This paper presents experimental and analytical impact factors for two existing curved steel box girder bridges. A Florida Department of Transportation test truck with a total weight of 468.8 kN applied the dynamic loading. The truck speed was incrementally increased from crawl to design speed. To evaluate the test results, the truck was simulated as a nonlinear vehicle model with 15 degrees of freedom. The bridge deck surface was assumed to be good and was simulated as a random process. Test and analytical results show that the impact factors of torsion are normally less than 30%, as are the impact factors of bending moment for bridges with span lengths less than 39 m. The impact factors of vertical shear are generally less than 15%, as are the impact factors of bending moment for bridges with span lengths greater than 50 m. The current AASHTO Guide Specifications for Horizontally Curved Highway Bridges appears to overestimate the dynamic loading of curved steel box girder bridges, especially for bridges with span lengths greater than 50 m. The research results are instructive and applicable to both bridge design and bridge load rating.