Study on Vehicle Driving Stability Under the Coupling of Ramp Circular Curve Radius and Pavement Wetness State

To clarify the coupled effects of ramp circular curve radius and pavement wetness state on vehicle driving stability, 26 ramp models with circular curve radii ranging from 30 to 280 m were constructed using Carsim software, and pavement friction coefficients were set as 0.85 (dry), 0.5 (wet), and 0.2 (rainfall) to simulate different wetness states. Lateral offset distance, lateral acceleration, and yaw rate were selected as key evaluation indicators to analyze vehicle dynamic response characteristics, and critical speed calculation models under different limit states were fitted. Two quantitative evaluation indicators, safety speed margin percentage (SSMP) and comfort speed margin percentage (CSMP), were proposed to characterize speed redundancy, and accident‐prone zones of ramps were identified. Finally, the driving safety of four typical ramps at the Kunming Northwest Ring Expressway Hub Interchange was comprehensively evaluated. The results show that the pavement friction coefficient is the core external factor determining the ramp safe speed; when the friction coefficient decreases from 0.85 to 0.5 and 0.2, the allowable maximum speed (AMS) decreases by an average of 22% and 50%, respectively. Small‐radius ramps ( R < 100 m) and low friction coefficients ( μ = 0.2) are dual risk factors, leading to severe vehicle yaw motion and increased skid/rollover risks. The main accident‐prone zones are the transition sections from transition curves to circular curves, with narrower speed tolerance ranges on rainy days. For the four typical ramps, under wet condition ( μ = 0.5), each has a certain safety speed margin, but under heavy rain ( μ = 0.2), the safety speed of small‐radius ramps (A, B) is lower than the design speed. The proposed SSMP and CSMP indicators and research results provide a theoretical basis and engineering reference for interchange ramp linear design, differentiated speed limit setting, and operational safety management.