Ranking Links in a Road Transport Network: A Practical Method for the Calculation of Link Importance

This paper deals with the issue of road network vulnerability and describes the implementation of a methodology which ranks the links of a network according to their importance for maintaining a proper connectivity between all origin-destination pairs. Such a ranking can be easily used by practitioners and decision-makers for prioritising maintenance investments along the links of a road transport network. In this regard, following a conceptual approach well consolidated in transport literature, vulnerability is assumed to be related to the concept of importance, i.e. a measure of the consequences of the collapse of a network's element. In the present study, the definition of importance – with respect to a given link –simultaneously includes two aspects: the level of usage, i.e. how many people typically use the link for their trips, and the impact that the closure of the link itself can have on the general functionality of the network as a whole. These two aspects are considered in the link importance index formulation, as two different functions that can be properly weighted by means of different coefficients. The methodology proposed has been implemented in the framework of Paramount EU project, to obtain a ranking of importance for the links of a real-scale network, i.e. the road network of Bolzano, a highly mountainous province located in the Italian Alps. The application of the methodology led to satisfactory results represented by a ranking of links, in decreasing order importance scores. Furthermore, Spearman's rank correlation coefficient has been used to quantify the variation of the importance ranking caused by the variation of its coefficients.