Dynamic responses of simply supported highway bridges are presented. The responses were measured across bridge spans. Dynamic tests were carried out on old highway bridges locating at several sites in Korea. Dynamic effects were obtained from the measured strain record and the position of record showing oscillation. Strain was measured at the characteristic point of a bridge which is normally at the midpoint of the measured span. Test results are discussed in the context of a design code for highway bridges. Test results show that most of the observed and / or computed dynamic load effects are less than those by the current KBDC(Korea Bridge Design Code). We could observe that the dynamic factor does not vary significantly along the bridge span as implied in KBDC. The results show that the dynamic effects are largely influenced by the suspension characteristics of the vehicle and more largely by the roadway surface roughness and are independent of span lengths. Finally, dynamic effects of highway bridges with respect to bridge types are studied.