Nowadays a permanent monitoring of structures such as bridges and viaducts is essential, due to their strategic interest and their essential role within the infrastructural network. These structures often exhibit recurring structural patterns and repetitive elements. In the absence of specific safety directives and with limited economic resources, selecting the portions of the structures to be permanently instrumented requires the definition of objective criteria. Ambient Vibrations Tests (AVTs) and Operational Modal Analysis (OMA) prove to be a valuable tool in this context, for the assessment of the structure’s actual behavior under operational conditions without requiring specific assumptions about its mechanical properties, construction details, or state of preservation. The proposed work describes the flow necessary to orientate in the selection of the most representative elements to be permanently monitored, in its application to a real case: the Ilde Viaduct, located on the A14 Highway. This Viaduct is a prestressed concrete bridge composed by 28 simply supported spans with pre-tensioned cables. The viaduct comprises two structurally independent carriageways that are interconnected through longitudinal expansion joints. The first step concerns the application of environmental vibration tests to an initial subset of 10 spans, each characterized by different defects (corroded reinforcement, degraded supports, etc.). Modal parameters are extracted from the 3600s recordings through OMA, using stabilization diagrams. Frequencies and modal shapes of the spans are compared and cross-referenced with observed defects. These outcomes are then implemented into a numerical model of the bridge to closely match the experimentally obtained modal properties. Finally, the most representative spans to be permanently monitored are selected to optimize resource allocation.