Collisions between vehicles and semi-trailers often lead to significant harm to both human life and property. This article investigates a specific scenario where a hatchback collides with the side of a semi-trailer, comparing outcomes with and without the use of a Side Underride Protection Device (SUPD) at a speed of 64 km/h. The study’s primary goal is to evaluate the SUPD’s effectiveness in enhancing passenger safety. Key metrics, including passenger compartment intrusion and foot acceleration, are analyzed in accordance with the guidelines set by the Insurance Institute for Highway Safety (IIHS). The findings reveal that while the presence of the SUPD slightly reduces the intrusion into the passenger compartment, the levels remain above acceptable limits. Moreover, the foot acceleration exceeds the recommended thresholds, indicating that the basic SUPD currently installed on the semi-trailer does not provide adequate protection for passengers under the tested conditions. These results highlight the necessity for further enhancements to the SUPD to meet safety standards and better safeguard vehicle occupants in side-impact collisions.