The paper investigates the dynamic amplification factors to be applied for critical loading events involving multiple vehicles in bridge design. Numerical simulation of dynamic responses of two typical highway bridges, a short span one and a medium to large span one, reported in the literature as part of the DIVINE research program was carried out for the purpose. During DIVINE field investigations, these bridges had responded very heavily to single truck passage to the extent that the current dynamic amplification factors in bridge design codes were exceeded. From the simulation studies it is observed that vehicles with air suspensions, generally supposed to induce lower dynamic loads compared to those with steel suspensions, are capable of generating amplified bridge responses whenever the combination of road profile and vehicle speed is capable of exciting the critical frequencies and there is condition of frequency matching between bridge and vehicle. Also, it was observed in the case of short span bridge that there can be many cycles of amplified bridge oscillations even when the vehicle is over the bridge, which may have implications in its fatigue life. Presence of multiple vehicles in bridge span reduced the dynamic amplification factors in case of both short span and medium span bridges, with the reduction being more pronounced near vehicle speeds capable of exciting critical vehicle frequencies. The reduction was more pronounced in the case of medium to large span bridge than for the one with short span
