The Swiss design code (SIA) model for fatigue design is less simplistic than the North American codes. Indeed, the definition of the fatigue correction factor is given by a multiplication of four factors which take into account the type of bridge and the type of traffic. The fourth factor takes into account the traffic on several lanes, in order to deal with the effect of simultaneous vehicle crossings. The North American fatigue correction factor is equal to 0.52 or 0.75 for the Canadian and the AASHTO (American) codes respectively and is a constant for all cases. The aim of this work is to demonstrate that this simplistic definition of the fatigue correction factor is not adequate and, among others, does not take into account the effect of simultaneous vehicle crossings.