EXPERIMENTAL RELATIONSHIPS BETWEEN OPERATING SPEEDS OF SUCCESSIVE ROAD DESIGN ELEMENTS IN TWO-LANE RURAL HIGHWAYS

Eboli, Laura; Guido, Giuseppe; Mazzulla, Gabriella; Pungillo, Giuseppe · 2015 · OpenAlex-citations

DOI: 10.3846/16484142.2015.1110831

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Summary

This study addresses the critical role of speed in road safety, specifically focusing on the inconsistency between geometric road design and actual driver behavior on two-lane rural highways. Inappropriate speeds are a primary cause of crashes, mortality, and disability. The authors investigate operating speed, defined as the 85th percentile speed, to evaluate design consistency. The research is motivated by the need for accurate experimental measures or reliable prediction models to identify safety issues arising from inconsistent roadway geometry, where drivers’ expectations do not align with the physical design. The study utilizes a 10 km segment of the Italian National Road No. 106 (SS106), a two-lane rural highway known for high accident rates. Data were collected using vehicles equipped with GPS-embedded smartphones, which recorded instantaneous speed and position at a frequency of 1 Hz. Twenty-seven drivers traversed the segment in both directions, providing continuous speed profiles for 29 road elements (15 tangents and 14 curves). The authors first performed a quantitative design consistency evaluation using Lamm’s safety criterion I, which compares operating speed ($V_{85}$) with design speed ($V_d$). This preliminary analysis revealed significant inconsistencies, particularly in the northern part of the segment, where operating speeds frequently exceeded design speeds, indicating potential safety risks. To address these inconsistencies, the authors developed two regression models to predict operating speeds based on geometric characteristics and the speed of the preceding road element. For horizontal curves, the model uses curve radius ($R$) and the operating speed of the previous element ($V_{85,i-1}$) as independent variables. The resulting equation is $V_{85,c,i} = 0.858 \cdot V_{85,i-1} + 0.037 \cdot R - 1.288$, which demonstrated a high predictive capability with an adjusted $R^2$ of 0.937. For tangent elements, the model incorporates the length of the tangent ($L$) and the previous element’s operating speed. The derived equation is $V_{85,t,i} = 0.762 \cdot V_{85,i-1} + 13.994 \cdot \log_{10}(L) - 10.721$, with an adjusted $R^2$ of 0.902. Both models showed that the speed of the preceding segment significantly influences current operating speed, alongside specific geometric features. The significance of this work lies in providing practical tools for road management and safety evaluation. The proposed models offer a simple, analytically sound method for predicting operating speeds, allowing technicians to assess design consistency and identify hazardous sections without extensive field measurements. The findings confirm that operating speed is heavily dependent on the continuity of the road alignment, validating the importance of considering successive road elements in safety analysis. These models can help align road geometry with driver behavior, thereby enhancing safety conditions on rural highways.

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Topics

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• road geometry
• speed choice