Truck Speed Profile Models for Critical Length of Grade

The climbing lane is an effective means by which to increase roadway capacity, improve safety, and provide smooth traffic operation on mountainous and rolling terrains where significant trucking prevails. Current critical length design guides for the climbing lane in the American Association of State Highway and Transportation Officials (AASHTO) Green Book is based on truck speed profile charts derived using the design standard of the weight-to-power ratio equal to 120 kg/kW (200 lb/hp). For locations where a significant mix of larger and heavier trucks with more axial loads exists, AASHTO suggests one apply a more representative weight-to-power ratio but gives no specific computational procedure other than a cautionary statement. This study proposes a well-defined approach and formulation to obtain truck speed profiles, based on the nominal dynamic, kinematic, and operating characteristics of trucks on grades. The design controls that regulate the resultant speed profiles include not only the weight-to-power ratio and the grade used in the current AASHTO manual, but also the design engine power of trucks (or the corresponding design load). The formulations, design charts, and tables derived from the design standards consistent with prevailing truck manufacturing specification allow designers to deduce the appropriate critical length for the climbing lane when significant deviation is found between the prevalent truck weight-to-power ratio and the current design standard. For practical design purposes, a tractable approximation model of reasonable accuracy that is suitable for manual calculation is also proposed, thus providing a feasible alternative to the look-up design chart currently employed by AASHTO.