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Speed is one of the most important issues that affect rural highways’ safety. Many researchers have reported that an increase in vehicles’ speed is accompanied by the increase of accident severity and probability of accidents occurring. Many models had been presented in previous studies to predict operating speed in four-lane highways. Most of these works had only used the geometric feature as the independent variable, with few of them conducted using roadside features to predict operating speed. Thus, the objective of this study is to propose a model which predict the operating speed of four-lane highways based on geometric and roadside features simultaneously. More than 13 800 spot speed data were gathered in tangent and curve segments in Kole jub-Borujerd and Borujerd-Khorramabad multilane highways in Iran and a linear regression model is developed with acceptable R-Square (0.779). The results showed that segment length, guardrail median, flat roadside configuration, and posted speed limit have a positive effect on operating speed; while, slope, the number of access, and the length of adjacent land use have a negative effect on operating speed. the coefficient of these variables indicated that the length of the adjacent land use has the greatest value (−7.435) and the sensitivity analysis demonstrate that a 500-meter increase in the length of the adjacent land use leads to almost 4 kilometers per hour decrease in operating speed. Therefore, this paper indicated that not only geometric features affect operating speed but also roadside features have a significant effect on operating speed. The outcomes of this study can be useful in design and safety planning studies of rural multilane highways.