Vertical and Horizontal Alignment

The writers of this paper see the current highway design standards as a two-dimensional projection which the horizontal and vertical alignments are treated separately. A model developed in this paper considers and study the sight distance as a three-dimensional alignment on the design requirements. The paper examines the required minimum radius of a horizontal curve combined with a crest or sag vertical curve in a cut section. Also, the required minimum length of a crest vertical curve combined with a horizontal curve in a fill section is studied. According to the result, the 3D design requirement differs significantly from the current 2D projections. Generic 2D design is highly dependent on the cut side-slope and vertical alignment, and it may under/overestimate the required radius. Also, the 2D alignment overestimates the required length of the crest vertical curve in fill section when combined with long horizontal curves. The 2D alignment can compromise both safety and economics of highway. Therefore, 3D-based design standards should be developed and used in highway geometric design.