Modified thrie-beam guardrail is a non-proprietary, longitudinal barrier system originally developed and evaluated under the National Cooperative Highway Research Program (NCHRP) Report 350 Test Level 3 (TL-3) and Test Level 4 (TL-4) safety performance criteria. Modified thrie-beam can be configured as both a single-sided roadside barrier and a dual-sided median barrier. The single-sided roadside configuration of the modified thrie-beam guardrail consists of 12-gauge thrie-beam panels mounted at a height of 864 mm and supported by 2,057-mm long W6 × 8.5 posts and W14 × 22 blockouts with an angled cutout in the web. The dual-sided modified thrie-beam guardrail is largely identical to the single-sided configuration, except that the blockouts and thrie-beam guardrail panels are mirrored on the backside of the system. Modified thrie-beam guardrail provides for increased vehicle capture and reduced deflection as compared to typical W-beam guardrails. In 2009, the American Association of State Highway and Transportation Officials (AASHTO) implemented an updated standard for the evaluation of roadside hardware. The new standard, the Manual for Assessing Safety Hardware (MASH), improved the criteria for evaluating roadside hardware beyond the previous NCHRP Report No. 350 standard through updates to test vehicles, test matrices, and impact conditions. A research effort was undertaken to evaluate the both the single-sided and dual-sided median barrier configurations of modified thrie-beam guardrail to MASH Test Level 3 through full-scale crash testing. This paper details the performance evaluation of the thrie-beam guardrail through MASH TL-3 full-scale crash tests and recommendations for implementation of the system.