The aging of bridges and viaducts, combined with new regulatory requirements for loads and resistances, necessitates a careful assessment of existing structures and the prioritization of corrective actions. Structural Health Monitoring (SHM) and its integration into standard evaluation practices present a significant challenge due to resistance to change, the lack of clear regulations and guidelines explaining the process and its specificities, and the absence of concrete examples demonstrating its benefits, including economic advantages. This paper explores the key challenges in integrating an SHM methodology for assessing existing structures, emphasizing the need for an objective system that enables accurate analysis for appropriate prognosis and decision-making procedures. Based on existing literature and orienting SHM methodology toward Performance Evaluation, a seven-level model is proposed to provide a clear, objective, and structured system for bridge performance assessment. Based on qualitative and quantitative data from structural monitoring and inspections, the procedure aims to provide a practical approach to clarifying the effects of potential damage/anomalies on the structure, identifying causes, and evaluating severity. The outcome, to be tested in future case studies, will provide operators with a clear understanding of the structure for identifying potential corrective actions and continuing monitoring procedures.