Design Requirements of Equal-Arc Unsymmetrical Vertical Curves

Abstract

Highway designers occasionally use unsymmetrical vertical curves in locations where the vertical clearance controls. In this case, the symmetrical curve, which has a constant rate of change of grade, is replaced with two arcs, which have two different rates of change of grade and a common point of tangency. Traditional unsymmetrical curves are designed so that the point of common tangency (point of compound curve, PCC) is at the same station of the point of intersection between the curve's two tangents. However, a better unsymmetrical curve can be achieved by locating the PCC in the middle of the curve. The resulting curve, called the equal-arc unsymmetrical (EAU) curve, enhances sight distance, highway aesthetics, driver comfort, and vertical clearance. In this paper, the sight distance profile on the EAU curve is analyzed and compared to that of the traditional curve. The EAU curve was found to improve the minimum available sight distance compared to the traditional curve, and thus the required length of an EAU curve to satisfy a specific sight distance would be shorter than that of a traditional curve. Also, design length requirements for the EAU curve that satisfy stopping (daytime and nighttime), passing, and decision sight distances are developed.