A parametric design analysis was performed in order to define the characteristics of protection devices which could be incorporated on the rear end of highway trailers in order to avoid automobile vehicle underride in a crash situation. Of particular concern in this study was the effect of weight and cost of the devices as they relate to parameters such as load capacity, guard height, restriction of the rear sliding bogie movement, and energy attenuation. Four separate design concepts were studied and compared on the basis of weight, cost, and slider restriction. Included were rigid as well as energy absorbing design approaches. Results have generally indicated that the energy absorbing guard approach has the best potential from a weight efficiency standpoint. The study also points out that significant modifications will be required to the trailer structure to absorb the impact loads under consideration.