Road bridges exposure and alternative routes: towards the definition of road network resilience

The road network represents both element-at-risk and key infrastructure to ensure the accessibility of peripheral areas. The overall risk levels of bridges depend on geophysical (mainly earthquakes and landslides) and hydrometeorological hazards (mainly floods), the physical vulnerability of structures, and the exposure (mainly resulting from traffic flows and the presence of alternative roads). In mountain contexts and peripheral territories, the accessibility of entire territorial system directly depends on few trunk and secondary roads. The risk assessment of bridges, according to the Ministerial Guidelines, provides inputs for a territorial risk assessment starting from the accessibility of peripheral territories and the redundancy of the road network as features to determine the resilience of the road network. This study aims at understanding the relationships between bridges as elements-at-risk part of the road network and the influence they have on the overall accessibility of a territory. In this study, through a GIS analysis of a set of selected roads of the Provinces of Caserta (Campania Region), the presence of bridges along the selected roads have been combined with data on the accessibility levels of municipalities they connect. At this purpose, reference to the classification provided by the National Strategy for Inner Areas (SNAI) has been made to understand how such a classification can change due to a road interruption because of a bridge interruption. The contribution aims at setting the stage for the implementation of a method to determine the presence of alternative roads based on travel times adopted by the SNAI classification and, as a result, the definition of unique threshold values that justifies the variation of the exposure class and allows to determine the resilience of the whole road network.