Simplified methodology for indirect loss–based prioritization in roadway bridge network risk assessment

Road networks are fundamental for the economic development of regions and play an important role in the day-to-day life of its users. Within the multiple elements of road networks, bridges are among the most vulnerable and recent collapses have increased awareness to both their deteriorated state and overall importance within transportation networks. Direct losses (related to the economic cost of the infrastructure alone) are relatively straightforward to estimate and have been commonly used in risk assessment to date. On the other hand, the indirect component of loss (cost deriving from the reduction in functionality of the road network) is usually neglected, mainly because of the technical challenge it represents and the great amount of information required to perform the road network modelling necessary to calculate these losses to a similar level of detail. This paper presents a detailed methodology to calculate indirect losses and proposes a simplified alternative, which evaluates the relative importance of bridges within a road system in terms of the indirect loss component. For this purpose, the delay that a single user would experience after the collapse of each bridge, when travelling in the network to and from each origin-destination pair, is used as a proxy for the indirect losses in the road network. The simplified alternative uses open-source software and datasets and does not depend on the availability of traffic demands, which can be difficult to obtain. A case study in Salerno, Italy, featuring the principal road network with 617 bridges was used to test both alternatives. The results indicate that the proposed proxy for relative importance of bridges within a given portfolio gives satisfactory estimates when compared to the more extensive calculation. This paves the way for a better and more accessible understanding of the potential impacts of road network interruptions in the overall context of resilience.