in generating different patterns of pathway movement and face-to-face encounter among pedestrians and other users.
Figure 1. Axial space as illustrated by a main shopping street in Mantova, Italy. Photo by Raymond Streeter.
An important quantitative measure in regard to axial spaces and pathway webs is integration, which Hillier defined as a measure of the relative degree of connectedness that a particular axial space has in relation to all other axial spaces in a particular pathway system. The assumption is that a pathway connected to many other pathways will be more traveled because users will need to traverse that pathway to get to other pathways and destinations in the settlement or region. Such a pathway is said to be strongly integrated in the movement field because many other pathways run into that well-connected pathway and potentially provide a large pool of users. In contrast, a segregated pathway has few or no other pathways running into it—for example, a dead-end street. All other things being equal, a segregated pathway will be the locus of less movement since it serves a more limited number of users in its immediate vicinity only.
Figure 2. View from the highest tower in the Italian hill town of San Gimignano. The pathway configuration of many pre-modern settlements like San Gimignano incorporate what in space-syntax theory is called a "deformed wheel"—a permeable street structure that supports a degree of interconnectedness and access. Photo by Raymond Streeter.
Through integration and other quantitative measures, Hillier developed a compelling understanding of the global pattern of a place—in other words, the way the particular spatial configuration of a place’s pathway fabric lays out a potential movement field that draws people together or keeps them apart. Natural movement is the term Hillier used to describe the potential power of the pathway network to automatically stymie or facilitate movement and the face-to-face interactions of pedestrians and other place users—for example, merchants, workers, and residents from shops, workplaces, and dwellings along the streets. With many people present involved in their own regular routines and activities, the result typically is animated pathways and exuberant local places (see fig. 2). Hillier recognized that other place elements like density, building types, and number, size, and range of functions and land uses also contribute to place vitality, but he argued that, ultimately, pathway configuration is most primary and most crucial.
In regard to cities, Hillier demonstrated that most urban pathway systems have traditionally been an integrated, interconnected fabric of variously scaled deformed grids—pathway systems in which the most active, integrated streets make a shape that roughly suggests a wheel of rim, hub, and spokes. Typically, each of these deformed grids is associated with some designated neighborhood or district—for example, London’s Soho, West End, or City (see fig. 3). In turn, the integrated pathway structure of these districts joins together to shape a much larger deformed grid that founds the movement dynamic of the city and London region as a whole. Hillier pointed out that twentieth-century urban design and planning regularly replaced integrated pathway configurations with treelike systems of segregated pathways that stymied or destroyed the intimate relationship between local and global integration and thereby eliminated much face-to-face interaction—for example, the “cul-de-sac and loop” pattern of low-density, automobile-dependent suburbs or the hierarchical circulation layouts of many modernist housing estates.
Figure 3. In this axial map of central London, the reddest lines represent the most integrated pathways and thus the streets of most pedestrian movement. Note that the yellow, orange, and red lines form rough “deformed wheels” indicating important London neighborhoods like Soho or The City. In turn, these smaller “wheels” weave themselves into a larger city web that allows for a great degree of permeability and ease of movement with and between neighborhoods and districts. Map image © Space Syntax Limited.
For future environmental design and policy, Hillier’s critique of modernist planning suggests that the possibility of individuals readily gathering in face-to-face encounter is greatly compromised because the particular pathway configuration does not channel the movements of many people into and along more integrated pathways. In other words, modernist pathway structure regularly separates pathway users rather than bringing them together, face to face, through an integrated pathway network of sidewalks, streets, and roads. Users that otherwise might feel a sense of spatial community—a situation that the deformed grid readily affords—remain apart, spatially and environmentally. They do not as readily meet in the course of a daily life, grounded in place regularity and routine. There is much less chance for what humanistic geographer Yi-Fu Tuan termed fields of care—places that come to be known affectionately through recurring, serendipitous, face-to-face meetings and experiences.
 Bill Hillier, Space Is the Machine (Cambridge: Cambridge University Press, 1996); Bill Hillier and Julienne Hanson, The Social Logic of Space (Cambridge: Cambridge University Press, 1984).
 Hillier, Space is the Machine, 161.
 Yi-Fu Tuan, “Space and Place: Humanistic Perspective.” Progress in Geography 6, (1974): 266-76.