A load factor is a number (normally greater than 1) which is multiplied by a design load in order to represent an extreme loading experienced by a structure. For example, the load factor 1.75 is multiplied by the standard H20-44 Truck Loading to represent an extremely heavy truck. In fact, it represents the heaviest truck that might reasonably be expected to cross the bridge in its lifetime.
Different kinds of have different load factors, because some loads are more unpredictable than others. For example, the weight of the bridge is more predictable than the weight of a heavy truck, so the load factor for bridge weight is much lower.