Lateral Clearance Needs on Compound Horizontal Curves

Abstract

The American Association of State Highway and Transportation Officials (AASHTO) and other models for evaluating the lateral clearance on horizontal circular curves are applicable only to simple curves. Employing these models for compound curves will underestimate or overestimate the lateral clearance needs, depending on whether the analysis is based on the flatter or the sharper arc of the compound curve. In this paper, the available sight distance on compound horizontal curves is formulated. The available sight distance is related to the parameters of the sharper and flatter arcs, the lateral clearance of the obstacle, its location within the sharper or flatter arc, and the locations of the driver and object. The minimum sight distance Sm and sight distance profile of the compound curve may be considerably different from those of the simple curve. To facilitate practical use, values of Sm are established for typical parameters of the compound curve.