Bridges are usually designed and evaluated on the basis of member strength and serviceability criteria; however, they are expected to have sufficient levels of reserve strength and multiple paths to resist collapse should any of their members get damaged or exceed their nominal strength capacity. The ability of a structural system to redistribute the load around damaged members is referred to as structural redundancy. To account for bridge redundancy during the design and safety evaluation process, the AASHTO load and resistance factor design and load and resistance factor rating provide a preliminary set of load modifiers or system factors, most of which were developed based on the code writer’s judgment and experience. This paper evaluated the redundancy of prestressed concrete box girder bridges under vertical loads and lateral loads. Pushover and pushdown analyses were performed by using frame and grillage models in SAP2000. The grillage models were validated by more advanced finite element models built in ABAQUS. The effects of different damage scenarios and types of connections between the box girder and columns on the structural redundancy level were investigated. On the basis of those results, a set of system factors for prestressed concrete box bridges accounting for the system redundancy is proposed. The proposed system factors can be used during the design and safety assessment of prestressed concrete box girder bridges subjected to transient lateral load and vertical vehicle load.