The construction and improvement of the local narrow roads that are mostly located in hilly and mountainous areas are not currently sufficient because traffic volume of such roads is small. One of the possible reasons for this is that the relationship between road improvement and the level of service (LoS) improvement is not well defined. This paper analyzes the relationship between road geometry and driving behavior and constructs a driving speed estimation model considering the road geometry. A speed estimation model is constructed mainly based on the sight distance, and the reproducibility of the model is confirmed. In addition, the road sections that should be improved from the viewpoint of the LoS improvement are selected based on the model and many of these are found to be the section that should also be improved from the viewpoint of traffic safety.