The primary objectives of transportation system development in European countries include improving transport efficiency, enhancing national economic competitiveness, and promoting sustainable mobility. A key aspect of achieving these goals is the evaluation of transport externalities, particularly external accident costs, which is critical for ensuring the economic and social sustainability of road transport development. This study comprehensively evaluates external road accident costs and proposes an innovative approach to internalizing these costs through a cost–benefit analysis (CBA). By integrating these external costs into CBA, policymakers can better understand the societal impact of road safety investments, allowing for more informed decisions regarding measures to reduce road accidents and fatalities. This paper also explores the role of specific safety investments, such as installing vehicle restrain systems (VRS), buffers, and terminals on the national road network, as well as installing speed cameras, in reducing accidents and mitigating external costs. The findings highlight road safety measures’ importance in enhancing economic efficiency and social well-being, providing a solid foundation for the sustainable and efficient development of transportation systems.