73,715 research outputs found
On the problem of boundaries and scaling for urban street networks
Urban morphology has presented significant intellectual challenges to
mathematicians and physicists ever since the eighteenth century, when Euler
first explored the famous Konigsberg bridges problem. Many important
regularities and scaling laws have been observed in urban studies, including
Zipf's law and Gibrat's law, rendering cities attractive systems for analysis
within statistical physics. Nevertheless, a broad consensus on how cities and
their boundaries are defined is still lacking. Applying an elementary
clustering technique to the street intersection space, we show that growth
curves for the maximum cluster size of the largest cities in the UK and in
California collapse to a single curve, namely the logistic. Subsequently, by
introducing the concept of the condensation threshold, we show that natural
boundaries of cities can be well defined in a universal way. This allows us to
study and discuss systematically some of the regularities that are present in
cities. We show that some scaling laws present consistent behaviour in space
and time, thus suggesting the presence of common principles at the basis of the
evolution of urban systems
Multiplex networks in metropolitan areas: generic features and local effects
Most large cities are spanned by more than one transportation system. These
different modes of transport have usually been studied separately: it is
however important to understand the impact on urban systems of the coupling
between them and we report in this paper an empirical analysis of the coupling
between the street network and the subway for the two large metropolitan areas
of London and New York. We observe a similar behaviour for network quantities
related to quickest paths suggesting the existence of generic mechanisms
operating beyond the local peculiarities of the specific cities studied. An
analysis of the betweenness centrality distribution shows that the introduction
of underground networks operate as a decentralising force creating congestions
in places located at the end of underground lines. Also, we find that
increasing the speed of subways is not always beneficial and may lead to
unwanted uneven spatial distributions of accessibility. In fact, for London --
but not for New York -- there is an optimal subway speed in terms of global
congestion. These results show that it is crucial to consider the full,
multimodal, multi-layer network aspects of transportation systems in order to
understand the behaviour of cities and to avoid possible negative side-effects
of urban planning decisions.Comment: 12 pages, 8 figures. Final version with an additional discussion on
the total congestio
Decoding the urban grid: or why cities are neither trees nor perfect grids
In a previous paper (Figueiredo and Amorim, 2005), we introduced the continuity
lines, a compressed description that encapsulates topological and geometrical
properties of urban grids. In this paper, we applied this technique to a large
database of maps that included cities of 22 countries. We explore how this
representation encodes into networks universal features of urban grids and, at the
same time, retrieves differences that reflect classes of cities. Then, we propose an
emergent taxonomy for urban grids
Spatial sustainability in cities: organic patterns and sustainable forms
Because the complexity of cities seems to defy description, planners and urban designers have
always been forced to work with simplified concepts of the city. Drawn from natural language, these
concepts emphasize clear hierarchies, regular geometries and the separation of parts from wholes,
all seemingly at variance with the less orderly complexity of most real cities. Such concepts are now
dominating the debate about sustainability in cities. Here it is argued that space syntax has now
brought to light key underlying structures in the city, which have a direct bearing on sustainability in
that they seem to show that the spatial form of the self-organised city, as a foreground network of
linked centres at all scales set into a background network of mainly residential space, is already a
reflection of the relations between environmental, economic and socio-cultural forces, that is
between the three domains of sustainability. Evidence that this is so in all three domains is drawn
from recent and new research, and a concept of spatial sustainability is proposed focused on the
structure of the primary spatial structure of the city, the street network
Metric and topo-geometric properties of urban street networks: some convergences, divergences, and new results
The theory of cities, which has grown out of the use of space syntax techniques in urban studies, proposes a curious mathematical duality: that urban space is locally metric but globally topo-geometric. Evidence for local metricity comes from such generic phenomena as grid intensification to reduce mean trip lengths in live centres, the fall of movement from attractors with metric distance, and the commonly observed decay of shopping with metric distance from an intersection. Evidence for global topo-geometry come from the fact that we need to utilise both the geometry and connectedness of the larger scale space network to arrive at configurational measures which optimally approximate movement patterns in the urban network. It might be conjectured that there is some threshold above which human being use some geometrical and topological representation of the urban grid rather than the sense of bodily distance to making movement decisions, but this is unknown. The discarding of metric properties in the large scale urban grid has, however, been controversial. Here we cast a new light on this duality. We show first some phenomena in which metric and topo-geometric measures of urban space converge and diverge, and in doing so clarify the relation between the metric and topo-geometric properties of urban spatial networks. We then show how metric measures can be used to create a new urban phenomenon: the partitioning of the background network of urban space into a network of semi-discrete patches by applying metric universal distance measures at different metric radii, suggesting a natural spatial area-isation of the city at all scales. On this basis we suggest a key clarification of the generic structure of cities: that metric universal distance captures exactly the formally and functionally local patchwork properties of the network, most notably the spatial differentiation of areas, while the top-geometric measures identifying the structure which overcomes locality and links the urban patchwork into a whole at different scales
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