Currently, all Inventory and Operating Ratings are expressed in terms of an equivalent HS-truck. Prior to about 1995, many ratings were for an equivalent H-truck. The H-truck directly corresponds to single-unit trucks, which used to be common on rural highways. Today, even rural Farm- or Ranch-to-Market highways and many off-system highways are exposed to much larger vehicles; therefore, the HS-truck is more realistic. Furthermore, AASHTO Legal Vehicles (TYPE 3 Trucks), Specialized Hauling Vehicles (SHV) and Emergency Vehicles (EV) are considered in addition to the HS-truck when load rating bridges.

All bridges carrying vehicular traffic shall have load ratings, which can be assigned, assumed, or calculated load ratings.