Modeling 85th percentile speed differential for evaluating geometric design consistency on two-lane rural highways

Operating speed is one of the essential criteria for evaluating mobility and safety on highways. Hence, the present study investigates highway design consistency using operating speed on a tangent-to-curve transition on two-lane rural highways. The speed data collected at various horizontal curve sections and the preceding tangents of the curve sections are analyzed to assess the 85th percentile speed of vehicle types such as Car, Two-wheeler (2W), Three-wheeler (3W), Light commercial vehicle (LCV), and Heavy commercial vehicle (HCV). The difference in operating speed on curve and approach tangent sections was estimated to find the differential speeds of each vehicle type. Further, the 85th percentile speed differential (∆85V) estimated at a location is compared with a differential of 85th percentile operating speed (∆V₈₅) measured at same location. Sensitivity analysis has been performed to verify the significant parameters on ∆₈₅V of various vehicle types. The study develops all subset regression models to forecast operating speed reduction on horizontal curve locations by examining the relation between ∆₈₅V and ∆V₈₅. The findings showed that the ∆V₈₅ undervalues ∆₈₅V by 5.0 kmph, 4.0 kmph, 3.0 kmph, 6.5 kmph and 9.0 kmph for Car, 2W, 3W, LCV and HCV, respectively. The operating speed differential model of vehicle type Car is used for developing the nomograms for assessing the road geometric safety on two-lane rural highways. The proposed nomograms are helpful to the highway practitioners for finding the possible combinations of various geometric parameters for better safety and consistency in highway geometric design.