Methods for Predicting Truck Speed Loss on Grades

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Methods for Predicting Truck Speed Loss on Grades

T. D. Gillespie

Truck speed loss on grades reduces highway capacity and increases the risk of accidents. The rational design of a truck climbing lane as a solution to this problem requires means for predicting truck speed changes on grades. Experimental measurements of the speed loss of trucks operating on highways were conducted at 20 sites throughout the country. These data were analyzed to compare performance to present guidelines for highway design embodied in the AASHTO Policy on Geometric Design of Highways and Streets. Methods were developed for modeling the hill-climbing performance of the four major truck classes at the 12.5 and 50 percentile population level using empirically determined weight-to-power values. Trucks pulling trailers, and doubles and triples are the truck classes with lowest hill-climbing performance. For the limited data obtained, the AASHTO model appears to provide a reasonable performance prediction for the 12.5 percentile population. Methods for estimating performance at the 12.5 percentile level for mixed truck populations is recognized.