DESIGN CONSIDERATIONS FOR COMBINED HIGHWAY ALIGNMENTS

According to current highway design standards, horizontal and vertical alignments are treated separately in two-dimensional (2D) projections . In this paper, a model developed by the writers is used to study the effects of considering three-dimensional (3D) alignment on design req uirements for sight distance. First, the paper examines the required m inimum radius of a horizontal curve combined with a crest or sag verti cal curve in a cut section. Also, the required minimum length of a cre st vertical curve combined with a horizontal curve in a hh section is studied. The results show that the 3D design requirements may differ s ignificantly from those in separate 2D projections. Depending on the v ertical alignment and on the cut side-slope, 2D design may underestima te or overestimate the required radius. In fill sections, the 2D align ment overestimates the required length of the crest vertical curve whe n combined with long horizontal curves. In summary, the 2D alignment c an compromise both safety and economics of highways. As a result, it i s concluded that 3D-based design standards should be developed and use d in highway geometric design.