Reliability-Based Specification on Critical Length of Highway Sections with Near-Maximum Grade
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2018, Korean Society of Civil Engineers. In Chinas current highway design specifications, the length of an upgrade segment is strictly limited when its grade is greater than or equal to 3%. For expressways in flat and rolling terrains, the design speed is commonly at 120 km/h [75 mph] and the corresponding maximum grade length must not exceed 900 m. However, there is no specific length limitation on any grade segments even though some of them may be just slightly below the maximum grade of 3%. This paper presents a probabilistic method to specify the proper critical segment lengths of Near-Maximum Grade (NMG) with a full consideration of expressway safety. Terminologically, the NMG in this paper refers to a set of longitudinal slopes with grades that are very close to the maximum grade of 3%, such as grades at 2.9%, 2.8%, and 2.7%. A long section with NMG on Nanjing-Hangzhou Expressway was chosen as the study case and its crash data, road geometry and vehicle operational performance were all carefully recorded. Reliability-based methods were employed to process the derived data to specify the critical length for this highway section. The results show that Chinas regulated values for grade lengths are slightly greater for trucks, partially attributed to poor dynamic performance including climbing and braking. Finally, a set of reliable limiting values for NMG section lengths are developed. These can provide more suitable parameters for highway design to achieve enhanced operational safety performance. The findings of this study are expected to provide a practical reference for future revisions of highway design specifications in China.
