Thrie-beam guardrail systems are used when enhanced Test Level 3 (TL-3) protection above that typically provided with a W-beam guardrail is desired. The currently available Manual for Assessing Safety Hardware ( MASH)-compliant modified thrie-beam system incorporates components that are more costly to fabricate, and the installation creates a wider footprint than a standard W-beam guardrail. The goal of this study was to develop a cost-effective thrie-beam guardrail system (TGS) for roadside and median applications under MASH TL-3 requirements. The roadside and median TGS configurations were developed using computer simulations, and their crashworthiness was evaluated through full-scale crash testing. Additionally, transition designs from a TGS to a standard W-beam system were developed. The TGS uses all the Midwest guardrail system (MGS) standard components except for the thrie-beam rail; it has a thrie-beam rail mounted at 34in. in height as opposed to the MGS, with a W-beam rail mounted at 31in. The two systems are attached using a symmetric W-to-thrie-beam transition segment. Based on the results of three successful full-scale crash tests and a computer simulation, both roadside and median TGS configurations meet all safety requirements for MASH TL-3.