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Trucks constitute a large and growing segment of the traffic on American highways. On many rural Interstate highways, trucks now constitute more than one-third of the total traffic stream. The increase in truck traffic is related to a strong and growing econ- omy, shifts in manufacturing patterns and inventory reduction through just-in-time delivery, and changing trade patterns resulting from the North American Free Trade Agreement (NAFTA). In addition to growth in truck volumes, the mix of truck types on U.S. highways has shifted toward larger vehicles. Trucks are an important consideration in geometric design of highways. Many high- way geometric design policies are based on vehicle characteristics. Truck characteris- tics are often a key consideration in determining the recommended values of such cri- teria. The research presented in this report reviews the characteristics of trucks in the current U.S truck fleet, as well as possible changes to the truck fleet, and recommends appropriate changes to highway geometric design policy to ensure that highways can reasonably accommodate trucks. The research found that NAFTA may lead to increased truck volumes using U.S. highways, but is unlikely to result in truck types not currently considered in highway geometric design policies entering the United States. Thus, geometric design must con- sider current trends in the United States truck fleet, but there is unlikely to be a need to accommodate truck configurations currently used in Canada and Mexico, but not cur- rently used in the United States. Several changes in the design vehicles presented in the AASHTO Policy on Geomet- ric Design of Highways and Streets, commonly known as the Green Book, are recom- mended. Specifically, it is recommended that the current WB-15 [WB-50] design vehicle be dropped because it is no longer common on U.S. roads. The kingpin-to-center-of-rear- tandem (KCRT) distance for the WB-19 [WB-62] design vehicle should be increased from 12.3 to 12.5 m [40.5 to 41 ft]. The WB-20 [WB-65] design vehicle should be dropped from the Green Book and the WB-20 [WB-67] design vehicle used in its place. In addition, a three-axle truck, the SU-8 [SU-25] design vehicle, and a Rocky Mountain Double, the WB-28D [WB-92D] design vehicle should be added to the Green Book. Four design vehicles, each larger than similar trucks currently on the road, were identified that have no current application, but might be needed if such trucks should SUMMARY REVIEW OF TRUCK CHARACTERISTICS AS FACTORS IN ROADWAY DESIGN
be permitted to operate, or to operate more extensively, on U.S. highways. These four design vehicles include a combination truck with a single 17.4-m [53-ft] semitrailer, designated the WB-22 [WB-71] design vehicle; a combination truck with two 10.1-m [33-ft] trailers, designated the WB-23D [WB-77D] design vehicle; a Turnpike Double combination truck, with two 16.1-m [53-ft] trailers, designated the WB-37D [WB-120D] design vehicle; and a B-Train double combination with one 8.5-m [28-ft] trailer and one 9.6-m [31.5-ft] trailer. There does not appear to be any need to update the current Green Book design cri- teria for sight distance, lane width, horizontal curves, cross slope breaks, or vertical clearance to better accommodate trucks. In each of these cases, an evaluation found that the current geometric design criteria can reasonably accommodate trucks. To assess the critical length of grade for trucks on long, steep upgrades, designers need a more flexible design tool than that available in the current Green Book. The cur- rently available design charts address only one particular truck weight/power ratio, one particular initial truck speed, and a constant percent grade. The research developed a spreadsheet program, known as the truck speed profile model (TSPM), that can esti- mate the truck speed profile on any specified upgrade, considering any truck weight/ power ratio, any initial truck speed, and any vertical profile. Field studies were also conducted to better quantify the weight/power ratios of the current truck fleet; the results of these field studies indicate that trucks in the western states have better per- formance than in the eastern states and the truck population on freeways generally has better performance than the truck population on two-lane highways. It is recommended that the Green Book provide additional guidance on the maxi- mum entry speeds and the diameter of the inscribed circle for roundabouts, as a func- tion of design vehicle characteristics. It is also recommended that designers be provided with additional information on the swept path widths of specific design vehicles for use in the design of double and triple left-turn lanes. The research results indicate that acceleration lane lengths designed to current Green Book criteria may accommodate average trucks, but may not fully accommodate heav- ily loaded trucks, such as the 85th percentile of truck performance. However, there is no indication that heavily loaded trucks are encountering any particular problems related to acceleration lane lengths. Therefore, no change in the design criteria for acceleration lane length is currently recommended, but further research on this issue, to document any problems actually encountered by trucks on acceleration lanes, is recommended. 2
