Innovative roadside design curve of lateral clearance: Roadway reverse horizontal curves

This paper presents a proposed concept of a spiraled roadside curve for determining the required lateral clearance that satisfies sight distance needs on a roadway reverse horizontal curve. The spiraled roadside curve was evaluated in the context of roadway plan view. The characteristics of its corresponding lateral offsets were analyzed. It was found that the ratio of the radii for the two reversing circular curves was the major factor that impacted the ratio of the required offset to the maximum offset of a circular curve. A single design chart and a design table were developed. The required offsets at alignment reversing sections were far less than those recommended by the American Association of State Highways and Transportation Officials (AASHTO) and those required at the approach and departure sections of a roadway simple curve. At the common point of the two reversing circular curves having a radius ratio R2/R1 ranging from 0.5 to 2, the required offsets were approximately 34%-66% of the maximum offsets recommended by AASHTO and approximately 41%-79% of those required in the case of a simple curve. The engineering implications of the proposed spiraled roadside curve are multifold: (a) it can be designed in a way similar to traditional roadway design, (b) it avoided the application of numerous design charts and extensive computations, (c) it can be easily staked on the field, (d) it not only greatly improved the AASHTO approximate approach, but also improved roadway design consistency.