Crest Vertical Curvature Safety Assessment through Variable Grade Stopping Sight Distance Control

Abstract

Stopping Sight Distance (SSD) is a vital design element, which directly affects the control values of critical road design parameters. Although grade marginally affects SSD values, most design guidelines disregard the grade impact during vehicle braking on variable grades. The paper investigates possible deficiency of this approach, regarding cases where the length of the vertical curve exceeds the control SSD values. The authors addressed the SSD calculation on variable grades during the braking process through a recently developed process that relates the point mass model and the laws of mechanics. The paper investigates the adequacy of the American AASHTO (2018), the Greek OMOE-X (2001) and the Italian DM, n.6792 (2001) design guidelines, during the determination of crest vertical curvature rates, from the variable grade SSD calculation point of view for rural arterials and freeways. The assessment was performed for high design speed values of 80km/h, 100km/h and 120km/h respectively, where for each case amended crest vertical curvature rates are delivered as a function of the crest vertical curve’s exit grade value, aiming to grant SSD adequacy throughout the braking process. The analysis, excluding the Greek OMOE-X design guidelines where costly overdesigned control crest vertical curvature rates are adopted, for AASHTO (2018) and DM, n.6792 (2001), revealed a SSD inadequacy on the downgrade area with exit grade values grades below -2% approximately.