Probabilistic assessment on the code-specified impact factors of RC decks is investigated by means of a three-dimensional traffic-induced dynamic response analysis of bridges combined with the Monte Carlo simulation technique. The random variables considered in the simulation are the roadway roughness, bump height, traveling position of vehicles, vehicle running speed and axle load of three-axle vehicles. Statistical parameters of the random variables are taken from surveying data onHanshin and Meishin Expressways in Japan. A simple span steel-girder bridge with RC decks that is experimentally verified is considered as a numerical example. This study demonstrates that the impact factor of the deck near expansion joints dominates the design impact factor due to the bump at the expansion joint of bridges.