Modeling network-based accessibility to analyze the transport network vulnerability: the case study of Catania (Italy)

In urban and suburban cities, the transport network is often characterized by main roads to handle incoming and outcoming flows and also crossing-traffic to reduce the number of vehicles affecting the city center. These high-speed roads, also called "ring roads", are identified as a ramp or a junction of the motorway and can constitute a connection between urban, commercial and industrial areas. To evaluate their functioning, it is important to consider both ordinary traffic conditions, characterized by peak and peak-off hours, but also anomalous events, both planned (e.g. maintenance work) and unplanned (e.g. accidents, weather conditions, etc.), that could temporarily reduce or nullify the transport network efficiency. In this regard, the paper aims to analyze and evaluate the transport network vulnerability by considering a composite indicator, i.e. the Vulnerability Network Index (VNI), which consider features related to network deployment, both in terms of link connectivity and flow entity. The methodological approach is based on the implementation of a macrosimulation model to derive the parameters for calculating VNI. It is composed by two terms: (i) the first one is the number of routes passing through each link, calculated after the zooning of the study area and perform the assignment procedureì; (ii) the second one evaluates the traffic congestion level in terms of vehicular traffic volumes assigned to each link. The methodology is implemented on a real case study coinciding with the city of Catania at different network levels (i.e. primary and secondary roads), by calibrating the simulation model with traffic counts provided by radar sensors installed within the study area. First results, both qualitative and quantitative, i.e. through numerical index and chromatic maps, show that the disruption of a link belonging to different routes and characterized by high traffic flows may have a greater impact on the network vulnerability. Future research will focus on assessing the relevance of a link related to the entire network and the impact of critical events on its transport features, allowing to compare ex-ante and ex-post network accessibility.