In many design codes for roadway bridges, fatigue design involves passing a load model over an influence line for a critical location on the bridge and then determining the resulting nominal stress range. For fatigue design in the finite-life domain, this stress range is then multiplied by a damage equivalence factor to account for differences in the fatigue damage because of the load model and the expected real traffic. In general, the effects of simultaneous vehicle crossings are not considered in the calibration of the damage equivalence factor. In this paper, fatigue design procedures in the U.S., Canadian, European, and Swiss codes applicable for the design of steel or aluminum roadway bridges are first reviewed. A simulation-based study, conducted to investigate the effects of simultaneous vehicle crossings on the damage equivalence factors for North American roadway bridges, is then presented. Based on the results of this study, recommendations are made for amplifying the North American damage equivalence factors in cases where the effects of simultaneous vehicle crossings are expected to be significant.