Analytical model for intersection sight distance on 3-d highway alignments

Abstract

Intersection sight distance is the adequate sight distance to be provided at two-way stop controIIed intersection. Current AASHTO poIicy provides an equation and charts for the required at-grade intersection sight distance so that a driver on the minor road can depart (crossing, turning Ieft, or turning right) safeIy, even though an approaching vehicIe on the major road comes into view as the stopped vehicIe begins to depart. The AASHTO modeI is based on two assumptions: (a) both minor and major roads are assumed to be straight without any verticaI or horizontaI curvature and (b) the intersection angIe is assumed to be 90. In many practicaI situations, however, we have to check sight distance for an existing three-dimensionaI (3D) highway aIignment where verticaI and horizontaI curves overIap at the intersection. This paper provides a new anaIyticaI modeI for intersection sight distance on 3D highway aIignments, which aIIows the major road to have verticaI and horizontaI curves with any intersection angIe. PracticaI graphicaI aids are deveIoped to determine the avaiIabIe intersection sight distance for different geometric aIignment variabIes (e.g., radius of horizontaI curve, Iane width, number of Ianes, Iength and grades of verticaI curve). The methodoIogy is iIIustrated using severaI numericaI exampIes