Abstract An assessment procedure based on structural system reliability principles, has been developed for highway bridges with concrete or composite decks. It concentrates on the ultimate flexural limit state of slabs and takes account of the fact that the failure of the structure is dependent on the combination of the applied loads, their variability, both spatial and in amplitude, and on the variability of geometric and material properties. Recognising that each of a number of topologically distinct collapse mechanisms may contribute to the overall probability of system failure, an optimisation technique is used in conjunction with structural reliability analysis in order to establish for each mechanism the critical geometry and the associated probability of occurrence. The procedure can be implemented in the assessment of existing bridge structures, where the use of plastic analysis may reveal strength reserves not utilised in design and, hence, alleviate the need for strengthening. By modelling the bridge as a virtual series system and analysing a finite number of collapse mechanisms, bounds on system failure probability in flexure can be estimated. An example of its intended use is presented for a solid slab concrete bridge deck.