Parametric analyses of large truck braking efficiencies

Abstract

Simplified braking models are used to analyze the braking efficiencies of a range of truck types under a variety of loading conditions and tire–pavement coefficients of friction. These analyses show that while most of the fully loaded trucks are capable of achieving a braking efficiency of 70%, the braking efficiencies of the unloaded or partially unloaded trucks deteriorate substantially. A bobtail tractor, an unloaded semi-trailer with tridem rear axle group and two air-lift belly axles, and a B-train double trailer with rear trailer unloaded have estimated braking efficiencies of 40–50%. The braking distances required by the unloaded and partially loaded vehicles from a 100 km/h operating speed are two to four times longer than those specified in the Canadian geometric design standards. Comparisons of the model-estimated braking distances with those obtained in one set of field tests show that the models are capable of reflecting accurately observed field behaviour. The results presented in the paper demonstrate the need for a systematic review of those highway design standards that are based on the braking capabilities of vehicles using the highway system. Key words: highway geometric design, truck braking efficiency, braking distances, highway safety.