Empirical modeling of the relationship between decision sight distance and stopping sight distance based on AASHTO

BASSAN, Shy · 2018 · Crossref

DOI: 10.5604/01.3001.0012.8362

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Summary

This paper addresses the need for a simplified empirical model linking Decision Sight Distance (DSD) to Stopping Sight Distance (SSD) in highway geometric design. While SSD ensures drivers can stop safely before obstacles, it is often insufficient for complex scenarios requiring maneuvers, such as interchanges, intersections, or lane drops, where drivers must process difficult-to-comprehend information. The study aims to calibrate a direct relationship between DSD and SSD based on American Association of State Highway and Transportation Officials (AASHTO) models, eliminating the need for strenuous estimation of specific maneuver and deceleration parameters. This approach simplifies the evaluation of DSD for rural, suburban, and urban roadways, filling a gap in literature where only rough approximations, such as those in British guidelines, previously existed. The methodology involves implementing AASHTO modeling assumptions for both SSD and DSD. SSD is calculated using vehicle speed, perception-reaction time (PRT), deceleration rate, and grade, with AASHTO specifying a constant deceleration of 3.4 m/s² and a PRT of 2.5 seconds. The paper reviews these parameters, noting that while 2.5 seconds is standard for rural roads, other guidelines suggest lower values (e.g., 2.0 seconds) for urban or alerted drivers. The study specifically models DSD for six avoidance maneuver types: three stopping maneuvers (Types A, A1, B) and three speed, path, and direction-changing maneuvers (Types C, D, E). These models account for pre-maneuver times and combined pre-maneuver plus maneuver times across three roadway categories. The empirical relationship is derived by calibrating these AASHTO-based computations to establish a direct correlation between the two sight distance metrics. The findings present an empirical model that allows engineers to determine the required DSD directly from computed SSD values for specific avoidance types and highway categories. The paper details the theoretical underpinnings of SSD, including the impact of perception-reaction time variations and deceleration rates influenced by pavement conditions and braking systems. It highlights that while AASHTO uses conservative values (3.4 m/s² deceleration), modern conditions and anti-lock braking systems may support higher rates. The proposed model simplifies the design process by removing the need to individually estimate complex maneuver time parameters for every scenario. Instead, designers can use the established correlation to ensure adequate sight distance for complex driving situations, such as ramp terminals and multiphase intersections, where standard SSD is inadequate. The significance of this work lies in its contribution to highway safety and design consistency. By providing a simplified, evidence-based correlation between DSD and SSD, the study facilitates more efficient and accurate geometric design for complex highway features. This ensures that drivers have sufficient visibility to execute necessary maneuvers safely, particularly in environments where information processing is challenging. The model supports balanced geometric design by aligning sight distance requirements with actual driver needs, thereby enhancing safety margins without requiring extensive computational resources for each specific maneuver scenario. This approach aids in maintaining design consistency and improving the overall safety of highway networks.

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• visibility analysis litigation