Deriving Sight Distance on a Compound Sag and Circular Curve in a Three Dimensional Space

ABSTRACTInsufficient roadway sight distance (SD) may become a contribution factor to traffic collisions or other unsafe traffic maneuvers. The sight distance (SD) for a two-dimensional (2-d) sag or circular curve has been addressed in detail in various traffic engineering literatures. Although three-dimensional (3-d) compound sag and circular curves are often found along ramps, connectors, and mountain roads, the sight distances for these compound curves are yet to be analyzed on an exact analytic setting. By considering human-vehicle-roadway interaction, the formulas for computing the SD on a 3-d curve are derived the first time on a unified analytic framework. The 2-d sag curve SD can also be deduced from these derived formulas as special limiting cases. Practitioners can easily program these formulas or equations on a user-friendly Microsoft Excel spread sheet to calculate 3-d SD on most roadways with roadside clearance. This framework can be extended to estimate SD on roadways with obstacles partially blocking vehicle headlight beams. 6.