The remaining service life of existing road bridges depends on the past, present and forecasted traffic loads. In view of increasing traffic loads, many existing bridges are not designed for the resulting fatigue stresses. For that reason, monitoring of existing, aged bridge structures becomes an economic as well as safety relevant task in civil engineering. This paper describes a monitoring program on a box girder structure. The measurements are carried out by a structural monitoring program consisting of strain gauges, displacement transducers and thermal sensors. The measured data are used to explore the appearance of passing vehicles and to determine their total vehicle weights. These results are used to determine the fatigue stresses caused by the current traffic loads. The remaining service life can be estimated by extrapolation of the collected load and vehicle data respectively the related fatigue stresses.