This article assesses the effectiveness of the side under-ride protection device (SUPD) by analyzing the structural response of a sedan in a side impact collision with a semi-trailer. The SUPD model evaluated is a basic type commonly found in trailer designs in Vietnam. Simulations utilize a finite element model of the sedan, which was set to collide with a stationary semi-trailer at a speed of 64 km/h, adhering to the 2021 IIHS (Insurance Institute for Highway Safety) side impact test standards. The study considers two scenarios: one where the semi-trailer is equipped with SUPD and one without it. Data on compartment intrusion at the driver’s position were collected at IIHS-recommended locations during the collision, from 0 to 0.2 s. The analysis centers on the structural deformation of the sedan in both scenarios. The findings reveal that while the SUPD system reduces intrusion, it does not fully achieve the recommended safety thresholds. The study emphasizes that although SUPD enhances protection, it is insufficient for complete passenger safety. Based on these findings, the article suggests further developments in SUPD and vehicle design to improve passenger protection. Future research should investigate how advanced materials and structural techniques can better balance foot acceleration and intrusion characteristics, addressing the shortcomings of current SUPD systems and enhancing overall crash safety.