The present study deals with logistics-related pathfinding topics in hilly or mountainous areas, where roads characterized by steep slopes and curves exist. Transportation utilizing this type of road yields negative impacts by greater fuel consumption, along with greater CO₂ emissions. In recent years, increasing tourist demand in Japan has highlighted capacity limitations and safety concerns. Meanwhile, old infrastructures such as arterial roads, expressways, and sewerage systems are reaching or exceeding their designed service years. In early 2025, a large road collapse accident due to drainage aging has occurred close to central Tokyo. Thus, well prioritized planning for reconstructing road infrastructures is necessitated. Along these lines, we are motivated to devise an environmental-friendly pathfinding algorithm for road (re)construction, which is beneficial for hilly or mountainous areas. This challenge involves terrain modeling and pathfinding. The former relates with a cartographic theory called Triangular Irregular Network (TIN) and Digital Elevation Model (DEM), while the latter with shortest-path algorithms. In the experiments, slope constraints and fuel efficiency are both considered. A case study on Izu-Oshima Island, an isolated island located southward from central Tokyo, was conducted. Validation was successful with savings of approximately 27% in path-length and some 10% in fuel consumption, adhering to the Japan’s maximum road slope standard.