Computer‐Aided Design for Horizontal Alignment

An accepted approach in conforming to the design standards for modern highways is to use a combination of arcs of circles and segments of clothoid spirals. The design of a horizontal alignment for a highway centerline can be accomplished manually by using circular and clothoid templates. This article illustrates how a computer may be used to automatically decompose a proposed centerline into arcs of circles and segments of clothoid spirals. True clothoidal transition spirals for the horizontal alignment of a highway centerline can be conveniently incorporated in interactive graphics‐based computer‐aided highway‐design software by using a control polygon approach. A control polygon approach applied to B‐spline and Bézier curves has already been used successfull in computer‐aided design for manufacture and has the advantage of providing a simple user interface. This article demonstrates that a similar approach is feasible for computer‐aided design of horizontal alignments. A description of prototype software as well as a demonstration example are included.