  Management of crowd information in public transportation (PT) systems is
crucial, both to foster sustainable mobility, by increasing the user's comfort
and satisfaction during normal operation, as well as to cope with emergency
situations, such as pandemic crises, as recently experienced with COVID-19
limitations. This paper presents a taxonomy and review of sensing technologies
based on Internet of Things (IoT) for real-time crowd analysis, which can be
adopted in the different segments of the PT system (buses/trams/trains,
railway/metro stations, and bus/tram stops). To discuss such technologies in a
clear systematic perspective, we introduce a reference architecture for crowd
management, which employs modern information and communication technologies
(ICT) in order to: (i) monitor and predict crowding events; (ii) implement
crowd-aware policies for real-time and adaptive operation control in
intelligent transportation systems (ITSs); (iii) inform in real-time the users
of the crowding status of the PT system, by means of electronic displays
installed inside vehicles or at bus/tram stops/stations, and/or by mobile
transport applications. It is envisioned that the innovative crowd management
functionalities enabled by ICT/IoT sensing technologies can be incrementally
implemented as an add-on to state-of-the-art ITS platforms, which are already
in use by major PT companies operating in urban areas. Moreover, it is argued
that, in this new framework, additional services can be delivered to the
passengers, such as, e.g., on-line ticketing, vehicle access control and
reservation in severely crowded situations, and evolved crowd-aware route
planning.
