Design Considerations of Sight Distance Red Zones on Crest Curves

The American Association of State Highway and Transportation Officials (AASHTO) presents general controls for the combination of horizontal and vertical alignments. Excellence in alignment coordination enhances safety, operation, and appearance. This paper quantifies one of the AASHTO general controls that is related to introducing a horizontal curve at or near the top of a crest vertical curve. It establishes the locations where a horizontal curve should not start in relation to a vertical curve because the available sight distance is less than (1) the stopping-sight distance (SSD); or (2) the preview sight distance (PVSD) that allows the driver to detect the horizontal curve and react properly and comfortably. The range of these locations, called the “red zone,” is established for a selected alignment using three-dimensional (3D) analysis of sight distance. Both daytime and nighttime cases are discussed. The sensitivity of the red zones for PVSD to the superelevation rate, turning direction of horizontal curve, vertical curvature, and spiral curves is examined. The effect of spiral curves and location of cut slopes on the red zone for daytime SSD is also examined. This paper shows how 3D analysis can be used to identify red zones for existing combined alignments or to avoid them for new alignments. As such, it should be of interest to the design engineers concerned with highway safety.