The evolution of European traffic, combined with the current works of revision of the Eurocode (CEN250 project 2nd Eurocode generation), represents an opportunity to improve the simplified method for fatigue checks (also known as the lambda method) for road bridges. This thesis is focused in particular on the development of the factor for the damage effect of traffic λ1 curves, which suffers unresolved inconsistencies and gaps regarding the types of models that have been used to obtain them, and for the total lack of correspondence between the results obtained with more recent models and the current curves present in the European Standard. The objective of this thesis is therefore to model the real fatigue behaviour of different static systems and to obtain results as similar as possible to reality by introducing different parameters that were not present in the previous simulations, including the passage of multiple vehicles at the same time, the congested traffic and the division of vehicles into axles. In addition to the development of the calculation code for single and multiple span bridges, work has been done in parallel for the study of the critical details and the case study of the Vasco da Gama cablestayed bridge in Lisbon. This additional work has made it possible to further investigate the behaviour of the factor λ1. This approach to the development of the new λ1 curves therefore represents the most advanced and detailed model performed so far. It is to be hoped that it will be taken into consideration to replace the current λ1 curves present in the Eurocode