Extensive tests was carried out on the rear protection equipment of a tipper-type silobus semi-trailer. This study includes the design and optimization process of an extensible rear underrun protective device for O4 category semi-trailers. Material selection and structural design features were evaluated within the framework of harmonization with regulation 58 of the European Commission. Design verification was done in 4 stages. Two of them were carried out by computer simulation and the other two were carried out as physical tests. The aim of the study is to increase the safety of the vehicle and other road users in accidents resulting in the under-vehicle entry. Ensuring a rear underrun protective device design that meets the test force requirements found in UNECE R58 is a key performance indicator in research. Also, it is aimed at reducing carbon emissions in vehicles where the rear underrun protective device will be used by providing the regulation conditions at a minimum level, simplifying the rear underrun protective device design, and simplifying the design. The output of the optimization process is that the extensible rear underrun protective device design is strong enough to adapt to regulation conditions and light enough to keep efficiency at the highest level.