Model for Sight-Distance Analysis of Uncontrolled Intersections

The American Association of State Highway and Transportation Officials (AASHTO) model of calculating sight distance at intersections without control (Case I) has been modified to explicitly incorporate the design speeds of both intersecting roads in computing the required sight distance for each road. It is shown in this paper that using design speeds of both roads in the modified model will result in an inadequate sight distance for most vehicles. To address this issue, this paper presents a revised model that is based on a small percentile speed of the intersecting road (such as the 1 or 5 percentile). Using the revised model, design requirements of intersection sight distance (ISD) are established for passenger cars and trucks. The results show that the practice of using design speed in ISD analysis may result in significant underestimation of ISD requirements, particularly when the speed of the approaching vehicle on the intersecting road is extremely low. Furthermore, the amount of underestimation increases with an increase in the difference between the design speeds of the intersecting roads. For intersections with sight-distance restrictions, the standard solution is to reduce vehicle speed. However, it is interesting that, for some obstruction locations, it is necessary to increase the speed to satisfy sight-distance needs. An analytical method and a worksheet for finding the required (reduced or increased) speed are presented.