Assessment of a single bridge and management system for all bridges in the network is still a major challenge, although much research has been carried out and implemented in existing networks over the last four decades. This paper presents a case study of a long-span arch bridge, the Maslenica Motorway Bridge, located in a multi- hazard maritime environment. Although special attention was paid to durability during design, the bridge required repair after 20 years of operation. The analysis includes an overview of the design project, structural health monitoring during construction and operation, numerous laboratory and in-situ testing, numerical analysis of structural capacity and remaining service life, and meteorological monitoring of the bridge site. A new approach to bridge assessment is presented that includes not only a deterioration index, but five groups of key performance indicators: (1) safety, reliability, and security ; (2) availability and maintainability ; (3) costs ; (4) the environment ; and (5) health and politics. Incorporating all available data and evaluating various aspects of bridge performance provides greater insight into the condition of the bridge, not only at the structure level but also at the network level. The method is applied to the reinforced concrete arch bridge in a harsh maritime environment and evaluation is provided based on the comprehensive data analysis. The key performance assessment procedure and lessons learned from this case study can be applied to a wide range of structures.