The design of interchange ramps and connectors, especially in large freeway-to-freeway interchanges, involves the use of stopping sight distance (SSD) criteria to determine horizontal and vertical geometries. Long connectors are usually required to avoid difficult horizontal and vertical obstructions. Therefore the use of minimum design standards for both horizontal and vertical geometries is quite common. The results of an investigation that evaluated SSD on interchange connectors by computerized three-dimensional (3-D) models are documented. Interchange connector models were developed by using combinations of minimal horizontal and vertical geometries with a longitudinal traffic barrier and a cross slope. A graphical procedure was used to measure SSD in a 3-D environment. The results revealed that the 3-D method of measuring SSD was not significantly different from the conventional two-dimensional method of measuring SSD. When all the 3-D models were rotated and viewed from different angles, the line of sight was always obstructed by the longitudinal barrier. Driver perspective views revealed that the cross slope affected the available SSD significantly. Therefore when a vertical curve is combined with a horizontal curve that requires a cross slope, the line of sight is not blocked by the roadway surface. This observation indicates an additional conservatism in the current crest vertical curve methodology. Designers should consider using computerized 3-D models in their normal design procedures. The use of models will allow designers to view different geometric configurations before deciding on the final combination. The models will allow designers to see the end result before the actual construction begins and thereby possibly eliminate costly field alterations.

The design of ramps and connectors is critical to the successful operation of a directional interchange. Direct connectors are usually long and geometrically complicated to avoid horizontal and vertical obstructions. Therefore, sight distance plays a critical role in the determination of a safe design. Typically designers develop horizontal and vertical geometries independently of one another on the basis of AASHTO sight distance requirements (1). When conservative values are used, the concern for the effect of their combination on sight distance is not generally an issue. With ever increasing traffic volumes and construction costs, a better understanding of the interactive effects of horizontal and vertical alignments on sight distance is needed to provide designers with the tools required to evaluate a variety of tight geometric. combinations.