Current fatigue load models (FLMs) for road bridges are typically calibrated based on steel and reinforced-concrete bridges. These models are not likely applicable to fiber-polymer composite bridges, primarily because the damage rate—defined as the slope of the fatigue load versus life curve of the composites—is significantly lower for composites than for steel. To address this limitation, a methodology is proposed to derive FLMs for composite road-bridge superstructures, and its application was demonstrated via a case study of a composite bridge deck. The adhesive deck joint was identified as the fatigue-critical location. Given the typically short spans of current composite bridges, a double-axle FLM was adopted, and the axle loads were calibrated using a composite road bridge that has been in service in Switzerland since 2012. Three different slopes of the F-N curve were assumed, all representative of composites. Calibration was based on a series of real traffic loads from the three countries, obtained from weigh-in-motion (WIM) measurements. The FLM axle loads were calibrated at specified numbers of cycles such that the damage generated by the FLM was equal to that in real traffic. Large axle loads were the predominant contributors to damage even at low cycle counts. The relationship between the axle loads and the number of cycles was established as a reference for testing across the three slopes. Characteristic axle loads increased with slope and decreased with cycle number. The results of this study represent an initial step toward developing an FLM for composite road bridge superstructures. Further calibration of bridges with varied geometries and composite materials is required to derive a generally applicable, parametrized model.