Bridge code calibration and assessment of existing bridges require an accurate knowledge of traffic load patterns. WIM data are useful for these applications, either to account for the current traffic loads or to assess the potential impact of future loads. Traffic load effects were calculated on two composite bridges, using WIM data of a French motorway, and the effect of an increase of the gross weight limit from 40 to 44 tons was assessed by a micro-simulation. Extreme values and fatigue lifetimes are calculated and compared for the current traffic and a traffic with the increased weight limit. The lifetime of details sensitive in fatigue could be decreased by 20% if the weight regulation is changed.