Transportation networks, from bicycle paths to buses and railways, are the
backbone of urban mobility. In large metropolitan areas, the integration of
different transport modes has become crucial to guarantee the fast and
sustainable flow of people. Using a network science approach, multimodal
transport systems can be described as multilayer networks, where the networks
associated to different transport modes are not considered in isolation, but as
a set of interconnected layers. Despite the importance of multimodality in
modern cities, a unified view of the topic is currently missing. Here, we
provide a comprehensive overview of the emerging research areas of multilayer
transport networks and multimodal urban mobility, focusing on contributions
from the interdisciplinary fields of complex systems, urban data science, and
science of cities. First, we present an introduction to the mathematical
framework of multilayer networks. We apply it to survey models of multimodal
infrastructures, as well as measures used for quantifying multimodality, and
related empirical findings. We review modelling approaches and observational
evidence in multimodal mobility and public transport system dynamics, focusing
on integrated real-world mobility patterns, where individuals navigate urban
systems using different transport modes. We then provide a survey of freely
available datasets on multimodal infrastructure and mobility, and a list of
open source tools for their analyses. Finally, we conclude with an outlook on
open research questions and promising directions for future research.Comment: 31 pages, 4 figure