![]() These methods assumed that the deck of the overpass structure lays horizontal and somewhat perpendicular to the underpassing road. Other closed-form methods were available in design guides ( AASHTO 2011). Easa ( 2009) presented a model to optimize the ASD in the design of underpasses. 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. However, overpass structures may limit sight distance on certain sections. In daylight, sight-distance restrictions are typically made on crests and curves. Most highway standards proposed 2D sight-distance estimation procedures ( Ministerio de Fomento 2016 AASHTO 2011). This model was then applied to a case study to analyze in full detail the effects of such a structure and the road geometry on the sight-distance conditions where the required stopping and passing sight distances are assessed. This paper first describes the computational process of 3D modeling of roadways and related elements and sight-distance estimations of them with an overpassing structure. Moreover, 3D methods avoid the need to rely on closed-form models typically devised from analytical 2D approaches ( Ismail and Sayed 2007). The elements used as inputs in 3D sight-distance models do not always permit the re-creation of overhanging structures. Nevertheless, a significant sight restriction may be created by overpasses at or near sag curves ( Easa 1992). Sag vertical curves do not usually present sight-distance issues in daytime however, in standards, the design of them is conditioned, to some extent, by nighttime driving performance ( Ministerio de Fomento 2016 AASHTO 2011). Furthermore, 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. The most significant include the use of two-dimensional (2D) models instead of 3D models, inaccurate 3D highway models, and the overlooking of structures and roadside elements. Possible sources of bias in sight-distance estimation are many and varied. Highway administrators have acknowledged the difficulties involved in estimating unbiased sight distance on in-service roadways. The three-dimensional (3D) nature of highways admittedly requires a 3D approach for the sight-distance estimation. The provision of adequate sight distance on highways is of utmost importance not only in the design phase but also during the operation of such roadways. Finally, the currently existing passing zones were evaluated by means of the results obtained, revealing possible deficiencies in the establishment of passing zones. 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. ![]() The effect of the observer and target heights on sight distance for underpasses proved the opposite of those on horizontal curves and crests. Stopping sight distance and passing sight distance were evaluated for a section of highway with specific sight-distance characteristics of overpasses. 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. 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. However, overhanging features may also affect sight distance. Horizontal curves and crest vertical curves were the common sight restrictions considered in highway design. The estimation of sight distance must be performed taking the three-dimensional (3D) nature of roadways and related features into account. When it comes to the safe design and operation of highways, sight distance is of utmost importance. ![]()
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