The dynamic amplification factor (DAF) is a significant parameter for new bridge design and characterization of existing bridges. The AASHTO code provides a predicted value based on an assumed structural model, but not one relative to actual bridge behaviors and field-testing results. Recently, the Safe and Efficient Transportation Act (2011) increased the traffic truck weight from 355.84 to 431.5 kN (80 to 97 kips). Thus, improved methods are needed to determine accurate DAF values for new bridge designs and to evaluate the site-specific DAF values of existing bridges. This paper presents a simulation method to evaluate the DAF values of existing bridges using bridge weigh-in-motion (B-WIM) data. This approach includes the static moment obtained from the field-calibrated simulation model and the dynamic moment obtained from the B-WIM experimental data. A model for multiple vehicles is developed to simulate the traffic intensity, and to predict the DAF value of an existing bridge. Presented are the experimental findings and determination of the site-specific DAF value. © 2014 American Society of Civil Engineers.