Rural Roads’ Passing Sight Distance Control Along Crest Vertical Curves

Passing sight distance (PSD) is a vital design element that directly imposes economic, as well as safety and operational, considerations. The provision of PSD is highly prioritized, at least for rural road sections without additional passing lanes. The paper investigates areas with PSD inadequacy on rural roads with crest vertical curves. The research is based on the German rural roads design guidelines, where PSD is currently dependent on the homogeneousness of the proposed road design classes and no longer on speed. Therefore, the required PSD for all the examined design classes was set to 600 m. The interaction between the road surface and the line of sight between the passing and the opposing vehicles was assessed through six different cases, while every case was associated with the resulting formulas. The analysis revealed that, excluding one situation for the EKL4 design class, the boundaries of PSD inadequacy were concentrated in advance and inside the vertical curve, and do not depend on the grade difference of the vertical curve but only on the crest vertical curvature rate value. The paper delivers a ready-to-use tool for engineers to identify areas with inadequate PSD in the early stages of the design process and avoid implementing costly additional passing lanes.