Access graph
A novel graph representation of public transport networks for accessibility analysis
Tina Šfiligoj (University of Ljubljana)
Aljoša Peperko (Institute of Mathematics, Physics and Mechanics Ljubljana, University of Ljubljana)
Oded Cats (TU Delft - Civil Engineering & Geosciences)
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Abstract
Accessibility, defined as travel impedance between spatially dispersed opportunities for activity, is one of the main determinants of public transport use. In-depth understanding of its properties is crucial for optimal public transport systems planning and design. Although the concept has been around for decades and there is a large body of literature on accessibility operationalisation and measurement, a standardised approach is lacking. To this end, we introduce a dedicated graph representation of public transport networks, termed the Access Graph, or A-space, based on the generalised travel times between nodes. We introduce an edge between two nodes in the access graph if the travel time between them is below a certain threshold time budget. In this representation, node degree directly measures the number of nodes reachable within a predetermined time, reproducing the cumulative opportunities measure of access at each specific value of the time budget. We study the threshold-dependent evolution of the degree distribution of the access graph and define a set of accessibility indicators. The indicators are observed at network-specific and passenger-based characteristic times. We apply the methodology to a dataset of 51 metro networks worldwide. For the Washington DC metro, we validate the approach by comparing supply to demand at the origin–destination level. The new representation addresses accessibility at the network structure level, offering a network science-grounded and flexible conceptual framework for accessibility studies.
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