Headlight sight distance on separate highway alignments: a new approach

Abstract

Headlight sight distance (HLSD) has been considered in the literature for only simple (isolated) sag vertical curves. This paper develops HLSD models for two-dimensional complex horizontal and vertical alignments. The horizontal alignment may consist of any combination of circular curves, spiral curves, and tangents. The HLSD model for horizontal alignment is based on the outside horizontal angles of headlight spread from the horizontal axes of the headlights. The available sight distance was modelled considering the effect of continuous (e.g., cut slope) and single obstructions. A design graph showing the adequacy of the available HLSD on simple circular curves is presented. The vertical alignment may consist of any combination of simple sag curves, simple crest curves, unsymmetrical (traditional or equal-arc) curves, and tangents. The HLSD model for vertical alignment is based on the traditional upward angle of headlight divergence from the horizontal axis of the vehicle. The model determines the available HLSD as well as headlight dips. Application of the models is illustrated using numerical examples. The presented models can be used to evaluate nighttime sight distance on complex horizontal alignments and to design sag curve as part of a complex vertical alignment, rather than as an isolated component. The HLSD models can help identify remedial measures to satisfy sight-distance needs such as advisory nighttime speeds. (A)