Determining Critical Length of Grade for Geometric Design of Vertical Alignments

In the 2001 Green Book, critical length of grade is generally defined as the length of grade that would produce a speed reduction of 15 km/h (10 mph) for a 120-kg/kW (200-lb/hp) truck. The Green Book does not provide an appropriate tool for highway engineers to calculate critical length of grade for locations where a truck with a weight/power ratio other than 120 kg/kW (200 lb/hp) is appropriate. Research was conducted to determine the distribution of truck weight/power ratios in the current truck fleet in several regions of the United States. The results suggest that, for design purposes, a truck with a weight/power ratio of 102 to 108 kg/kW (170 to 180 lb/hp) would be appropriate for freeways in California and Colorado, although a weight/power ratio of 126 kg/kW (210 lb/hp) would be more appropriate in Pennsylvania. For two-lane highways, a 108-kg/kW (180-lb/hp) design vehicle may be appropriate in Colorado, although less powerful design vehicles in the range of 150 to 168 kg/kW (250 to 280 lb/hp) may be appropriate for California and Pennsylvania. A truck speed profile model also was developed that enables highway designers to calculate the speed profile of trucks with any given weight/power ratio and for any given vertical alignment. This design tool will improve the ability to calculate critical length of grades for any situation.