Intersection sight distance (ISD) is an important design element. A stopped vehicle on the minor road needs sufficient sight distance to depart (cross, turn left, or turn right) safely, even though an approaching vehicle on the major road comes into view. Current AASHTO policy assumes that both minor and major roads are straight and intersect at right angles. Previous research has addressed ISD for stop-control intersections on three-dimensional (3D) alignments for obstructions inside the horizontal curve, and intersection and major road vehicle (object) on the curve. This paper extends previous research work by: (a) allowing the object to be anywhere on the horizontal curve or tangent, (b) allowing the horizontal and vertical curves to partially overlap, and (c) considering the case where the obstruction lies outside of the horizontal curve. The obstruction location was formulated using a simple variable that takes the values of 1 or -1 for an obstruction inside or outside the horizontal curve, respectively. Design aids for the required minimum lateral clearances (from the minor and major roads) are presented for different radii of horizontal curve and major road design speeds. Application of the model is illustrated using a numerical example. The presented model and guidelines, which are general and easy to use, should be of interest to highway designers.
Abstract