Evaluating the Relationship between Operating Speed and Collision Frequency of Rural Multilane Highways Based on Geometric and Roadside Features

Mehrabani, Behzad Bamdad; Mirbaha, Babak · 2018 · Crossref

DOI: 10.28991/cej-0309120

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Summary

This study investigates the simultaneous relationship between operating speed and collision frequency on rural multilane highways, specifically analyzing how geometric and roadside features influence these factors. Motivated by the recognition that speed is a primary determinant of road safety and that previous research often failed to model the direct and indirect effects of environmental variables simultaneously, the authors aim to provide a comprehensive safety assessment. The research focuses on rural multilane highways in Iran, utilizing data from the Boroujerd-Khoramabad highway. The methodology involved collecting data from 103 homogeneous segments of the highway. Operating speed was measured using laser guns, recording the speeds of 100 vehicles per segment under free-flow conditions, resulting in 10,300 total spot speed observations. Collision frequency data were gathered from a one-year period (March 2014–March 2015). The study employed Structural Equation Modeling (SEM) to analyze the complex interactions between variables. Two latent variables were defined: "geometric effect," comprising segment length, longitudinal slope, paved shoulder presence, and curvature; and "roadside effect," comprising the number of accesses and residential land use. This approach allowed for the simultaneous estimation of how these features affect operating speed and, subsequently, collision frequency. The results indicate distinct impacts of the latent variables on safety metrics. The "roadside effect" was found to increase collision frequency significantly, with a standard regression weight of 3.455, while simultaneously reducing operating speed by a weight of –0.385. Conversely, the "geometric effect" demonstrated an opposite pattern, decreasing collision frequency by a weight of –5.313 and increasing operating speed by 0.730. Crucially, the analysis revealed that lower operating speed leads to a reduction in collision frequency, evidenced by a standard regression weight of 7.734 for the effect of speed on collisions. These findings suggest that while certain roadside features may lower speeds, they do not necessarily improve safety outcomes, whereas favorable geometric designs can enhance safety despite potentially higher speeds. The significance of this study lies in its application of SEM to capture the mediating role of operating speed between physical road characteristics and crash outcomes. By integrating both geometric and roadside factors, the research provides a more nuanced understanding of road safety dynamics than previous studies that often isolated these variables. The findings offer practical implications for road designers and safety agencies, highlighting the need to consider the interplay between speed reduction and collision risk when modifying roadside environments or geometric designs. This approach supports more effective safety interventions by identifying which features genuinely reduce crash frequency versus those that merely alter driving behavior without improving overall safety.

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• Empirical Findings: crash risk outcomes