The current AASHTO stopping sight distance (SSD) model has two components: (1) perception-reaction time, which determines the distance a vehicle travels at a fixed speed while these actions occur, and (2) braking distance, the distance the vehicle travels during the braking maneuver. This model has been altered only slightly since its inception in the 1940s, and it continues to result in well-designed roads. However, the worst-case scenario--with its conservative assumptions of reaction time, low assumptions of pavement friction values, and unproven driver visual capabilities--combined with an assumed below average driver, results in a model that provides a considerable margin of safety but is difficult to justify or defend as representative of either a real-life environment or a safe driving behavior.
Texas A&M Research Foundation has evaluated, on the basis of the impact on vertical and horizontal curve design, the current AASHTO methodology and alternative approaches to establishing stopping sight distance. Based on a review of current and alternative practices, they have updated representative vehicle-performance characteristics, roadway and pavement characteristics, and driver-behavior data, and have recommended design procedures for specific applications.
The recommended SSD model, published in NCHRP Report 400, "Determination of Stopping Sight Distances" in late January 1998, remains conceptually the same as the existing AASHTO model, that is, SSD equals perception-reaction distance plus braking distance, but with initial speed equal to design speed and design deceleration substituted for friction coefficient. As with the current model, the minimum SSD, driver eye height, and object height values are used to calculate required minimum length of vertical curve (VC) and required minimum rate of curvature or lateral clearance on horizontal curves. The recommended changes result in proposed SSDs that fall between current AASHTO minimum and desirable design values. This research was undertaken, not because of safety concerns with the current AASHTO SSD model, but to propose scientifically-based, reproducible, rational SSD design values reflecting driver capabilities and performance.
The unpublished appendices are available here.