Speed management and control is an effective way to ensure vehicle safety on horizontal curves. In this paper, the speed limit strategy of horizontal curves on downgrades based on a modified transient bicycle model is established. A convenient expression of the friction demand versus friction supply lateral friction margin is derived based on the friction ellipse. The maximum longitudinal friction coefficient is obtained from an empirical formula, and the maximum lateral friction coefficient is derived from a fixed relationship with it. The available longitudinal and lateral friction coefficients are determined by introducing the lateral friction coefficient utilization. The lower value between the critical speed of sideslip and the critical speed of safe stopping is selected as the speed limit value based on the restriction of the friction ellipse. The comparison of the transient bicycle model and the CarSim simulation model confirms the rationality of transient bicycle model. The critical speed of sideslip based on transient bicycle model is compared with that based on the steady-state bicycle model. Results show that for stopping sight distance deceleration (3.5 m/s2), the gap is more than 30 km/h, illustrating the necessity of considering transient effects in setting the speed limit of horizontal curves on downgrades.
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