National Academies Press: OpenBook

Review of Truck Characteristics as Factors in Roadway Design (2003)

Chapter: Chapter 4 - Design Vehicles

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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 4 - Design Vehicles." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

22 CHAPTER 4 DESIGN VEHICLES This chapter reviews the design vehicles used in the 2001 Green Book and presents recommended changes to the design vehicles for consideration in future editions of the Green Book. This chapter also describes the recommended changes in design vehicles and documents the reasons for these rec- ommended changes. OVERVIEW OF DESIGN VEHICLES The physical characteristics and proportions of vehicles of various sizes that use the highway represent a key control in highway geometric design. Specific design vehicles are pre- sented in the Green Book to represent classes or categories of vehicles. A design vehicle is not intended to represent an aver- age or typical vehicle in its class but, rather, to have larger physical dimensions and a larger minimum turning radius than most vehicles in its class. Thus, geometric design of the roadway to accommodate a specific design vehicle should accommodate most vehicles in the same class as the design vehicles, as well as nearly all vehicles in classes composed of smaller vehicles. The 2001 Green Book presents design vehicle dimensions and turning radii for 19 design vehicles, including 8 trucks. The trucks addressed in the Green Book are as follows: • Single-Unit Truck, SU; • Intermediate Semitrailer, WB-12 [WB-40]; • Intermediate Semitrailer, WB-15 [WB-50]; • Interstate Semitrailer, WB-19 [WB-62]; • Interstate Semitrailer, WB-20 [WB-65 or WB-67]; • “Double-Bottom”-Semitrailer/Trailer, WB-20D [WB- 67D]; • Turnpike Double-Semitrailer/Trailer, WB-33D [WB- 109D]; and • Triple-Semitrailer/Trailers, WB-30T [WB-100T]. Table 19, based on Green Book Exhibit 2-1, presents the dimensions of the design vehicles, and Table 20, based on Green Book Exhibit 2-2, presents their minimum turning radii. The appropriateness for the current and future truck fleet of each of the truck design vehicles, shown in Tables 19 and 20, are discussed below. In addition, other classes of trucks that may merit inclusion are discussed. The Green Book does not specify which design vehicle should be selected for the design of any specific highway proj- ect. This is, and should be, a choice left to the designer who is familiar with local highway and traffic conditions. How- ever, the Green Book does provide some general guidelines to designers on the appropriate selection of design vehicles. Green Book Chapter 2 indicates that [how much is quote?] • A passenger car may be selected when the main traffic generator is a parking lot or series of parking lots. • A single-unit truck may be used for intersection design of residential streets and park roads. • A city transit bus may be used in the design of state highway intersections with city streets that are desig- nated bus routes and that have relatively few large trucks using them. • Depending on expected usage, a large school bus (84 pas- sengers) or a conventional school bus (65 passengers) may be used for the design of intersections of highways with low-volume county highways and township/local roads under 400 ADT. The school bus may also be appropriate for the design of some subdivision street intersections. • The WB-20 [WB-65 or 67] truck should generally be the minimum size design vehicle considered for inter- sections of freeway ramp terminals with arterial cross- roads and for other intersections on state highways and industrialized streets that carry high volumes of traffic and/or that provide local access for large trucks. The Green Book could provide guidance to assist design- ers in selecting trucks as design vehicles in other instances. Such instances are discussed below. The text of potential future Green Book changes is presented in Appendix F. FUTURE CHANGES TO THE U.S. TRUCK FLEET This research was charged with assessing the effect on geometric design of both current and future truck popula- tions. The current truck population has been documented from existing data sources and field data collection (see Chapter 3). Future truck populations are not known and can only be hypothesized. The factors reasonable to consider in

23 TABLE 19 Design vehicle dimensions from the 2001 Green Book (1) Metric Dimensions (m) Overall Overhang Design Vehicle Type Symbol Height Width Length Front Rear WB1 WB2 S T WB3 WB4 Typical Kingpin to Center of Rear Axle Passenger Car P 1.3 2.1 5.8 0.9 1.5 3.4 – – – – – – Single Unit Truck SU 3.4-4.1 2.4 9.2 1.2 1.8 6.1 – – – – – – Buses BUS-12 3.7 2.6 12.2 1.8 1.9e 7.3 1.1 – – – – – Inter-city Bus (Motor Coaches) BUS-14 3.7 2.6 13.7 1.8 2.6e 8.1 1.2 – – – – – City Transit Bus CITY-BUS 3.2 2.6 12.2 2.1 2.4 7.6 – – – – – – Conventional School Bus (65 pass.) S-BUS 11 3.2 2.4 10.9 0.8 3.7 6.5 – – – – – – Large School Bus (84 pass.) S-BUS 12 3.2 2.4 12.2 2.1 4.0 6.1 – – – – – – Articulated Bus A-BUS 3.4 2.6 18.3 2.6 3.1 6.7 5.9 1.9a 4.0a – – – Trucks Intermediate Semitrailer WB-12 4.1 2.4 13.9 0.9 0.8e 3.8 8.4 – – – – 8.4 Intermediate Semitrailer WB-15 4.1 2.6 16.8 0.9 0.6e 4.5 10.8 – – – – 11.4 Interstate Semitrailer WB-19* 4.1 2.6 20.9 1.2 0.8e 6.6 12.3 – – – – 13.0 Interstate Semitrailer WB-20** 4.1 2.6 22.4 1.2 1.4-0.8e 6.6 13.2-13.8 – – – – 13.9-14.5 “Double-Bottom”-Semitrailer/Trailer WB-20D 4.1 2.6 22.4 0.7 0.9 3.4 7.0 0.9b 2.1b 7.0 – 7.0 Triple-Semitrailer/ Trailers WB-30T 4.1 2.6 32.0 0.7 0.9 3.4 6.9 0.9c 2.1c 7.0 7.0 7.0 Turnpike Double-Semitrailer/Trailer WB-33D* 4.1 2.6 34.8 0.7 0.8a 4.4 12.2 0.8d 3.1d 13.6 – 13.0 Recreational Vehicles Motor Home MH 3.7 2.4 9.2 1.2 1.8 6.1 – – – – – – Car and Camper Trailer P/T 3.1 2.4 14.8 0.9 3.1 3.4 – 1.5 5.8 – – – Car and Boat Trailer P/B – 2.4 12.8 0.9 2.4 3.4 – 1.5 4.6 – – – Motor Home and Boat Trailer MH/B 3.7 2.4 16.2 1.2 2.4 6.1 – 1.8 4.6 – – – Farm Tractorf TR 3.1 2.4-3.1 4.9g – – 3.1 2.7 0.9 2.0 – – – NOTE: Since vehicles are manufactured in U.S. Customary dimensions and to provide only one physical size for each design vehicle, the values shown in the design vehicle drawings have been soft converted from numbers listed in feet, and then the numbers in this table have been rounded to the nearest tenth of a meter. * = Design vehicle with 14.63 m trailer as adopted in 1982 Surface Transportation Assistance Act (STAA). ** = Design vehicle with 16.16 m trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA). a = Combined dimension is 5.91 m and articulating section is 1.22 m wide. b = Combined dimension is typically 3.05 m. c = Combined dimension is typically 3.05 m. d = Combined dimension is typically 3.81 m. e = This is overhang from the back axle of the tandem axle assembly. f = Dimensions are for a 150-200 hp tractor excluding any wagon length. g = To obtain the total length of tractor and one wagon, add 5.64 m to tractor length. Wagon length is measured from front of drawbar to rear of wagon, and drawbar is 1.98 m long. • WB1, WB2, and WB4 are the effective vehicle wheelbases, or distances between axle groups, starting at the front and working towards the back of each unit. • S is the distance from the rear effective axle to the hitch point or point of articulation. • T is the distance from the hitch point or point of articulation measured back to the center of the next axle or center of tandem axle assembly. (continued on next page)

24 TABLE 19 (Continued) US Customary Dimensions (ft) Overall Overhang Design Vehicle Type Symbol Height Width Length Front Rear WB1 WB2 S T WB3 WB4 Typical Kingpin to Center of Rear Axle Passenger Car P 4.25 7 19 3 5 11 – – – – – – Single Unit Truck SU 11-13.5 8.0 30 4 6 20 – – – – – – Buses BUS-40 12.0 8.5 40 6 6.3e 24 3.7 – – – – – Inter-city Bus (Motor Coaches) BUS-45 12.0 8.5 45 6 8.5e 26.5 4.0 – – – – – City Transit Bus CITY-BUS 10.5 8.5 40 7 8 25 – – – – – – Conventional School Bus (65 pass.) S-BUS 36 10.5 8.0 35.8 2.5 12 21.3 – – – – – – Large School Bus (84 pass.) S-BUS 40 10.5 8.0 40 7 13 20 – – – – – – Articulated Bus A-BUS 11.0 8.5 60 8.6 10 22.0 19.4 6.2a 13.2a – – – Trucks Intermediate Semitrailer WB-40 13.5 8.0 45.5 3 2.5e 12.5 27.5 – – – – 27.5 Intermediate Semitrailer WB-50 13.5 8.5 55 3 2e 14.6 35.4 – – – – 37.5 Interstate Semitrailer WB-62* 13.5 8.5 68.5 4 2.5e 21.6 40.4 – – – – 42.5 Interstate Semitrailer WB-65** or WB-67 13.5 8.5 73.5 4 4.5-2.5 e 21.6 43.4-45.4 – – – – 45.5-47.5 “Double-Bottom”-Semitrailer/Trailer WB-67D 13.5 8.5 73.3 2.33 3 11.0 23.0 3.0b 7.0b 23.0 – 23.0 Triple-Semitrailer/ Trailers WB-100T 13.5 8.5 104.8 2.33 3 11.0 22.5 3.0c 7.0c 23.0 23.0 23.0 Turnpike Double-Semitrailer/Trailer WB-109D* 13.5 8.5 114 2.33 2.5e 14.3 39.9 2.5d 10.0d 44.5 – 42.5 Recreational Vehicles Motor Home MH 12 8 30 4 6 20 – – – – – – Car and Camper Trailer P/T 10 8 48.7 3 10 11 – 5 19 – – – Car and Boat Trailer P/B – 8 42 3 8 11 – 5 15 – – – Motor Home and Boat Trailer MH/B 12 8 53 4 8 20 – 6 15 – – – Farm Tractorf TR 10 8-10 16g – – 10 9 3 6.5 – – – * = Design vehicle with 48 ft trailer as adopted in 1982 Surface Transportation Assistance Act (STAA). ** = Design vehicle with 53 ft trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA). a = Combined dimension is 19.4 ft and articulating section is 4 ft wide. b = Combined dimension is typically 10.0 ft. c = Combined dimension is typically 10.0 ft. d = Combined dimension is typically 12.5 ft. e = This is overhang from the back axle of the tandem axle assembly. f = Dimensions are for a 150-200 hp tractor excluding any wagon length. g = To obtain the total length of tractor and one wagon, add 18.5 ft to tractor length. Wagon length is measured from front of drawbar to rear of wagon, and drawbar is 6.5 ft long. • WB1, WB2, and WB4 are the effective vehicle wheelbases, or distances between axle groups, starting at the front and working towards the back of each unit. • S is the distance from the rear effective axle to the hitch point or point of articulation. • T is the distance from the hitch point or point of articulation measured back to the center of the next axle or center of tandem axle assembly.

25 hypothesizing future truck populations are (1) the current truck population, (2) current trends in the truck population, and (3) the likelihood of specific future changes in truck size and weight laws or regulations. As noted in Chapter 2, the 1982 STAA required all states to permit trucks with single 14.6-m [48-ft] trailers and twin 8.7-m [28.5-ft] trailers to operate on the National Truck Network. Since 1982, combi- nation trucks with single 16.2-m [53-ft] trailers have become common on the National Network (NN) in many states, and a few states permit combinations with trailers as long as 18.1 m [59.5 ft]. There has been some recent interest in Con- gress in eliminating trailers longer than 16.2 m [53 ft]. Many states, particularly in the West, allow so-called Longer Combination Vehicles (LCVs) to operate, often under permit. LCVs include doubles combinations with trailers longer than 8.7 m [28.5 ft], B-train doubles (doubles combi- nations connected with a B-dolly), Rocky Mountain doubles (combinations with two trailers of unequal length), Turnpike doubles (combinations of two long trailers), and triple-trailer combinations. LCVs are used primarily by segments of the TABLE 20 Minimum turning radii of design vehicles from the 2001 Green Book (1) Metric Design Vehicle Type Pas- senger Car Single Unit Truck Inter-city Bus (Motor Coach) City Transit Bus Conven- tional School Bus (65 pass.) Large2 School Bus (84 pass.) Articu- lated Bus Intermed- iate Semi- trailer Intermed- iate Semi- trailer Symbol P SU BUS-12 BUS-14 CITY-BUS S-BUS11 S-BUS12 A-BUS WB-12 WB-15 Minimum Design Turning Radius (m) 7.3 12.8 13.7 13.7 12.8 11.9 12.0 12.1 12.2 13.7 Center- line1 Turning Radius (CTR) 6.4 11.6 12.4 12.4 11.5 10.6 10.8 10.8 11.0 12.5 Minimum Inside Radius (m) 4.4 8.6 8.4 7.8 7.5 7.3 7.7 6.5 5.9 5.2 Design Vehicle Type Interstate Semi-trailer “Double Bottom” Combina- tion Triple Semi- trailer/ trailers Turnpike Double Semi- trailer/ trailer Motor Home Car and Camper Trailer Car and Boat Trailer Motor Home and Boat Trailer Farm3 Tractor w/One Wagon Symbol WB-19* WB-20** or WB-20 WB-20D WB-30T WB-33D* MH P/T P/B MH/B TR/W Minimum Design Turning Radius (m) 13.7 13.7 13.7 13.7 18.3 12.2 10.1 7.3 15.2 5.5 Center- line1 Turning Radius (CTR) 12.5 12.5 12.5 12.5 17.1 11.0 9.1 6.4 14.0 4.3 Minimum Inside Radius (m) 2.4 1.3 5.9 3.0 4.5 7.9 5.3 2.8 10.7 3.2 NOTE: Numbers in table have been rounded to the nearest tenth of a meter. * = Design vehicle with 14.63 m trailer as adopted in 1982 Surface Transportation Assistance Act (STAA). ** = Design vehicle with 16.16 m trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA). 1 = The turning radius assumed by a designer when investigating possible turning paths and is set at the centerline of the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum design turning radius minus one-half the front width of the vehicle. 2 = School buses are manufactured from 42 passenger to 84 passenger sizes. This corresponds to wheelbase lengths of 3,350 mm to 6,020 mm, respectively. For these different sizes, the minimum design turning radii vary from 8.78 m to 12.01 m and the minimum inside radii vary from 4.27 m to 7.74 m. 3 = Turning radius is for 150–200 hp tractor with one 5.64 m long wagon attached to hitch point. Front wheel drive is disengaged and without brakes being applied.

26 trucking industry that haul bulky, low-density freight. The ability of states to permit new LCV operations has been frozen by Congress (limited to operations that were legal prior to the early 1990s), but LCV volumes are growing where they are permitted and could grow more if the Congressional freeze were ended. The economics of the trucking industry strongly influence the demand for highway agencies to permit larger and heav- ier trucks to operate. Serious consideration has been given in recent years to allowing an increase in truck loads, with- out increase in axle loads, by adding more axles and spac- ing the axles differently to minimize potential impacts on structures and pavements; an example of this is the so-called Turner Truck proposal, named after former Federal High- way Administrator Frank Turner (12). The state of Michi- gan already allows trucks with up to 11 axles to operate with much higher gross weights than are normally allowed by other states. TRB Special Report 267, Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles (13), recently rec- TABLE 20 (Continued) US Customary Design Vehicle Type Pas- senger Car Single Unit Truck Inter-city Bus (Motor Coach) City Transit Bus Conven- tional School Bus (65 pass.) Large2 School Bus (84 pass.) Articu- lated Bus Intermed- iate Semi- trailer Intermed- iate Semi- trailer Symbol P SU BUS-40 BUS-45 CITY-BUS S-BUS36 S-BUS40 A-BUS WB-40 WB-50 Minimum Design Turning Radius (ft) 24 42 45 45 42.0 38.9 39.4 39.8 40 45 Center- line1 Turning Radius (CTR) 21 38 40.8 40.8 37.8 34.9 35.4 35.5 36 41 Minimum Inside Radius (ft) 14.4 28.3 27.6 25.5 24.5 23.8 25.4 21.3 19.3 17.0 Design Vehicle Type Interstate Semi- trailer “Double Bottom” Combina- tion Triple Semi- trailer/ trailers Turnpike Double Semi- trailer/ trailer Motor Home Car and Camper Trailer Car and Boat Trailer Motor Home and Boat Trailer Farm3 Tractor w/One Wagon Symbol WB-62* WB-65** or WB-67 WB-67D WB-100T WB-109D* MH P/T P/B MH/B TR/W Minimum Design Turning Radius (m) 45 45 45 45 60 40 33 24 50 18 Center- line1 Turning Radius (CTR) 41 41 41 41 56 36 30 21 46 14 Minimum Inside Radius (m) 7.9 4.4 19.3 9.9 14.9 25.9 17.4 8.0 35.1 10.5 * = Design vehicle with 48 ft trailer as adopted in 1982 Surface Transportation Assistance Act (STAA). ** = Design vehicle with 53 ft trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA). 1 = The turning radius assumed by a designer when investigating possible turning paths and is set at the centerline of the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum design turning radius minus one-half the front width of the vehicle. 2 = School buses are manufactured from 42 passenger to 84 passenger sizes. This corresponds to wheelbase lengths of 132 in to 237 in, respectively. For these different sizes, the minimum design turning radii vary from 28.8 ft to 39.4 ft and the minimum inside radii vary from 14.0 ft to 25.4 ft. 3 = Turning radius is for 150-200 hp tractor with one 18.5 ft long wagon attached to hitch point. Front wheel drive is disengaged and without brakes being applied.

27 ommended that Federal law be changed to allow two specific truck types to operate under state permits: • Six-axle tractor-semitrailers with a maximum weight of 35,400 kg [90,000 lb]; and • Double-trailer configurations with each trailer up to 10.1 m [33 ft] long; with seven, eight, or nine axles; and with a weight governed by the present Federal bridge formula As noted in Chapter 3, NAFTA is likely to result in increased volumes of trucks entering the United States from Canada and Mexico, but NAFTA does not change the limits imposed by existing U.S. truck size and weight regulations. SINGLE-UNIT TRUCKS Figure 3, based on Green Book Exhibit 2-4, illustrates the dimensions of the current single-unit (SU) truck design vehi- cle. The SU design vehicle is a two-axle truck with an overall length of 9.2 m [30 ft] and a turning radius of 12.8 m [42 ft]. It is potentially confusing that the single-unit truck appears in the upper portion of Table 19, rather than in section of the table labeled “Trucks.” It is recommended that the “Trucks” section of the table should be renamed “Combination Trucks.” There is concern that the AASHTO SU design vehicle is not representative of larger single-unit trucks. The vast major- ity of single-unit trucks on the road are two-axle trucks. How- ever, the truck population includes a substantial number of three- or four-axle SU trucks. The population of these trucks is small compared with the population of two-axle SU trucks, but large when compared with the population of truck types larger than an SU truck. The current SU design vehicle is representative of the largest two-axle trucks currently in use. Table 21 compares the current SU design vehicle with several representative three-axle SU trucks that were evaluated in the FHWA Com- prehensive Truck Size and Weight (CTSW) study (5). Seven of the 10 representative single-unit trucks shown in the table are larger than the SU design vehicle, including the larger van and all of the tank, garbage, grain, and concrete mixer trucks. Based on the data in Tables 11, 13, and 21, the research team recommends that a three-axle SU design vehicle with a wheel- base of 7.6 m [25.0 ft] be added to the Green Book, in addition to the current two-axle SU design vehicle. Figure 4 illustrates the dimensions of the recommended design vehicle. TRUCK TRACTORS There is no AASHTO design vehicle representing a truck tractor (without a trailer), but Figure 5 depicts dimensional data for truck tractors shown in Green Book Exhibit 212. No changes in this exhibit are recommended. Figure 5 depicts the fifth wheel as located directly over the rear axles of each Figure 3. Dimensions of single-unit (SU) truck design vehicle in current Green Book (1). TABLE 21 Dimensions of single-unit trucks—SU design vehicle vs. other representative vehicles Body type Overall lengths m [ft] Box length m [ft] Number of axles Spacing between axles or axle groups m [ft] SU Design Vehicle Van 9.2 [30.0] – 2 6.1 [20.0] Other Representative Vehiclesa Van 9.0 [29.5] 6.1 [20.0] 3 7.6 [25.0] Van 12.0 [39.5] 9.2 [30.0] 3 7.6 [25.0] Tank 9.8 [32.0] 6.9 [22.5] 3 6.3 [20.5] Tank 11.6 [38.0] 8.7 [28.5] 3 7.3 [24.0] Dump 7.5 [24.5] 4.6 [15.0] 3 7.3 [24.0] Dump 9.0 [29.5] 6.1 [20.0] 3 5.8 [19.0] Garbage 9.9 [32.5] 7.0 [23.0] 3 6.3 [20.8] Grain 12.0 [39.5] 9.2 [30.0] 3 7.6 [25.0] Concrete mixer 9.8 [32.0] 6.9 [22.5] 3 6.3 [20.5] Concrete mixer 11.6 [38.0] 8.7 [28.5] 3b 7.3 [24.0] a Representative vehicles taken from Reference 15. b Larger concrete mixers may have four or more axles.

28 tractor; in actual practice, the kingpin is often set forward about 0.3 m [1 ft] from the axle centerline. This forward dis- placement of the kingpin generally has only a small effect on offtracking and swept path width and, therefore, this effect has not been addressed in this report. Figure 5 refers to tractors for Rocky Mountain and Turn- pike doubles configurations. It should be noted that Rocky Mountain doubles combinations are mentioned nowhere else in the Green Book, but should be because they may be more common than Turnpike doubles combinations. SINGLE-TRAILER COMBINATIONS (FIVE-AXLE TRACTOR-SEMITRAILERS) The Green Book includes four design vehicles that are single-trailer combination trucks. These are as follows: • WB-12 [WB-40] design vehicle, with a 10.1-m [33-ft] trailer; • WB-15 [WB-50] design vehicle, with a 13.0-m [42.5-ft] trailer; • WB-19 [WB-62] design vehicle, with a 14.6-m [48-ft] trailer; and • WB-20 [WB-65 or WB-67] design vehicle, with a 16.2-m [53-ft] trailer. The dimensions and turning paths of these vehicles are shown in Figures 6 through 9, based on Green Book Exhibits 2-13 through 2-16. The AASHTO design vehicles are drawn showing these trucks with van-type trailers. In fact, there are many other types of tractor-semitrailer combinations, such as flat-bed, dump, tanker, and container-carrying trailers. Because of vehicle-weight limitations, these other trailer types tend to be shorter than those used for van trailers. Thus, it is reasonable to envision the design vehicles as van trailers. The WB-12 [WB-40] and WB-15 [WB-50] are rarely seen today on highways, with some exceptions discussed below. The AASHTO Green Book states that these design vehicles may be appropriate for design of local roads and streets. Note that this is only true if the locations under consideration do not serve the larger and more common combinations with 14.6-m [48-ft] and 16.2-m [53-ft] trailers. Another use for the WB-12 [WB-40] design vehicle, although with a trailer other than a van, is as a container- carrying vehicle. These trailers are similar to flat-bed trail- ers, but are designed for carrying containers such as are com- monly loaded on ships and trains. It is recommended that the WB-12 [WB-40] design vehicle be retained. The Green Book should state that this design vehicle is appropriate for local streets not used by larger tractor-semitrailers and for access roads to ports and train yards where container traffic may predominate. The WB-15 [WB-50] design vehicle has a 13.0-m [42.5-ft] trailer. This trailer size, or similar trailers with lengths of 12.2 m [40 ft] or 13.7 m [45 ft] was quite common prior to the 1982 STAA. However, since the 1982 STAA mandated that states allow 14.6-m [48-ft] trailers on the NN, trailers in the 12.2 m [40 ft] to 13.7 m [45 ft] length range have largely disappeared. Table 22, based on VIUS data for single-semitrailer trucks by trailer length, shows that trucks in the length range of the WB- 15 [WB-50] truck constitute only 8% of the population of sin- gle-semitrailer combination trucks. By contrast, single-semi- trailer combination trucks with trailer lengths of 13.7 m [45 ft] or more, typically represented in design by either the WB- 19 [WB-62] or a larger design vehicle, constitute more than 65% of the single-semitrailer truck population. Given that the situations in which the WB-15 [WB-50] is an appropriate design vehicle are very limited, it is recommended that this design vehicle be eliminated from the Green Book. The WB-19 [WB-62] design vehicle is a tractor-semitrailer with a 14.6-m [48-ft] trailer. This was, at one time, nearly Width: 8.00 ft Track: 8.00 ft Steering Angle: 31.80° Figure 4. Dimensions of recommended three-axle single-unit (SU-8 [SU-25]) design vehicle.

29 the largest tractor-semitrailer on the highway. The WB-20 [WB-65 or WB-67] with a 16.2-m [53-ft] trailer is now per- haps more common than the 14.6-m [48-ft] trailer. Consider- ation might be given to dropping the WB-19 [WB-62] design vehicle because it has become less common, but it is recom- mended that it be retained because it represents a vehicle size limit specified in Federal law to be allowed to operate any- where on the NN and it represents very closely the offtrack- ing performance of longer trucks with their rear axles pulled forward to meet state KCRT distance requirements. The cur- rent WB-19 [WB-62] design vehicle has a KCRT distance of 12.3 m [40.5 ft]. The most common KCRT distance is 12.5 m [41 ft] because 19 states limit the KCRT distance to about 12.5 m [41 ft] (see Table 2). Therefore, it is recommended that the WB-19 [WB-62] design vehicle be modified slightly in the next edition of the Green Book to incorporate this Figure 5. Lengths of commonly used truck tractors (1).

30 Figure 6. Dimensions of intermediate semitrailer (WB-12 [WB-40]) design vehicle in current Green Book (1). Figure 7. Dimensions of intermediate semitrailer (WB-15 [WB-50]) design vehicle in current Green Book (1). Figure 8. Dimensions of intermediate semitrailer (WB-19 [WB-62]) design vehicle in current Green Book (1).

31 12.5-m [41-ft] dimension. The revised WB-19 [WB-62] design vehicle is illustrated in Figure 10. A variant of the WB-19 [WB-62] configuration is becom- ing more common, especially on flat-bed trailers, and to some extent on van trailers. This variant involves the use of a split or spread tandem axle set at the rear of the trailer. The normal tandem axle set has a nominal spacing of 1.2 m [4 ft] between axles. The spread option moves these axles apart to distances up to 3.1 m [10 ft]. This increases the load-carrying capacity of the tandem from 15,500 to 18,200 kg [34,000 to 40,000 lb], in accordance with the Federal bridge formula. Increases of spread-axle spacing beyond 3.1 m [10 ft] do not provide any increase in load-carrying capacity because the single-axle load limit of 9,100 kg [20,000 lb] becomes a constraint. This increase in loading of the rear tandem axle by spread- ing the rear axles farther apart does not permit the gross vehi- cle weight (GVW) of the truck to legally exceed 36,400 kg [80,000 lb], but it is popular with truckers in that it provides Figure 9. Dimensions of interstate semitrailer (WB-20 [WB-65]) design vehicle in current Green Book (1). . . Figure 10. Recommended revision in the dimensions of interstate semitrailer (WB-19 [WB-62]) design vehicle. TABLE 22 Distribution of trailer lengths for tractor-semitrailers from 1997 VIUS data Number of tractor-semitrailers Estimated vehicle miles traveled (VMT) Length of trailer 1 axle on 1 semi- trailer 2 axles on 1 semi-trailer 3 axles on 1 semi-trailer Total Percentage 1 axle on 1 semi-trailer 2 axles on 1 semi-trailer 3 axles on 1 semi-trailer Sum of row Percentage 1-20 ft 4,913 12,305 1,297 18,515 1.3 49,980,417 295,974,708 28,481,053 374,436,178 0.4 21-28 ft 46,507 48,175 5,244 99,926 6.9 1,290,291,305 1,540,429,521 186,823,947 3,017,544,773 3.3 29-35 ft 20,606 70,709 12,600 103,915 7.2 444,861,237 2,231,595,411 543,069,119 3,219,525,767 3.5 36-40 ft 6,638 184,967 15,753 207,358 14.4 141,988,342 8,114,772,425 597,410,431 8,854,171,198 9.6 41-44 ft 1,460 112,331 6,980 120,771 8.4 41,260,168 6,600,238,638 376,937,640 7,018,436,446 7.6 45-47 ft 6,910 195,385 40,473 242,768 16.9 344,488,954 11,404,888,159 3,107,112,371 14,856,489,484 16.1 48-52 ft 2,733 405,086 24,108 431,927 30.0 159,852,808 32,530,535,430 1,487,776,128 34,178,164,366 37.1 53 ft or more 1,303 188,841 22,905 213,049 14.8 140,054,006 19,312,020,893 1,250,456,865 20,702,531,764 22.4 Totals 91,070 1,217,799 129,360 1,438,229 100.0 2,612,777,237 82,030,455,185 7,578,067,554 92,221,299,976

32 more flexibility in loading the truck, and in keeping the load on the steering axle of the tractor to a more driver-friendly level. With a normal tandem spacing, the maximum GVW of 36,400 kg [80,000 lb] is met by loading both the trailer and tractor tandem axles to their maximum of 15,500 kg [34,000 lb], and the steering axle to 5,500 kg [12,000 lb] (which is about the maximum that truckers want, for comfort and ease of driving.) With a spread tandem at the rear of the trailer, the 36,400-kg [80,000-lb] load can be moved some- what rearward, keeping all axle loads at or below their indi- vidual maxima. As an additional (albeit minor) benefit, spread- ing the axles by an additional 1.8 m [6 ft] also reduces the KCRT distance by 0.9 m [3 ft], reducing the total offtracking. Because spreading the rear axles results in a slight reduc- tion in offtracking and swept path width, the current design vehicles with a 1.2-m [4-ft] axle spacing are more appropri- ate for use as design vehicles than the comparable trucks with 3.1-m [10-ft] spread tandem axles. Therefore, there is no need to consider a tractor-semitrailer design vehicle with a rear spread tandem axle. The WB-20 [WB-65] design vehicle, shown in Figure 9, has a 16.2-m [53-ft] trailer. Tractor-semitrailers with 16.2-m [53-ft] trailers can operate in most states; all but three juris- dictions (i.e., Alaska, the District of Columbia, and Rhode Island) allow trailers of 16.2 m [53 ft] or larger to operate on the NN. All but an additional six states allow them on all state roads. Table 22 shows 22.4% of veh-mi by trucks with single semitrailers with length of 16.2 m [53 ft] or more. However, field studies conducted for this research at rural sites in three states found that trucks with 16.2-m [53-ft] trailers constitute approximately 47% of all combination trucks (see Appendix B). A principal reason why 16.2-m [53-ft] trailers are used so commonly is that a significant number of loaded trucks do not weigh 80,000 lb. Trucks carrying low- or medium-density cargo often “cube out” (their volume becomes filled) before they “gross out” (reach the gross vehicle weight limit). The current Green Book incorporates a design vehicle very similar to the WB-20 [WB-65], known as the WB-20 [WB-67] design vehicle, which also has a 16.2-m [53-ft] trailer. The WB-20 [WB-67] design vehicle, illustrated in Figure 11, is identical to the WB-20 [WB-65] design vehi- cles, except that the rear tandem axle of the WB-20 [WB-67] is positioned closer to the rear of the truck. There is no need for both of these design vehicles to be included in the Green Book, given that they are variations of one another. It makes more sense to include the WB-20 [WB-67] design vehicle, rather than the WB-20 [WB-65], because the WB-20 [WB-67] design vehicle has a greater turning radius, greater offtrack- ing, and greater swept path width. Although the inclusion of the WB-20 [WB-67] design vehicle in the Green Book is recommended, this design vehi- cle will not be applied as widely as might be expected because 19 states limit the KCRT distance to a maximum of about 12.5 m [41 ft]. One state (California) is more stringent, two states (Illinois and Maine) are more liberal, and the remain- ing states do not limit kingpin-to-rear-axle distance. Beyond simply complying with state regulations, many truckers move the rear axles of the trailer forward to improve maneuverability. (Indeed, it is even common to see 14.6-m [48-ft] trailers with their rear axles moved forward, resulting in kingpin-to-rear-axle distances of 11.6 m [38.0 ft] or less.) Although many truckers prefer to move the rear axles of the trailer forward, where practical, the use of 16.2-m [53-ft] trail- ers introduces a potential axle load limitation. If a 36,400-kg [80,000-lb] truck carries a 22,700-kg [50,000-lb] payload, and if that load is spread evenly along a 14.6-m [48-ft] trailer, the individual axle loads are close to the maximums described earlier. However, if such a load is spread evenly along the length of a 16.2-m [53-ft] trailer with the rear trailer axles moved forward, the trailer axle load would reach about 19,500 kg [43,000 lb], an overload of 4,100 kg [9,000 lb]. Stated differently, to keep the trailer axle load within legal limits, a 16.2-m [53-ft] trailer with the axles moved forward could carry no more than about 18,200 kg [40,000 lb] if loaded evenly along the length of the trailer with the axles forward. Figure 11. Recommended dimensions of interstate semitrailer (WB-20 [WB-67]) design vehicle.

33 For other configurations, the use of trailer types other than vans have been discussed. However, it is doubtful that there are many 16.2-m [53-ft] trailers other than vans, because trailer types other than vans are used primarily for higher density cargo for which 16.2-m [53-ft] trailers provide no advantage. If a designer is considering the offtracking and swept path of a tractor-semitrailer combination truck with a 16.2-m [53-ft] trailer with the axles moved forward to maintain a 12.5-m [41-ft] KCRT distance, the WB-19 [WB-62] design vehicle should be used. The offtracking and swept path width of the WB-20 [WB-67] design vehicle with its axles pulled forward is identical to the offtracking and swept path width of the revised WB-19 [WB-62] shown in Figure 10. The rear swingout of the WB-20 [WB-67] with its axles pulled for- ward exceeds the rear swingout of the WB-19 [WB-62] design vehicle by approximately 0.15 m [0.5 ft] for a 90-deg turn with a 15-m [50-ft] radius (see the discussion of this issue in Chapter 5 of this report). Where the length of a tractor-semitrailer combination truck with a 16.2-m [53-ft] trailer is critical to design, its overall length of 22.4 m [73.5 ft] should be used. Eight states permit trucks with trailer lengths greater than 16.2 m [53 ft] to operate on the NN. However, even in these states, trucks with trailer lengths greater than 16.2 m [53 ft] were found to be very rare. Appendix C reports the results of field studies in two states that permit trailer lengths greater than 16.2 m [53 ft]—Kansas and Texas. The field studies con- ducted at weigh stations on major interstate highways found that only 0.7 percent of trucks in Kansas and 4.4 percent of trucks in Texas had trailers whose lengths exceeded 16.2 m [53 ft]. A recent paper by Clayton et al. (14) reported that a field study at a different Texas location found only 0.5 percent of single-semitrailer trucks had lengths over 16.2 m [53 ft]. Based on the data reported above, the inclusion in the Green Book of a design vehicle with a trailer length greater than 16.2 m [53 ft] currently is not recommended. However, an appropriate design vehicle has been developed for future consideration, should the number and proportion of trucks with trailers greater than 16.2 m [53 ft] in length increase. This design vehicle for possible future use is designated as the WB-22 [WB-71] design vehicle and is illustrated in Figure 12. The WB-22 [WB-71] design vehicle has a 17.4-m [57-ft] trailer and a KCRT distance of 15.1 m [49.5 ft] and will off- track substantially more than the other tractor-semitrailer design vehicles (see Chapter 5 of this report). However, many trucks with 17.4-m [57-ft] trailers are operated with maximum 12.5-m [41-ft] KCRT distances to meet state limitations. The offtracking and swept path width of the WB-22 [WB-71] design vehicle with the axles pulled forward can be considered in design using the WB-19 [WB-62] design vehicle. The WB-22 [WB-71] with the axles pulled forward will have rear swingout that exceeds the WB-19 [WB-62] by 0.23 m [0.76 ft] for a 90-deg turn with a 15-m [50-ft] radius (see Chapter 5 of this report). Where the length of the vehicle is critical to design, the overall length of 23.6 m [77.5 ft] for the WB-22 [WB-71] design vehicle should be used. SINGLE-TRAILER COMBINATIONS (SIX-AXLE TRACTOR-SEMITRAILERS) A future change in Federal law may allow states to issue permits for operation of six-axle tractor-semitrailers with max- imum weights up to 35,400 kg [90,000 lb]. Six-axle tractor- semitrailers can operate legally now. However, within current gross vehicle weight and axle limits, there is no particular advantage to using a six-axle combination rather than a five- axle combination. Six-axle tractor-semitrailers are likely to come into common use only if Federal law were to permit Figure 12. Dimensions of long semitrailer (WB-22 [WB-71]) design vehicle for possible future use.

34 six-axle trucks to carry greater loads than five-axle trucks. Such a change in Federal law has been recently recommended in TRB Special Report 267 (13). A six-axle tractor- semitrailer would have a single tractor steering axle, a tan- dem tractor drive axle, and a tridem (triple) axle at the rear of the trailer. Although such a vehicle would be an important factor in pavement and bridge design, it would have little effect on geometric design because six-axle tractor semi- trailers actually offtrack about 5 percent less than compara- ble five-axle tractor-semitrailers (15). Therefore, the possi- ble wider use of six-axle tractor-semitrailer trucks does not constitute a reason to add a new Green Book design vehicle. SINGLE-TRAILER COMBINATIONS (TRUCK/FULL TRAILER COMBINATIONS) A truck/full trailer combination is created when an SU truck has a full trailer attached to it. Trucks pulling small to modest trailers (4.6 m [15 ft] to 6.1 m [20 ft]) are seen quite commonly in the United States. Conceivably, however, such a combination could include a long trailer and exhibit sub- stantial offtracking (more than a tractor-semitrailer combi- nation). In practice, however, such combinations are not used routinely, but rather as needed. SU trucks are not designed to routinely pull a large full trailer—their engines lack the horsepower of over-the-road tractors, and their drivers are usually not accustomed to driving them on a daily basis. The Green Book does not include a truck/full trailer combination as a design vehicle, and there does not appear to be any strong reason to add one. DOUBLE-TRAILER TRUCKS Tractor/Semitrailer/Full Trailer Combinations The Green Book includes two double-trailer trucks as design vehicles. These are as follows: • WB-20D [WB-67D] with “twin” 8.7-m [28.5-ft] trailers and • WB-33D [WB-109D] with two 14.6-m [48-ft] trailers, known as a Turnpike Double. Figures 13 and 14 illustrate their dimensions. Both double- trailer design vehicles represent combination trucks consist- ing of a tractor, coupled to a semitrailer, followed by a towed full trailer. Figure 13. Dimensions of double-trailer combination (WB-20D [WB-67D]) design vehicle in current Green Book (1). Figure 14. Dimensions of turnpike-double combination (WB-33D [WB-109D]) design vehicle in current Green Book (1).

35 The WB-20D [WB-67D] “twin-trailer” truck with two 8.7-m [28.5-ft] trailers has been permitted to operate freely on the NN since 1982 and has become a very common truck on intercity roads. It has smaller offtracking and swept path width than the 14.6-m [48-ft] and 16.2-m [53-ft] tractor-semitrailer combinations discussed above. An advantage of having two shorter trailers, rather than one larger trailer, for over-the-road operations, is that a tractor pulling a single 8.7-m [28.5-ft] trailer can serve as a highly maneuverable pick-up and deliv- ery vehicle. The dimensions of the WB-20D [WB-67D] design vehicle are comparable with those used in the FHWA CTSW Study. The circumstances in which the WB-20D [WB-67D] would be appropriate as a design vehicle are probably quite limited because it has less offtracking and swept path width than the WB-19 [WB-62] and WB-20 [WB-65 and WB-67] tractor-semitrailers that are more numerous and generally travel the same roads. However, it is recommended that the WB-20D [WB-67D] be retained as a design vehicle in the Green Book because it represents a maximum vehicle size limit specified in Federal law. If the current LCV freeze were lifted, one vehicle that might possibly be legalized is a “twin trailer” truck with two 10.1-m [33-ft] trailers. TRB Special Report 267 (13) recently recommended that Federal law be changed to allow such trucks to operate under state permits. It is not recommended that a design vehicle with two 10.1-m [33-ft] trailers be included in the Green Book at this time, but an appropriate design vehicle has been developed for consideration, should such trucks be permitted and become common in the future. This design vehicle for possible future use is designated as the WB-23D [WB-77D] and is illustrated in Figure 15. The WB-33D [WB-109D] or Turnpike Double design vehi- cle consists of a truck with two 14.6-m [48-ft] trailers. This design vehicle acquired its name because it was first permit- ted to operate on a number of Eastern toll roads and turn- pikes. Turnpike Doubles generally operate only under permit on specific roadways approved for their use. The situations in which a Turnpike Double is the appropriate design vehi- cle are typically quite limited because Turnpike Doubles are often made up and broken down at staging areas at the entrances to or exits from specific highway facilities; they do not typically operate beyond that point onto the local road sys- tem. Situations in which a Turnpike Double might be expected to make a right or left turn at an at-grade intersection are rela- tively rare. However, for those roadways where Turnpike Doubles operate in substantial numbers, they may be an appro- priate design vehicle because they will almost certainly be the largest and least maneuverable vehicle on the road. The dimensions of the Turnpike Double combination are reasonable, except that the Green Book uses a cab-over trac- tor with a 3.7-m [12-ft] wheelbase. A conventional tractor with a larger wheelbase—4.9 m [16 ft] or more—would prob- ably be more realistic. However, the effect of the larger trac- tor on offtracking would be minimal, and it is not recom- mended that any change in the WB-33D [WB-109D] design vehicle be made at this time. If the current LCV freeze were lifted, there might be inter- est in the trucking industry for use of a Turnpike Double truck with two 16.2-m [53-ft] trailers. The trucking industry might find it economically advantageous to use such trucks to move low-density commodities because so many 16.2-m [53-ft] trailers are currently in use in single tractor-semitrailer combinations. However, it is far from certain whether such trucks would be permitted to operate by states, even if allowed by Federal law, because such trucks would offtrack more than even the Turnpike Double with 14.6-m [48-ft] trailers. The Turnpike Double with 16.2-m [53-ft] trailers cannot make Figure 15. Dimension of double-trailer combination (WB-23D [WB-77D]) design vehicle for possible future use.

36 a 90-deg right turn with a 22.9-m [75-ft] radius, while a Turn- pike Double with 14.6-m [48-ft] trailers can. It is not recom- mended that a design vehicle with two 16.2-m [53-ft] trailers be included in the Green Book at this time, but an appropriate design vehicle has been developed for future consideration, should such trucks be permitted and become common in the future. This design vehicle for possible future use is designated as the WB-37D [WB-120D] and is illustrated in Figure 16. In addition to the twin- or double-trailer truck and the Turnpike Double truck in the Green Book, there is another combination, the Rocky Mountain Double, that is also fairly common. It appears as a cross between twin-trailer and Turn- pike Double trucks, combining a longer semitrailer and a shorter full trailer. A typical Rocky Mountain Double com- bination has a 14.6-m [48-ft] semitrailer followed by an 8.7-m [28.5-ft] full trailer. Rocky Mountain Doubles currently oper- ate in 20 states (mostly in the western United States), includ- ing 3 states where Turnpike Doubles are not permitted and 6 states where triples are not permitted. In these states, Rocky Mountain Doubles may offtrack more than any other relatively common truck type. There- fore, a Rocky Mountain Double design vehicle is recom- mended for inclusion in the Green Book for potential appli- cation by state highway agencies that need it. The Rocky Mountain Double is designated the WB-28D [WB-92D] design vehicle. The recommended design vehicle has nine axles and an overall length of 30.0 m [98.3 ft] as shown in Figure 17. Figure 16. Dimensions of larger Turnpike Double combination (WB-37D [WB-120D]) design vehicle for possible future use. Figure 17. Recommended dimensions of Rocky Mountain Double combination (WB-28D [WB-92D]) design vehicle.

37 Tractor/Semitrailer/Semitrailer Combinations (B-Trains) B-Train double-trailer trucks are fairly common in Canada and are used to some extent in some of the northern tier of the United States. B-Train doubles are also operating under permit between Monterey, Mexico, and Brownsville, Texas. A B-Train has a hitching mechanism that differs from the common double trailers seen in the United States. The hitch of the U.S. double trailer is typically referred to as an A-hitch. It is essentially a tow bar, fastened at one end to the dolly under the front of the full trailer being towed by the first trailer and at the other end by an eye which is hooked over a pintle hook attached to the first trailer. The second trailer of a B-Train is connected to the first by a fifth-wheel arrange- ment mounted on a dolly, which protrudes from the rear of the first trailer. Thus, the rear trailer in a B-Train double is a semitrailer rather than a full trailer. B-Trains are heavier than their twin-trailer counterparts. Having one less articulation point, B-Trains offtrack slightly more than the U.S. twin-trailers, but are more easily backed up. Most importantly, however, they can carry heavier loads (with their extra axles), so are used for bulk and other types of loads that are particularly heavy. (In the United States, therefore, they are only used in areas where heavier loads are legal.) There is no design vehicle in the Green Book correspond- ing to the B-Train, and because of its limited use in the United States, there is no compelling reason to add it at this time. How- ever, a potential B-Train design vehicle is presented in Fig- ure 18 should future changes in U.S. truck size and weight laws encourage its use. The B-Train is designated the WB-23BD [WB-75BD] design vehicle. Its dimensions are based on a current Canadian design vehicle (16), but might need to be adapted for future U.S. application, depending on which con- figurations become most common in the United States. TRIPLE-TRAILER TRUCKS The Green Book includes one triple-trailer truck as a design vehicle. This is the WB-30T [WB-100T] with three 8.7-m [28.5-ft] trailers—one semitrailer and two full trailers. Fig- ure 19 shows the dimensions for this design vehicle. The WB-30T [WB-100T] is representative of the most common triple-trailer combination on the road today. Larger triple-trailer combinations are not generally permitted, so the WB-30T [WB-100T] is an appropriate design vehicle, and no changes in this design vehicle are recommended. SUMMARY OF DESIGN VEHICLE RECOMMENDATIONS The following changes in or additions to the Green Book design vehicles are recommended. Single-Unit Trucks • The current two-axle SU design vehicle should be retained and designated the SU-30 design vehicle. • A longer three-axle SU design vehicle should be added and designated the SU-8 [SU-25] design vehicle. Single-Trailer Combinations (Five-Axle Tractor-Semitrailers) • The WB-12 [WB-40] should be retained for applica- tion to container trucks and local pickup and delivery operations. • The WB-15 [WB-50] is no longer common and should be dropped. Figure 18. Dimension of B-Train double combination (WB-23BD [WB-75BD]) for possible future use.

38 • The WB-19 [WB-62] design vehicle represents a truck configuration specified in Federal law and should be retained. The KCRT distance for this design vehicle should be increased from 12.3 to 12.5 m [40.5 to 41 ft] to correspond to the maximum limits applicable in 19 states. The WB-19 [WB-62] design vehicle should be used for design of offtracking and swept path width for all longer tractor-semitrailer combinations that are configured with a 12.5-m [41-ft] maximum kingpin-to-center-of-rear- axle-set distance. • The WB-20 [WB-65] design vehicle represents an “in between” axle placement that is neither a best nor a worst case for design. This design vehicle should be dropped. • The WB-20 [WB-67] design vehicle should be retained, but the KCRT distance should be increased from 13.3 to 13.9 m [43.5 to 45.5 ft] to represent a “worst case” condition. • A tractor-semitrailer design vehicle with a trailer length greater than 16.2 m [53 ft] is not needed at this time. However, a WB-22 [WB-71] design vehicle has been developed for future application should such a truck become more common. Single-Trailer Combinations (Six-Axle Trailer-Semitrailer) • No six-axle tractor-semitrailer design vehicle is needed because such trucks are not common at present and because a six-axle tractor-semitrailer is not a critical design consideration because it would offtrack less than a comparable five-axle tractor-semitrailer. Double-Trailer Trucks • The WB-20D [WB-67D] design vehicle with two 8.7-m [28.5-ft] trailers should be retained because it represents a truck configuration specified in Federal law. • A twin-trailer truck with two 10.1-m [33-ft] trailers is not needed at this time but might become more common if Federal law permitted such a truck to operate at gross vehicle weights over 36,400 kg [80,000 lb]. A WB-23D [WB-77D] design vehicle has been developed for poten- tial future application should such a truck become more common. • The Turnpike Double design vehicle with two 14.6-m [48-ft] trailers, known as the WB-33D [WB-109D] design vehicle, should be retained. • A Turnpike Double design vehicle with two 16.2-m [53-ft] trailers is not needed at this time. However, a WB- 37D [WB-120D] design vehicle has been developed for future application should such a truck become common. • A Rocky Mountain Double design vehicle with a 14.6-m [48-ft] semitrailer and a 8.7-m [28.5-ft] full trailer cur- rently operates under permit in 20 states. The addition of a Rocky Mountain Double WB-30D [WB-92D] design vehicle is recommended. • A B-Train double design vehicle is not needed at this time. However, a B-Train WB-23BD [WB-75BD] design vehicle has been developed for future application should this truck become more common. Triple-Trailer Trucks • The current triple-trailer WB-30T [WB-100T] design vehicle with three 8.7-m [28.5-ft] trailers should be retained. Figure 19. Dimensions of triple-trailer combination (WB-30T [WB-100T]) design vehicle in current Green Book (1).

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 505: Review of Truck Characteristics as Factors in Roadway Design presents guidance to roadway geometric designers on how to accommodate large trucks on the U.S. highway system.

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