Traffic safety remains a major challenge in transportation engineering, and vehicle speed is a key factor influencing both the occurrence and severity of crashes. Drivers’ speed behavior is affected by environmental conditions and roadway geometry. This study aims to analyze operating speed and its influencing factors on four-lane rural highways in Lorestan Province. Free-flow speed data were collected covertly, and geometric characteristics were obtained through field surveys. A total of 201 segments, including tangent and curve sections, were identified, and the free-flow speed of 100 passenger cars was recorded for each segment, resulting in 20,100 valid observations. Multiple linear regression was used to model operating speed. The novelty of this study lies in its empirical assessment of drivers’ compliance with various posted speed limits and the comparison of speed behavior between tangent and curved sections—an aspect rarely addressed in previous domestic and international studies. Results showed that compliance was observed only at posted limits of 110 km/h and 95 km/h, while decreasing the speed limit increased the gap between operating and posted speeds by up to 2.2 times on tangents and 2 times on curves. On tangents, the 85th-percentile speed of the preceding curve had the strongest positive effect (β = 0.55), whereas upgrades reduced speed by an average of 25.8 km/h and edge line markings increased speed by 3.7 km/h. On horizontal curves, the preceding tangent’s 85th-percentile speed (β = 0.63) was the most influential factor, and each 100-m increase in curve radius raised speed by 1.2 km/h. Upgrades reduced curve speeds by 3.96 km/h. The coefficient of determination was 0.721 for tangents and 0.786 for curves, indicating good predictive performance. Findings highlight the need for consistent geometric design, targeted traffic-calming measures, and enhanced intelligent control systems to manage speeding and improve rural highway safety.