Abstract
When it comes to the safe design and operation of highways, sight distance is of utmost importance. The estimation of sight distance must be performed taking the three-dimensional (3D) nature of roadways and related features into account. Horizontal curves and crest vertical curves were the common sight restrictions considered in highway design. However, overhanging features may also affect sight distance. This paper presents a 3D virtual model for evaluating sight distance on sites where overpass structures restrict sight distance and for detecting sight-distance-related issues. The procedure and inputs for the computation of sight distance are described as are the results, which were validated and applied to a case study of an in-service highway underpass. Stopping sight distance and passing sight distance were evaluated for a section of highway with specific sight-distance characteristics of overpasses. The effect of the observer and target heights on sight distance for underpasses proved the opposite of those on horizontal curves and crests. Near an overpass, the heights should be switched from those used on curves and crests to evaluate stopping and passing sight distance according to operational and safety criteria. Finally, the currently existing passing zones were evaluated by means of the results obtained, revealing possible deficiencies in the establishment of passing zones.
Highlights
The provision of adequate sight distance on highways is of utmost importance in the design phase and during the operation of such roadways
The digital terrain models (DTMs) used consisted of a surface built up from a point cloud arranged in a 1-m grid derived from an aerial light detection and ranging (LiDAR) survey (IGN 2010)
The curved stripe was discontinuous beyond 830 m because of the sight lines slipping through the loophole created between the concrete parapet and the top metal rail (Fig. 3)
Summary
The provision of adequate sight distance on highways is of utmost importance in the design phase and during the operation of such roadways. The available sight distance (ASD) is closely related to the signposting of speed limits and the design of passing zones, which are fundamental for the safe and efficient operation of highways. Most highway standards proposed 2D sight-distance estimation procedures (Ministerio de Fomento 2016; AASHTO 2011). Easa (1992) formulated a 2D analytical method to calculate the sight distance on highways with noncentered overpasses where the underpassing roadway profile conformed to a grade–sag–grade sequence. Other closed-form methods were available in design guides (AASHTO 2011) These methods assumed that the deck of the overpass structure lays horizontal and somewhat perpendicular to the underpassing road. The validity of 2D procedures is limited because deviations from those assumptions may arise (e.g., nonperpendicular overpass structures or more complex layouts in highway alignment), affecting the sight-distance results
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