The key to extending the service life of a historical masonry bridge is an accurate examination of its actual performance. This paper presents a risk-based safety approach which introduces a risk matrix illustrating the risk presented by a structure as a function of the failure probability of the structure and consequences of structural failure. The failure probability of a structure is established on the basis of condition surveys to evaluate the condition state of the structure. The latter is represented by the arch deviation compared to its original shape and, therefore, by the number of hinges visible along the arch. The risk, reflected in the matrix, indicates the sophistication of the analysis and the importance of the likely intervention measures that need to be taken. As a preliminary step in the structural analysis of a single span masonry arch bridge, a simple analytical model to determine its ultimate resistance has been developed. The application of the model is illustrated with an example