Today all major standards implicitly or explicitly include an “impact factor”, “dynamic amplification factor” (DAF) or “dynamic load allowance”. The amount of dynamic traffic action represented by the factor varies considerably [6, 7]. The values are usually derived from experiments with isolated trucks crossing a bridge with or without an artificial obstacle. For every new road profile, vehicle and bridge a new factor is found. It can be concluded that this approach does not converge in a solution and moreover it does not answer a series of questions relevant for the structural safety of road bridges: - How to apply the results obtained with a single, conventional truck to an overloaded truck, considering the presence of other vehicles? - How to conclude from the measurement of dynamic deflections on dynamic section forces at mid-span and at the support? - How to conclude from measurements on bridge girders on the behaviour of bridge decks? - How to conclude from measurements at service state to the applicable factor for the ultimate limit state?