Search

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. Keywords: Roadside curve, Lateral clearance, Reverse curves, Offset

Roundabouts are a superior solution to traffic operation issues at intersections with complex geometry, such as skewed and offset approaches where there are property constraints or where realigning approaches is costly. Although roundabouts with offset, skewed approaches are unavoidable, the current design standards are typically established based on standard roundabouts having radial approach alignments and perpendicular approaches. This work aimed to (1) examine the geometric characteristics of roundabouts with offset or skewed approaches or both, (2) establish the relationships of the design parameters with the approach offset and angle, (3) develop geometric optimization models, and (4) recommend geometric design guidelines for inscribed circle radius, entry curve radius, and exit curve radius. The approach or intersection angle ranges are also recommended for single-lane and two-lane roundabouts under urban and rural conditions. Design graphs and tables of the minimum required radii were developed. They not only help highway designers and engineers to select the appropriate parameters and reduce trial-and-error efforts during design, but also offer a guide to property needs during the planning stage. [ABSTRACT FROM AUTHOR]

Turbo roundabouts, emerging as a solution to conventional multilane roundabouts’safety and operations concerns, are prevalent in Europe and are still new to North America. However, few studies and guidelines on intersection sight distance (ISD) requirements exist. This paper focuses on the basic turbo roundabouts, examines the circulatory roadway and approach design features, and develops ISD models with North American practices. The ISD requirements were identified for the vehicle approaching and entering the roundabout, which conflicts with the circulating vehicle in the circulatory roadways, and the entering vehicle at the upstream entry approach. Analytical models were developed to identify the required lateral clearance at the central island and clear sight triangles at the roundabout quadrants to satisfy the required ISD. The analyses were completed for four typical sizes of basic turbo roundabouts per Dutch design criteria. The design guidelines are provided for the required lateral clearance at the central island and the required clear sight triangles at the roundabout quadrants. They would be beneficial in establishing landscaping restrictions and determining the roadway right of way and property impacts at the intersections during the roadway planning stage. [ABSTRACT FROM AUTHOR]

This paper presents an innovative design method for determining lateral clearance needs on a spiraled horizontal curve to satisfy sight distance requirements. The roadside lateral clearance is represented by a spiraled horizontal curve that is easy to implement in practice. The design parameters were determined. The characteristics of the corresponding lateral offsets were explored for the influential roadway factors, including required sight distance or design speed, curve radius, curve length, spiral curve length, and station location expressed in fraction of sight distance. The results show that ratio of spiral curve length to required sight distance is the major factor that affects the ratio of the lateral offset to the maximum offset at a circular curve. A single design chart and a design table also are provided as alternative design tools to determine the offset for a specific obstruction location. The proposed design method not only greatly improves the AASHTO approximate approach, but also provides an alternative approach to improving design consistency on horizontal alignments. This paper complements another paper on lateral clearance needs for simple horizontal curves. DOI: 10.1061/JTEPBS.0000046. [ABSTRACT FROM AUTHOR]