Structural assessment of existing bridges is one of the most challenging aspects for the correct and sustainable management of civil infrastructures that emerged in the last few years. Specifically, Italy has a substantial number of bridges over 50 years old, mainly represented by simply supported prestressed concrete I-girder-type with a cast-in-situ deck. Proof load testing represents an empirical alternative to the standard calculation methods commonly adopted for safety assessment of existing structures. This article investigates the reliability of a bridge in different configurations including damaged and undamaged scenario through sitespecific traffic data collected by Weigh-In-Motion (WIM) systems in the Netherlands. Assuming a target load corresponding to characteristic load combination according to Eurocode provisions, the proof load test resulted into slightly reduced reliability indexes during the test while achieving higher values under successful completion. Preliminary results contribute to demonstrate proof load test can represent a valuable method for assessment of existing bridges.