Research on Average Longitudinal Slope Length of Expressway Based on Braking Behavior

Excessive temperature of the truck brake hub is a primary factor leading to brake failure. To enhance the safety of trucks traveling on continuous downhill sections, this study refined the average longitudinal slope design parameters and investigated the correlation between driver braking behavior and the temperature rise characteristics of truck brake hubs. Based on driver braking behavior, it proposed a reliability design method for longitudinal slope length. Firstly, a continuous downhill section of an expressway in the western mountainous area was selected for real vehicle test, and the road longitudinal slope parameters and driver braking behavior data were collected. Secondly, according to the measured data, the evaluation indexes were proposed: displacement intensity coefficient and brake hub temperature gradient. This study investigated the relationship between displacement intensity coefficient and road profile, both longitudinal and transverse, as well as the relationship between displacement intensity coefficient and brake hub temperature gradient through regression analysis. Finally, a reliability model was constructed based on the driver’s braking behavior and critical temperature. Using the Monte Carlo simulation, critical slope lengths corresponding to different average longitudinal slopes on continuous downhill sections were determined and compared with the specification. The results indicated that there is a weak correlation between the radius of the circular curve and the displacement intensity coefficient, while there is a significant positively correlation between the longitudinal slope gradient and the displacement intensity coefficient, and the goodness of fit r² is 0.95. When the longitudinal slope gradient is greater than 2%, the braking measures taken by drivers are mostly sustained braking, which is more different from the braking behavior of drivers mostly taking point braking when the longitudinal slope gradient is less than 2%. The displacement intensity coefficient and the temperature gradient of the braking hub are significantly positively correlated with the goodness-of-fit r² is 0.845. When the proportion of the driver braking for 85%, the braking behavior of the driver at this time with the specification of defining the conditions of the slope length is basically the same. When the reliability is 0.95, the average longitudinal slope is 2.1%~3.0%, and the critical value of the continuous slope length is 14.95~30.12 km. The given reference values take into account the randomness in the real driving environment, and provide a basis for the design of the slope length with an average slope of less than 2.5%.