China has a large mileage base of low-grade highways, and accidents caused by brake failure of heavy-duty trucks occur frequently. To conduct an in-depth study on the braking behavior of truck drivers on continuous downhill sections of low-grade highways, this research, based on the field-measured data of six-axle articulated trains on low-grade highways during long downhill driving, identified the driver’s braking timing on downhill sections with complex horizontal and vertical alignments, analyzed and quantified the characteristics of truck drivers’braking behaviors on continuous downhill sections of low-grade highways, established a joint simulation model of driver behavior in TruckSim simulation software by integrating Matlab/Simulink, and, finally, verified the reliability of the model through comparison. The results show that, on horizontal curve sections, truck drivers are influenced by the curves and will exhibit relatively high-intensity braking behaviors at the end of straight sections and the beginning of the curved sections. On straight downhill sections with a gradient of 2%~9%, the drivers’ braking magnitude is positively correlated with the gradient; According to the radius, the curves are classified into sharp curves, medium curves and gentle curves, and the average brake pedal stroke before entering sharp curves reaches 220 mm, while there is no significant difference in the average brake pedal stroke between medium curves (176 mm) and gentle curves (174 mm). As compared with the brake drum temperature rise prediction models in previous studies that did not consider horizontal alignment factors, the simulation model established in this research is of higher prediction accuracy for brake drum temperature, thus providing a theoretical basis for the traffic safety guarantee on low-grade highways.

The cross-section speed data of a continuous long downhill section of an expressway in Southwest China was collected, in order to study the operating characteristics of trucks on the long downhill section, improve the effectiveness of the operating speed prediction model, and ensure that vehicles can drive safely on the long downhill section. The temporal and spatial distribution characteristics of truck speed and the degree of dispersion of vehicle speed were analyzed, and the speed distribution characteristics of trucks on the long and large downhill road was tested through Q-Q probability diagram and single-sample K-S test, thus the speed distribution characteristics of trucks on the long downhill section were obtained. The error analysis of the operating speed model was carried out according to the distribution characteristics, so as to determine the cause of the error and correct the variable. Finally, a prediction and correction model of truck operating speed was established, and the effectiveness of the mo-del was compared and analyzed before and after the correction. The results show when the truck is traveling on a long downhill section, the speed gradually decreases first and then tends towards stability as the downhill distance increases; the degree of dispersion of vehicle speed increases with the increase of downhill distance and traffic vo-lume. Truck speed characteristics do not change significantly over time; truck speed in the long and large downhill sections conforms to the law of Logistic distribution, and the speed is highly concentrated and symmetrical to the peak speed value. The existing speed prediction model has deviations mainly because these models do not consider the influence of traffic density. The relative error of the truck running speed prediction correction model established based on this is reduced by 4% to 14%, which significantly improved the effectiveness. This can provide a theore-tical basis for the study of the long downhill running speed and improve the downhill safety of trucks.

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%.