||Corrugated steel W-beam guardrail system is the most popular energy absorbing system along roadside throughout the world. It helps to dissipate a vehicle's kinetic energy in impact events so as to reduce the damage to the car occupants and the car itself. Therefore, its performance and characteristics are of great importance for road safety. Experimental study was conducted on a scaled-down W-beam guardrail with a scaling factor of 1:3.75. Downscaled W-beam samples were tested under quasi-static and impact three-point bending to reveal its energy absorption characteristics and the large deformation mechanism. The load-deflection curves, the global flexural profiles and the cross-sectional distortion were all recorded to gain the information on the deformation mechanism and energy dissipation of the W-beams. The effects of three different supporting conditions and end constraints on the large deformation behavior of the beams were also examined. From the experimental results, it was found that the input energy is dissipated by the W-beam through the cross-sectional distortion, the global flexural deformation and as well as the local deformation at the supports. The experimental observations and measurements of the samples under impact loading were compared with its static counterparts, and a "dynamic enhancement factor" is defined to represent the combined effects of strain-rate sensitivity, structural inertia and so on that greatly influence the dynamic performance of the W-beams. Furthermore, experiments were conducted on a scaled-down system consisting of a guardrail beam and two supporting posts to explore the interaction between the beam and the posts during an impact event. The energy partitioning between the beam and the posts has been explored, indicating that the beam absorbs most of the input energy for minor impact, but the post will play a significant role when the input energy is large. Finally, a simple mass-spring model has been developed to estimate the dynamic behavior of a W-beam guardrail system under impact loading. In this model, the load-displacement characteristics obtained from above down-scaled W-beam tests is used to specify the elastic-plastic property of a spring; whilst the deformability of the supporting posts is represented by another non-linear spring. The predictions on the final displacement of the guardrail system from the model show a good agreement with the experimental results, and this simple model is capable of approximately predicting the dynamic deformation of a real guardrail system when it is subjected to an oblique collision of a vehicle. At the end of the thesis, relevant problems to be further studied have been identified.