Three-Dimensional Model for Stop-Control Intersection Sight Distance

Abstract

Intersection sight distance (ISD) is the sight distance to be provided at intersections between a minor road and a major road. Current AASHTO policy provides an equation and charts for the required at-grade ISD so that a driver on the minor road can depart (crossing, turning left, or turning right) safely, even though an approaching vehicle on the major road comes into view as the stopped vehicle begins to depart. The AASHTO model is based on two assumptions: (1) both minor and major roads are assumed to be straight without any vertical or horizontal curvature; and (2) the intersection angle is assumed to be 90°. In many practical situations, however, sight distance is required to be checked for an existing or proposed 3D intersection alignment where vertical curves (crest or sag) and horizontal curves overlap. This paper presents a new mathematical model for the analysis of stop-controlled ISD on 3D highway alignments that allows the major road to have vertical and horizontal curves with skewed angle, and the minor road to have a longitudinal grade. Design aids are developed to determine the available ISD for different geometric alignment variables (e.g., radius of horizontal curve, lane width, number of lanes, and vertical curve parameters). Application of the methodology is illustrated using numerical examples.