An algorithm based on the PPC (Polynomial Parametric Curve) for designing horizontal highway alignments

Criteria for geometric design of highway alignments have been studied in various researches so far, providing new methods, as alternative to traditional approaches. They can be useful for obtaining better solutions, characterized by high performances in terms of motion safety and comfort. In particular, continuous curvature alignments, based on polynomial functions, are highly consistent solutions with users' driving behaviour and vehicle trajectories. Moreover, alignment geometry optimization allows engineers to minimize dynamic effects on drivers with considerable advantages in terms of comfort and safety. This paper carries on the research of a previous study, in which a polynomial parametric curve, called "PPC", useful for horizontal highway alignment design, has been proposed. Besides, this curve has been shown to be suitable for solving cases with complex geometry, such as the road interchanges. In these particular situations and in the design of horizontal curves in highway alignments, the computational advantages related to the use of a single curve, compared to traditional procedures based on a composition of different elements (clothoids and circular curves), are very clear. In particular, in this paper, an original algorithm, for simplifying the PPC introduction in the highway alignment design, is proposed. To verify the PPC improvements, an exhaustive numerical example, with several comparisons to traditional approach (characterized by straight sections, circular curves and clothoids as transition curves), has been performed. A both point by point and global examination of kinematic and dynamic variables directly involved while driving has attested the method validity.