Often an accident reconstructionist is asked to determine vehicle speeds where the vehicle impacts a roadside object. One class of roadside objects that can be involved in the impact are the end treatment energy absorbing systems of “Jersey” barriers, guard rails and bridges. One such end treatment system is known as the Guard Rail Energy Absorbing Terminal, G-R-E-A-T. The G-R-E-A-T system consists of 6 to 8 foam and metal honeycomb cartridges surrounded by a framework of triple-corrugated steel guardrail. The number of cartridges used determine the maximum impact speed the barrier will safely handle. This type of accident cannot be analyzed as a fixed barrier impact.A method has been developed to determine the impact speed of a vehicle that has hit a constant force ( constant deceleration ) energy absorbing end treatment system. By determining the distance the subject vehicle crushed the energy absorbing system and the mass of the subject vehicle, the impact speed of the subject vehicle can be determined. This involves the application of the work - energy principle. The method described involves the analysis of high speed films taken during certification testing of the energy absorption system to determine the impact speed of a subject vehicle.