There are a lot of bridges in Egypt; most of them are used sto cross the River Nile or its branches and also to cross the streets to reduce the traffic jam. Due to some problems in the bridge itself or in the road after the bridge, it is required to reduce the velocity of vehicles on the bridge. In many cases obstacles are constructed on the bridge to reduce the velocity of the vehicles. These obstacles cause impact forces in the structural elements of the bridge. Also, due to the rough pavement or due to the bad maintenance of the expansion joints, the impact forces happen when the vehicles pass over the bridge.  In this research the vibration responses of a typical R.C bridge, due to truck passing an obstacle on the bridge, were measured using accelerometer transducers connected to a data logger system. Signal processing operations were carried out on the recoded signals to enhance the data quality and to obtain the velocity and displacement from the acceleration response.  Finite element model was developed for the bridge to evaluate the effect of vibration. The numerical model was subjected to the real recorded signals (displacement response). The stresses due to these vibrations were calculated and compared to the original stress of the bridge due to static loads. Then, an oscillator beam model was used to model the bridge, the obstacle, and the truck taking into consideration the interaction between them. The results of this model were compared to that obtained from the measured response to assure that the stresses due to the experiment are matched with that obtained from the oscillator beam model. Then a parametric study was performed to study the effect of the obstacle height, the wheel load, and the span of the bridge on the results. The impact factor obtained from the Egyptian Code for Loads was compared to that obtained from the numerical results to investigate the excessive stresses induced in the bridge due to the existence of an obstacle on the bridge.  As a result of this study, recommendations and guidelines were suggested.