The assessment of the structural safety of existing bridges and viaducts becomes increasingly important in many countries due to the age of the structures and an increase in traffic loads. Many structures need to be reassessed in order to find out whether the safety requirements are met. Most existing standards, however, are developed for the design of new structures. This paper summarises the recent developments with respect to the specification of the target reliability levels for existing structures. It appears from total life cost optimisation that application of the same target reliability levels for existing structures as for new structures is uneconomical. Further, in some cases the cost optimisation seems to yield rather low reliability levels and human safety criteria become decisive for specification of the target reliabilities of existing structures. In this paper old concrete slab bridges without shear reinforcement are studied. Probabilistic calculations are performed in order to calibrate partial factors satisfying the target reliabilities under traffic load. These partial factors can be used by engineers in level I probabilistic calculations. In this way the often over-costly application of safety standards intended for new structures can be avoided in the reassessment of existing structures.