Optimizing freeway maximum gradient and slope length using specific power analysis of five- and six-axle articulated vehicles

Currently, six-axle articulated trucks on Chinese freeways exhibit lower climbing performance than the design vehicle specified in the 'Design Specification for Highway Alignment' (China). A simulative vehicle model for freeway longitudinal section design criteria was developed using specific power data collected from 1,758 tractor units listed on the Truck Home website. This study established a simulation scene using Trucksim software to model climbing on a single longitudinal slope section, it helped determine control values for the maximum longitudinal slope gradient/length based on equilibrium speed, and speed reduction. The research results show that 82.9% of tractor specific power data is concentrated between 6 to 9 kW/t, recommending the selection of a six-axle articulated truck with a specific power of 6.94 kW/t as the representative model for freeway longitudinal section design; there is a significant difference in specific power distribution between five-axle and six-axle articulated trucks, with the mode (6.14 kW/t) and mean (6.77 kW/t) specific power of five-axle articulated trucks both lower than the mode (6.92 kW/t) and mean (7.13 kW/t) specific power of six-axle articulated trucks. For freeways with design speeds of 120, 100, 80, and 60 km/h corresponding to unlimited lengths, the maximum longitudinal slopes are 2.0%, 2.5%, 3.0%, and 3.5% respectively. Except for the simulation condition with no limit on slope length, the recommended slope lengths for the other simulation conditions are reduced to varying degrees compared to the standard values, with the highest reduction reaching up to 33.75%.