Dynamic interaction between moving vehicle and a bridge is, however, a complex phenomenon and is less understood in horizontally curved bridges. The dynamic response is traditionally considered by multiplying the static response with a single impact factor. This approach is not very realistic and, in many cases, misrepresentation of the sophisticated phenomenon involved in the bridge-vehicle interactions. In this study, extended three-dimensional finite element models of horizontally curved twin I-girder bridge by considering superelevation are developed for the bridge-vehicle interaction analysis. The bridge is modeled in detail with solid and shell elements for concrete deck and steel members, respectively. Vehicle is idealized as a 3D nonlinear model consisting of several lumped masses connected by rigid beams and supported by spring-dampers. Gap and actuator elements are included into the tire models to imitate the separation between the tires and road surface, and road surface roughness. Four different classes of road surface roughness profiles generated from power spectral density and cross spectral functions for very good, good, average and poor roads are used in this study. The models are capable to consider both bridge and vehicle responses with virtually no limitation on the complexity of vehicle and bridge. By using the bridge-vehicle model, the dynamic responses of the horizontally curved twin I-girder bridge are extensively investigated under conditions of various roughness classes, vehicle speeds, lane positions, amount of superelevation etc. From the numerous calculated results, some useful information regarding to the impact factor of the studied bridge is presented.