Modeling of required preview sight distance

Poor coordination of horizontal and vertical alignments can create locations where the available sight distance drops below the required sight distance. Therefore, current design guides have recommended a number of guidelines to enhance the alignment coordination. A better and more quantified approach for alignment coordination can be achieved using a concept, called sight distance red zones, based on 3D analysis. A red zone, based on preview sight distance (PVSD), is defined as a section of the road where a horizontal curve should not start relative to a vertical curve. This paper presents a framework to estimate the required PVSD, which is the sight distance required to see, perceive, and react to a horizontal curve before its beginning. The required PVSD consists of two parts: PVSD on tangent and PVSD on curve. A simple analytical model of PVSD on tangent is presented based on the laws of kinematics. The PVSD on curve was investigated empirically using physical modeling and computer animation. Curves with different radii (500-2,000 m), turning directions (left and right), and configurations (with and without spirals) were simulated. Using the collected data, the effect of curve parameters was examined, regression models for the required PVSD on curve were developed, and preliminary design values for the required PVSD are presented.