Starting from the three-dimensional definition of the concept of sustainability (social, economic and environmental perspectives), this paper proposes a methodological approach based on the theory of fuzzy systems and the theory of possibility, such as will allow the determination of a single overall indicator for the sustainability of possible action plans on transport and territory. The methodology can be used to formalize the conceptual schema of the three dimensions of sustainable

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... t widely referred to in literature as the "three pillars of sustainability". The method works on three different levels. At the first level, three fuzzy inference systems produce three sustainability indicators (social, economic and environmental), each using respective input variables (indicators commonly used for evaluation processes in the field of transportation systems analysis), and respective inference rules formulated by experts. At the second level, the fuzzy variables representing social, economic inference systems (based on rules formulated by experts), the outputs of the inference systems are indicators of equity (social/economic dimension), viability (economic/environmental dimension) and bearableness (social/environmental dimension). At the third level, the fuzzy variables produced from the previous level become input variables for an inference system, again based on set rules, which can be used to compose the different dimensions (social, economic and environmental) and deliver, as the final output, a fuzzy indicator of the durability and sustainability offered by the action plan.