Inventory Rating, as defined by the current AASHTO

Manual for Bridge Evaluation

,¹ is that load, including loads in multiple lanes, that can safely utilize the bridge for an indefinite period of time.

Operating Rating, defined by the same manual, is the maximum permissible live load that can be placed on the bridge. This load rating also includes the same load in multiple lanes. Allowing unlimited usage at the Operating Rating level will reduce the life of the bridge.