Investigation of passing sight distance sufficiency on rural roads with crest vertical curves

The paper investigates areas with passing sight distance (PSD) inadequacy on rural roads with crest vertical curves. The research is based on German (RAL 2012) rural roads design guidelines for EKL2, EKL3 and EKL4 design classes. The interaction between the road surface and line of sight between the passing and the opposing vehicles was assessed through 6 different cases, while every case was associated with the resulting formulas. The line of sight between the passing and the opposing vehicle was set to the RAL 2012 required PSD criterion of 600m. The analysis revealed that, excluding one situation for 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 results are presented in graphs that deliver a useful tool for engineers in order to identify areas with inadequate PSD in their road projects, and avoid the design of costly additional passing lanes.