Three-Dimensional Stop-Control Intersection Sight Distance: General Model

Intersection sight distance 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 sight distance for stop-control intersections on three-dimensional alignments for obstructions inside the horizontal curve and for intersection and major-road vehicle (object) on the curve. The results of the research presented in this paper extend 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 overlap partially, and (c) considering the case in which the obstruction lies outside the horizontal curve. The obstruction location was formulated through use of a simple variable that takes the value of +1 or −1 for an obstruction, respectively, inside or outside the horizontal curve. 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 through a numerical example. The presented model and guidelines, which are general and easy to use, should be of interest to highway designers.