Network centrality measures and their correlation to mixed-uses at the pedestrian-scale

Abstract

Street network analysis holds appeal as a tool for the assessment of pedestrian connectivity and its relation to the intensity and mix of land-uses; however, application within urban-design triggers a range of questions on implementary specifics due to a variety of theories, methods, and considerations and it is not immediately clear which of these might be the most applicable at the pedestrian scale in relation to land-uses. It is, furthermore, difficult to directly evaluate differing approaches on a like-for-like basis without recourse to the underlying algorithms and computational workflows. To this end, the cityseer-api Python package is here used to develop, compute, and compare a range of centrality methods which are then applied to the Ordnance Survey Open Roads dataset for Greater London. The centralities are correlated to high-resolution land-use and mixed-used measures computed from the Ordnance Survey Points of Interest dataset for the same points of analysis using a spatially precise methodology based on network distances to premise locations.

The comparisons show that mixed-uses correlate more strongly against closeness than betweenness centralities; segmented measures tend to offer slightly stronger correlations than node-based equivalents; weighted variants offer correlations similar to unweighted versions, but with a greater degree of spatial specificity; simplest-path methods confer an advantage in the context of local high-street mixed-uses but not necessarily for district-wide mixed-uses or land-use accessibilities; and the application of centrality measures to the dual network does not offer tangible benefits over the primal network.

Introduction: Street networks as emergent artefacts

Streets interconnect assortments of people and places. Whereas their purpose seems obvious, their emergent properties are less so. We intuitively sense that particular sidewalks are busier than others, that certain land-uses may have a proclivity towards distinct locations, or that highly successful streets can become public destinations in their own right. Although architects, urban designers, and planners may argue an instinctive appreciation for creating and nurturing such streets, history has often proved otherwise. Planned streets — whether intentionally so or not — have in many cases come to function antithetically to their historical purposes by prioritising connections to distant places at the expense of pedestrians and local connectivity. Streets and interstitial spaces can thus cease functioning as spatially connective tissue, leading to the implosion of public space while contributing to cities’ social and functional fragmentation, a symptom of the broader paradigm of lower-density car-centric development characterised by a paucity of pedestrian-accessible land-uses. Nevertheless, even where the intention is to create more dense and walkable forms of urban development, it is not easy, without recourse to the incremental and evolutionary development typical of historical towns and cities, to anticipate the emergent characteristics of planned street networks and how these might relate to the potential vibrancy of land-uses.

Historical forms of urbanism evolved as an accretion of exchanges competing for access to the heart of villages, towns, and cities. An agglomerative dynamic, these processes feed — and are fed by — flows of people, goods, capital, and information cascading through the innumerable tendrils of urban networks reaching near or far. In congealed form, this logic is fine-grained and porous, with characteristically small urban blocks enmeshed by street networks interleaving numerous assortments of land-uses. Jacobs (1969) is quick to assure that any semblance of inefficiency is, actually, a hallmark of complex systems and the particular reason why evolutionary forms of urbanism can be persistently vibrant and resilient [@Jacobs1961; @marshall2009]. Nevertheless, buoyed by the spirit of modernity, early 20th century architects, planners, and officials set out to tear these locally integrative networks asunder, mandating the rearrangement of cities around motor vehicles and the separation of land-uses [@Hilberseimer1955; @Garnier1989; @Corbusier1967]. Though rationalised as ordered, efficient, and hygienic, the tree-like network structures deployed by modernists amounted to reductionism and a “compulsive desire for neatness and order” (Alexander 1967, p.11). By order of magnitude, these networks precluded the local combinatorial possibilities available on the densely interconnected semi-lattice structures of historical urban networks (Alexander 1964). A key ingredient had thus gone missing: planned street networks which prioritised motor vehicles seldom facilitated locally complex and evolving arrangements of interactions, the requisite lifeblood of healthy neighbourhoods and vibrant street-fronts (Harvey 1989; Jacobs 1961).

The utopian cities envisioned by modernists were motivated by the zeitgeist of the times: belief in the power of rational thought and the invincibility of technological progress. New forms of mobility and communications technologies would compress space and time and (were supposed to) unify and connect the disparate communities and functionally segregated regions of planned cities. In reality, these networks marginalised pedestrians and, as epitomised by Robert Moses’ heavy-handed slum-clearances in New York City, went so far as to lay waste to entire communities (Flint 2011; Harvey 1989; Lyon 1999; Ellin 1999). The salient issue is that networks optimising space-time compression for certain citizens frequently create barriers for others. Problematically, an inordinate amount of power is wielded by those deciding or influencing the formation of such infrastructures and their consequent flows. These patterns represent an exclusionary model of urban development premised on the dissolution of public space, a state of urban disembodiment engendered through the network mechanisms of spatial bypass, selective access, and prohibitive pedestrian distances — patterns we have come to associate with suburbia. (Graham & Marvin 2001; Pope 1996). These forms of planning not only remain prevalent but have found new life in the more recent Smart Cities hype (Greenfield 2013; Hill 2013; Townsend 2013; Sterling 2014).

If planned street networks are brittle and biased or exhibit unpredictable emergent properties, then how should planned development proceed to preserve the cohesiveness of the public realm and enhance access for pedestrians? It is here where network analytic methods hold appeal. By allowing the opportunity to evaluate both existing and proposed street configurations, it becomes easier to make observations about their potential emergent properties as systems of streets. Thus personal intuition can be bridged to demonstrable, rigorous, and scalable forms of analysis while retaining integrity in light of complex systems interpretations of urbanism.

Please see the linked preprint paper for additional information.