In the Netherlands, the shear capacity of a large number of existing reinforced concrete solid slab bridges is subject to discussion, as initial assessments indicated that their capacity was insufficient. In certain cases, the deterministic value of the moment capacity is larger than the deterministic value of the shear capacity. However, when the variability of the material properties, and of the capacity models themselves are factored in, a probability of a certain failure mode can be calculated. Here, a method is introduced to calculate the chance that a cross-section fails in shear before it fails in bending. The method that is derived here is applied to the Ruytenschildt Bridge. This case study is a reinforced concrete solid slab bridges that was tested to failure in two spans during the summer of 2014. The relative probability of failure in shear of the bridge was determined. The predictions indicated a smaller probability of a shear failure than of a bending moment failure. In the first tested span, failure was not reached, but indications of flexural distress were observed. In the second span, a flexural failure was achieved, in line with the probabilistic predictions. The presented method can be used in the assessment of existing bridges to determine which failure mode is most probable, taking into account the variability of materials and capacity models.