Abstract
We study the temporal evolution of the structure of the world's largest
subway networks in an exploratory manner. We show that, remarkably, all
these networks converge to a shape that shares similar generic features
despite their geographical and economic differences. This limiting shape
is made of a core with branches radiating from it. For most of these
networks, the average degree of a node (station) within the core has a
value of order 2.5 and the proportion of k = 2 nodes in the core is
larger than 60 per cent. The number of branches scales roughly as the
square root of the number of stations, the current proportion of
branches represents about half of the total number of stations, and the
average diameter of branches is about twice the average radial extension
of the core. Spatial measures such as the number of stations at a given
distance to the barycentre display a first regime which grows as r(2)
followed by another regime with different exponents, and eventually
saturates. These results-difficult to interpret in the framework of
fractal geometry-confirm and yield a natural explanation in the
geometric picture of this core and their branches: the first regime
corresponds to a uniform core, while the second regime is controlled by
the interstation spacing on branches. The apparent convergence towards a
unique network shape in the temporal limit suggests the existence of
dominant, universal mechanisms governing the evolution of these
structures.
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