The service life of existing bridges depends on the previous, present and predicted fatigue stresses. These are primarily caused by passing heavy gross vehicles. Prerequisite for statements about the reliability and durability are realistic load models of heavy traffic. Information about the heavy traffic conditions, the average daily traffic volume and the axle loads and gross vehicle weights of the heavy gross vehicles can be determined by Weigh-In-Motion systems. Such a system consisting of optical and tactile measuring devices has been realized at the superstructure and the expansion joint of two existing bridge structures. Extracts of the measurement and evaluation results will be presented to show that current traffic loads can also be quantified in this manner.