Modeling the Operational Speed of Tangents and Curves in Four-lane Highways Based on Geometric and Roadside Factors

Operational speed is an index that represents drivers’ speeding behaviors and shows the comfort and safety levels they experience. Many models had been proposed to predict operational speed in tangent and curve segments of highways. Most of these works had used geometric, with few of them conducted using roadside variables to predict operational speed. Also, the operational speed study in multilane rural highways had been gained less attention by researchers in recent years and most of them had been conducted in two-lane highways. In this study, two four - lane rural highways had been surveyed for analyzing the operational speed of tangent and curve segments. More than 13,800 spot speed data were gathered in 108 tangent and 30 curve segments in Kole jub-Borujerd and Borujerd-Khorramabad multilane highways. Two linear regression model were developed to predict operational speed in the tangent and curve segments using geometric and roadside factors simultaneously with acceptable R-square statistic (0.730 and 0.854 respectively). The results showed that segment length, guardrail median, and flat roadside configuration have a positive effect while slope, accesses density, curvature, and adjacent land use length have a negative effect on operational speed. Moreover, the sensitivity analysis demonstrates that the effect of slope on operational speed in curves is twice comparing to its effect in tangents; while, operational speed in tangents is approximately 2.5 times more sensitive to access density than operational speed in curves. The outcomes of this study can be useful in design and safety planning studies of rural multilane highways.