Abstract: The unreasonable setting of longitudinal slope is one of the fundamental reasons for frequent accidents in long longitudinal slope sections. An army of scholars have done some research on whether it is necessary to set up a transitional gradient between two steep slopes. Quiet a few scholars believe that setting transitional gradient between steep slopes may give drivers the illusion of entering flat or reverse slopes, while others believe that the transitional gradient must be set to help the trucks recover their uphill speed. Therefore, based on the vehicle climbing performance and vehicle braking theory in vehicle dynamics, 21 truck drivers were selected to conduct field driving experiments on a long longitudinal slope section in Shaanxi Province to obtain the driver's slope perception data and summarize the law between perception gradient and actual gradient. The results show that in the case of long longitudinal slope, the transitional gradient between steep slopes can recover the speed of the truck, and as long as the gentle slope is set reasonably, the slope illusion will not occur. However, too long transitional gradient will increase the overall design length of the highway, which can not meet the requirements of Engineering economy; too short transitional gradient will have no obvious effect on the speed recovery of the truck, and can not fully reflect the purpose of setting transitional gradient. Therefore, the length of the transitional gradient should take into account the requirements of engineering economy and the setting of vehicle climbing performance. The research results have theoretical guiding significance for road designers in longitudinal slope design. Key words: alignment design, transitional gradient, long longitudinal slope, truck, slope perception, uphill speed recovery

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