Sight distance assessment on left horizontal curves over crest vertical curves on divided highways through reliability-based analysis

This paper delivers a realistic representation of the Stopping Sight Distance (SSD) adequacy investigation for cases of left horizontal curves over crest vertical curves on divided highways. The research is based on geometric control parameters set by the German Design Guidelines (RAA, 2008). The authors extend previous research of theirs based on an algorithm that calculates the available and demanded SSD (ASD, SSD, respectively). SSD assessments were performed through 5 road geometric parameters, namely, speed (V), horizontal radius (R), crest vertical curvature rate (Hk), entry grade (s1) and exit grade (s2). In general, major SSD inadequacy areas were found to be associated with high speeds (120km/h and 130km/h). In total, approximately 590,000 alignments were examined. Such an excessive data amount was efficient for the authors to perform a reliability analysis approach, by expressing the percentage of non-compliance for cases with SSD inadequacy. In order to assess more thoroughly the undesired cases where SSD>ASD, and quantify the frequency and intensity of SSD inadequacy, a new factor was introduced: the ratio of overlap, defined as the ratio of crest vertical curve length without the required Stopping Sight Distance (SSD). The results can be categorized and presented in color coded presentations, where the designer of compound highway sections can understand the degree of the required geometric interventions in order to achieve SSD adequacy. However, prior to implementing the proposed methodology, the approach should be also examined in relation to crash databases, where a more clear view of the situation can be obtained.