Evaluation of available sight distance estimation method on rural roads according to Croatian regulation

Stančerić, Ivica; Bezina, Šime; Stepan, Željko; Džambas, Tamara · 2024 · Crossref

DOI: 10.5592/co/cetra.2024.1532

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Summary

This study evaluates the adequacy of current sight distance estimation methods for rural roads under Croatian regulations, specifically comparing traditional two-dimensional (2D) checks against three-dimensional (3D) simulations. The research addresses a critical safety gap: while regulations mandate that available sight distance (ASD) must meet or exceed stopping sight distance (SSD), standard 2D methods assess horizontal and vertical alignments separately. This approach ignores cross-sectional features, such as curb heights and verge slopes, which can obstruct a driver’s field of vision. The authors aim to demonstrate that 2D compliance does not guarantee actual visibility safety, particularly in complex geometric alignments. The methodology involved designing 60 road models using Autodesk Civil 3D and Bentley OpenRoads Designer software. These models adhered to Croatian rural road regulations, featuring horizontal curves with minimum radii for design speeds of 50 to 90 km/h and deflection angles of 30°, 60°, and 90°. Vertically, the models utilized crest curves with minimum radii calculated for grade changes ranging from 2% to 8%. Crucially, all models were designed in a "cut" cross-section, where the road is below terrain level, to simulate conditions where roadside elements significantly impact visibility. The study compared the ASD results from standard 2D regulatory calculations against simultaneous 3D simulations that accounted for the full geometric profile, including cross-sectional dimensions like lane width, gutter, and curb height. The results revealed that passing the mandatory 2D sight distance test does not ensure sufficient ASD in 3D simulations. In many cases, particularly at higher speeds and steeper grade changes, the ASD in the 3D models was less than the required SSD, despite the designs meeting 2D criteria. The study found that simply widening the road verges to satisfy horizontal sight line offsets was insufficient to compensate for visibility losses caused by vertical curve geometry and cross-sectional features. To achieve adequate ASD in 3D simulations, the radii of the crest vertical curves often needed to be significantly larger than the minimums prescribed by 2D regulations. For instance, at 90 km/h with an 8% grade change, the applied vertical radius required for 3D safety was substantially higher than the regulatory minimum, and in some high-speed scenarios, no feasible radius within standard limits provided sufficient sight distance. The significance of this research lies in its recommendation to adopt 3D sight distance testing in road design and safety assessments. The findings indicate that 2D methods overlook critical cross-sectional factors that negatively impact driver visibility, potentially leading to unsafe road designs. The authors conclude that planners and decision-makers must move beyond mandatory 2D checks and utilize specialized software for 3D analysis. This approach ensures the correct selection of horizontal and vertical elements, thereby improving the reliability of safety estimates and reducing the risk of accidents on rural roads.

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Topics

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• road geometry
• visibility analysis litigation