DESIGN OF SAG VERTICAL CURVES IN 3-DIMENSIONAL ALIGNMENTS

Headlight sight distance has been an important element in designing sa g vertical curves because nighttime drivers need sufficient sight dist ance for stopping, and also because they depend on the road marking to maintain uniform speed and proper lane positioning. Considering an is olated sag vertical curve in a two-dimensional projection based on the sight distance needs for stopping, the current design standards recom mend a minimum length of the sag curve. However, little research work has been directed toward determining the available headlight sight dis tance in three-dimensional highway alignments. A new analytical model developed by the writers for 3D analysis of headlight sight distance h as shown that the 2D analysis may overestimate the available headlight sight distance on sag vertical curves combined with horizontal curves . In this paper, a computer program based on the 3D analytical model i s used to evaluate the current 2D standards for sag vertical curves on fill sections, The results show that the current 2D design standards overestimate the available headlight sight distance and therefore unde restimate the required length of sag vertical curves combined with hor izontal curves. In this paper more accurate values for the required le ngth of sag vertical curves combined with horizontal circular curves a re developed. A more economical design is also suggested through the u se of asymmetrical (compound) sag vertical curves.