Developing safe and effective guardrail terminals has been a high priority of roadside safety researchers for several decades. Numerous full-scale crash tests have been performed, and many types of new terminals have been developed. In recent years, the Federal Highway Administration has formalized the evaluation and certification process for roadside safety hardware, meaning that all guardrail terminals used on the National Highway System must satisfy the full-scale crash test and evaluation requirements of NCHRP Report 350. Although the newer guardrail terminals undoubtedly have better full-scale crash test performance, the relevancy to reducing serious and fatal injuries in real-world guardrail terminal collisions is not quite as clear. The in-service performance of guardrail terminals was evaluated to determine if upgrading the terminals to NCHRP Report 350 standards would reduce serious and fatal injury accidents.