/network-centrality
1st May, 2021Gareth Simons

An overview of centrality measures and their correlations 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 landuses; however, application within urban-design triggers a range of questions pertaining to implementary specifics due to a variety of theories, methods, and considerations and it isn’t immediately clear which of these might be the most applicable at the pedestrian scale in relation to landuses. 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-apiPython 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 landuse 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. Distance-weighted mixed-use richness and Food Retail accessibility at a network-distance threshold of 300m is used as a proxy for high street mixed-uses, whereas distance-weighted mixed-use richness and Commercial landuse accessibility at 1000m is used as a proxy for wider mixed-use districts. The comparisons show that mixed-uses correlate more strongly against localised closeness than betweenness centralities; segmentised 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.

Link to preprint paper pending.

Introduction: Street networks as emergent artefacts

Streets interconnect assortments of people and places. Whereas their general purpose seems obvious, their emergent properties are less so. We intuitively sense that certain sidewalks are busier than others; that particular landuses may have a proclivity towards particular locations; or that highly successful streets can become public destinations in their own right. Yet, although architects, urban designers, and planners may argue an instinctive appreciation for how to create and nurture 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 to function as spatially connective tissue, leading to the implosion of public space while contributing to social and functional fragmentation of cities. This is, in part, a symptom of a wider paradigm of lower-density car-centric development characterised by a paucity of pedestrian-accessible landuses; however, even where the intention is to create more dense and walkable forms of urban development it can be difficult, 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 landuses.

Historical forms of urbanism evolved as an accretion of exchanges competing for access to the heart of villages, towns, and cities. This is an agglomerative dynamic: it feeds — and is fed by — flows of people, goods, capital, and information cascading through the manyfold 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 innumerable assortments of landuses. Jane Jacobs1 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 resilient2|3. Yet, buoyed by the spirit of modernity, early 20th century architects, planners, and officials literally set-out to tear these locally integrative networks asunder by mandating that cities be rearranged around motor-vehicles and the separation of landuses4|5|6. 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”7. By an order of magnitude, these networks precluded the local combinatorial possibilities available on the densely interconnected semi-lattice structures of historical urban networks8. A key ingredient had thus gone missing: planned street networks which prioritised motorvehicles seldom facilitated locally complex and evolving arrangements of interactions, the requisite lifeblood of healthy neighbourhoods and vibrant street-fronts9|2.

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. Yet, 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 communities10|9|11|12. The salient issue is that networks optimising space-time compression for some oftentimes creates barriers for others; problematically, an inordinate amount of power is wielded by those who get to decide or influence the formation of such infrastructures and their consequent flows. This is 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.13|14. These forms of planning not only remain prevalent, but have found new life in the more recent smart cities hype15|16|17|18.

If planned street networks are brittle and biased, or exhibit unpredictable emergent properties, then how should planned development proceed so as to preserve the cohesiveness of the public realm and enhance access for pedestrians? This is where network analytic methods hold appeal. By affording the opportunity to evaluate both existing and proposed street configurations it becomes easier to make observations about their likely emergent properties as systems of streets. This provides an important link from personal intuition to demonstrable, rigorous, and scalable forms of analysis that retain integrity in light of complex systems interpretations of urbanism.

Link to preprint paper pending.


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