Abstract
Lateral clearance on the inside of horizontal curves is required by all geometric design guidelines in order to provide at least stopping sight distance. There already exist graphical models, analytical models, and design charts for determining minimum clearance offsets to meet the requirement. Some of these models determine the offsets based on constant design sight distance values on the assumption that drivers negotiate horizontal curves at constant speed. Therefore, those models are suitable for sites where there is no difference in speeds between tangent and curved sections. Past studies have reported that there are sites where drivers decelerate on entering horizontal curves and accelerate on departing from the curves. At those sites stopping sight distance for a given driver varies with driver location due to variable speed. This paper presents an analytical model and a chart for determining minimum offsets needed to provide desired sight distances at horizontal curves with variable operating speeds. At those sites the offsets yield roadside clearance boundaries that have transition arcs with performances that are similar to those of elliptical arcs. Therefore, practitioners may choose to use elliptical equations or equations and the chart developed herein. Results of this study will be of value to practitioners in the area of roadside design.
Highlights
Provision of sufficient sight distance is one of the requirements for safe and efficient operation of highways
The graphical method in the AASHTO’s Green Book [1], design charts by Raymond [2] and Glennon [3], and the analytical model by Mauga [4] are known for determining minimum offsets that provide a given fixed or constant design sight distance
Design guidelines require that the roadside area on the inside of horizontal curves be cleared of sight obstructions
Summary
Provision of sufficient sight distance is one of the requirements for safe and efficient operation of highways. The graphical method in the AASHTO’s Green Book [1], design charts by Raymond [2] and Glennon [3], and the analytical model by Mauga [4] are known for determining minimum offsets that provide a given fixed or constant design sight distance. Sections near beginnings and ends of curves need wider clearance offsets so as to provide longer sight distances that match high operating speeds. The only current model that may be addressing wide clearance offsets for beginnings and ends of horizontal curves is the simple analytical model by the Green Book [1] as presented by Equation (1) This model is mainly suitable for middle sections of curves but the guideline suggests that it be applied uniformly along curves. To identify a known curve that approximates the roadside clearance envelopes that result when those minimum offsets are implemented in the field
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