Design considerations for highway sight-hidden dips

Vertical alignment in which a short section of roadway is hidden from the driver’s view should be avoided in design because hidden-dip profiles contribute to passing maneuver accidents. These profiles can be examined only graphically because no analytical method is available. In this paper, an analytical method for evaluating hidden-dip profiles is developed. The locations on the traveled path where the sight-hidden dip starts and ends are related to the roadway and vehicle parameters. The roadway parameters are the algebraic difference in grades and lengths of the crest and sag arcs, the common tangent length, and the second tangent length. The vehicle parameters are the driver’s eye height and the height of the roof of the opposing vehicle. Exact relationships and graphical aids for finding the locations of the start and end of the sight-hidden dip for existing profiles and the combinations of parameters that avoid these dips in design are developed. The adequacy of the minimum rates of vertical curvature of the American Association of State Highway and Transportation Officials is examined. In practice, the locations of the start and end of sight-hidden dips can be delineated on the highway using appropriate signing to achieve safer operations.