Operating speed prediction model as a tool for consistency based geometric design of four-lane divided highways

Sil, Gourab; Maji, Avijit; Nama, Suresh; Maurya, Akhilesh Kumar · 2019 · DOAJ

DOI: 10.3846/transport.2019.10715

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Summary

This study addresses the lack of operating speed prediction models for four-lane divided highways, particularly in developing countries where weak lane discipline influences driving behavior. While existing models predominantly focus on two-lane rural highways, the authors argue that vehicles on multi-lane divided highways are affected by lead vehicles in adjacent lanes, rendering traditional lane-based free-flow assumptions inadequate. The research aims to develop a methodology for identifying free-flowing vehicles and a corresponding linear operating speed prediction model based on geometric consistency. The study was conducted at 18 horizontal curve sites on National Highways 3 and 40 in India, characterized by flat terrain and uniform cross-sections. To ensure data represented free-flow conditions, the authors established a rigorous tracking methodology using synchronized video cameras at three points: 50 meters before the curve, at the point of curvature, and at the curve center. A vehicle was classified as free-flowing only if it met two conditions: maintaining a time headway of at least 5 seconds from any lead vehicle (regardless of lane) at all observation points, and exhibiting no passing maneuvers within the study zone. Speed data for passenger cars were extracted from video logs using a 15-meter trap length at the curve center. Fifteen sites were used for model development, and three for validation. The analysis revealed that operating speed (V85) increases with curve radius but plateaus for curves with a radius of 360 meters or more, indicating that sharper curves significantly influence driver speed choices. The developed linear prediction model identifies curve radius and the length of the preceding tangent as the primary determinants of operating speed. Specifically, longer preceding tangents were found to yield higher operating speeds at the curve center. The model was validated against the remaining three sites, confirming its applicability. The significance of this work lies in providing a specialized tool for the geometric design and consistency evaluation of four-lane divided highways. By accounting for weak lane discipline and inter-vehicle interactions across lanes, the model offers a more accurate representation of driver behavior than existing two-lane models. The authors propose two nomograms based on the derived model to assist engineers in conventional design, consistency-based design, and safety evaluation of horizontal curves, ultimately aiming to reduce single-vehicle crashes associated with speed misjudgments on sharp curves.

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• road geometry