In view of the current lack of stability of the highway superelevation transition section and the poor characteristics of the zero-slope section drainage, an relative gradient model of the cubic parabola, the lower half-wave cosine curve and the upper half-wave sinusoidal curve is established. Taking the long easing curve of the multi-lane expressway and the S-curve section as the experimental road conditions, the relative gradation rate at the zero-slope section is analyzed. Based on the construction of experimental road conditions under the dynamic software CarSim simulation platform, the stability of driving under different relative gradient modes was verified, and the corresponding stability parameter changes were output. The comparative analysis shows that the maximum relative gradation rate of the cubic parabola and the lower half-wave cosine curve transition form is 1.5 times and 1.57 times of the linear transition relative gradation rate respectively. The maximum relative gradient of the upper half-wave sinusoidal curve is the same as the linear transition. When the curve gradual transition is adopted, the curves of the lateral acceleration and yaw angular velocity, which near the start and end point of the superelevation transition are relatively smooth and continuous. It is suggested that the maximum relative gradation rate is combined with the zero-slope section position in the transition section,which can reduce the length of the gentle slope section and enhance the drainage of the road surface. The curve superelevation gradient model is of great significance for improving the stability and drainage performance of multi-lane highways and S-curve superelevation gradient sections. Key words: Road engineering; Curve superelevation gradient model; Driving stability; Drainage performance; CarSim simulation;

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