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

Chapter: Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets

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Suggested Citation:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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:"Appendix F - Recommended Changes to the AASHTO Policy on Geometric Design of Highways and Streets." 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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F-1 APPENDIX F RECOMMENDED CHANGES TO THE AASHTO POLICY ON GEOMETRIC DESIGN OF HIGHWAYS AND STREETS This appendix presents recommended changes in the AASHTO Policy on Geometric Design of Highways and Streets (1), commonly known as the Green Book, based on the research presented in this report. The appendix presents appropriate changes to the text of the Green Book and sug- gested modification to Green Book exhibits. For key rec- ommended changes to the Green Book, the modified text is presented in redline format, with additions underlined and deletions indicated with strikethroughs. The rationale for these changes is presented in Chapters 4 and 6 of this report. The appendix is arranged by Green Book chapters, in page- order sequence based on the 2001 edition of the Green Book. Chapter 1—Highway Functions No changes recommended. Chapter 2—Design Controls and Criteria The text of the sections on general characteristics and minimum turning radius of design vehicles from p. 15 to 43 should be revised as follows: GENERAL CHARACTERISTICS Key controls in geometric highway design are the physical characteristics and the proportions of vehicles of various sizes using the highway. Therefore, it is appropriate to examine all vehicle types, establish general class groupings, and select vehicles of representative size within each class for design use. These selected vehicles, with representative weight, dimensions, and operating characteristics, used to establish highway design controls for accommodating vehicles of designated classes, are known as design vehicles. For purposes of geometric design, each design vehicle has larger physical dimensions and a larger minimum turning radius than most vehicles in its class. The largest design vehicles are usually accommodated in freeway design. Four general classes of design vehicles have been established, including: (1) passenger cars, (2) buses, (3) trucks, and (4) recreational vehicles. The passenger-car class includes passenger cars of all sizes, sport/utility vehicles, minivans, vans, and pick-up trucks. Buses include inter-city (motor coaches), city transit, school, and articulated buses. The truck class includes single-unit trucks, truck tractor-semitrailer combinations, and truck tractors with semitrailers in combination with full trailers. Recreational vehicles include motor homes, cars with camper trailers, cars with boat trailers, motor homes with boat trailers, and motor homes pulling cars. In addition, the bicycle should also be considered a design vehicle where bicycle use is allowed on a highway.

Dimensions for 20 design vehicles representing vehicles within these general classes are given in Exhibit 2-1. In the design of any highway facility, the designer should consider the largest design vehicle likely to use that facility with considerable frequency or a design vehicle with special characteristics appropriate to a particular intersection in determining the design of such critical features as radii at intersections and radii of turning roadways. In addition, as a general guide, the following may be considered when selecting a design vehicle: • A passenger car may be selected when the main traffic generator is a parking lot or series of parking lots. • A two-axle single-unit truck may be used for intersection design of residential streets and park roads. • A three-axle single-unit truck may be used for design of collector streets and other facilities where larger single-unit trucks are likely. • A city transit bus may be used in the design of state highway intersections with city streets that are designated bus routes and that have relatively few large trucks using them. • Depending on expected usage, a large school bus (84 passengers) 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-67] truck should generally be the minimum size design vehicle considered for intersections of freeway ramp terminals with arterial crossroads 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. In many cases, operators of WB-20 [WB-67] and larger vehicles pull the rear axles of the vehicle forward to maintain a kingpin-to-rear-axle distance of 12.5 m [41 ft], which makes the truck more maneuverable for the operator and is required by law in many jurisdictions. Where this practice is prevalent, the WB-19 [WB-62] may be used in the design of turning maneuvers, but the WB-20 [WB-67] should be used in design situations where the overall length of the vehicle is considered, such as for sight distance at railroad-highway grade crossings. In addition to the 20 design vehicles, dimensions for a typical farm tractor are shown in Exhibit 2-1, and the minimum turning radius for a farm tractor with one wagon is shown in Exhibit 2-2. Turning paths of design vehicles can be determined from the dimensions shown in Exhibit 2-1 and 2-2 and through the use of commercially available computer programs. F-2

F-3 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 1.9a 7.3 1.1 – – – – – Inter-city Bus (Motor Coaches) BUS-14 3.7 2.6 13.7 1.8 2.6a 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 – – – Combination Trucks Rocky Mountain Double-Semitrailer/Trailer WB-28D 4.1 2.6 13.9 0.7 0.9 5.3 12.3 – 13.0 Intermediate Semitrailer WB-12 4.1 2.4 16.8 0.9 0.8a 3.8 8.4 – – – – 7.8 Interstate Semitrailer WB-19* 4.1 2.6 20.9 1.2 0.8a 6.6 12.3 – – – – 12.5 Interstate Semitrailer WB-20** 4.1 2.6 22.4 1.2 1.4-0.8a 6.6 13.2-13.8 – – – – 13.9 “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 – 6.4 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 6.4 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 – 12.3 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. Single Unit Truck (three-axle) SU-8 3.4-4.1 2.4 12.0 1.3 3.2 25.0 – – – – – – 0.9b 2.1b 7.0 EXHIBIT 2-1 Design vehicle Dimensions—REVISED

F-4 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 Tandem Axle Passenger Car P 4.25 7 19 3 5 11 – – – – – – Single Unit Truck SU 11-13.5 8.0 30 4 6 20 – – – – – – Single Unit Truck (three-axle) SU-25 11-13.5 8.0 39.5 4 10.5 25 – – – – – – Buses BUS-40 12.0 8.5 40 6 6.3a 24 3.7 – – – – – Inter-city Bus (Motor Coaches) BUS-45 12.0 8.5 45 6 8.5a 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 – – – Combination Trucks Rocky Mountain Double-Semitrailer/Trailer WB-92D 13.5 8.5 98.3 2.33 3 17.5 40.5 3.0b 7.0b 23.0 – 42.5 Intermediate Semitrailer WB-40 13.5 8.0 45.5 3 2.5a 12.5 27.5 – – – – 25.5 Interstate Semitrailer WB-62* 13.5 8.5 68.5 4 2.5a 21.6 40.4 – – – – 41.0 Interstate Semitrailer WB-67** 13.5 8.5 73.5 4 4.5-2.5a 21.6 43.4-45.4 – – – – 45.5 “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 – 21.0 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 21.0 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 – 40.5 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. EXHIBIT 2-1 Design vehicle Dimensions—REVISED (continued)

Recent research has developed several design vehicles larger than those presented here, with overall lengths up to 39.3 m [129.3 ft]. These larger design vehicles are not generally needed for design to accommodate the current truck fleet. However, if needed to address conditions at specific sites, their dimensions and turning performance can be found in NCHRP Report 505. MINIMUM TURNING PATHS OF DESIGN VEHICLES Exhibits 2-3 through 2-23 present the minimum turning paths for 20 typical design vehicles. The principal dimensions affecting design are the F-5 Metric Design Vehicle Type Passenger Car Single Unit Truck Single Unit Truck (Three Axle) 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 Symbol P SU SU-8 BUS-12 BUS-14 CITY-BUS S-BUS11 S-BUS12 A-BUS WB-12 Minimum Design Turning Radius (ft) 7.3 12.8 15.7 13.7 13.7 12.8 11.9 12.0 12.1 12.2 Center-line1 Turning Radius (CTR) 6.4 11.6 14.5 12.4 12.4 11.5 10.6 10.8 10.8 11.0 Minimum Inside Radius (ft) 4.4 8.6 11.1 8.4 7.8 7.5 7.3 7.7 6.5 5.9 Design Vehicle Type Interstate Semi-trailer “Double Bottom” Combina- tion Rocky Mtn Double 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-28D WB-30T WB-33D* MH P/T P/B MH/B TR/W Minimum Design Turning Radius (ft) 13.7 13.7 13.7 25.0 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 23.8 12.5 17.1 11.0 9.1 6.4 14.0 4.3 Minimum Inside Radius (ft) 2.4 1.3 5.9 25.1 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. EXHIBIT 2-2 Minimum turning radii of design vehicles—REVISED

minimum centerline turning radius (CTR), the out-to-out track width, the wheelbase, and the path of the inner rear tire. Effects of driver characteristics (such as the speed at which the driver makes a turn) and of the slip angles of wheels are minimized by assuming that the speed of the vehicle for the minimum turning radius is less than 15 km/h [10 mph]. The boundaries of the turning paths of each design vehicle for its sharpest turns are established by the outer trace of the front overhang and the path of the inner rear wheel. This turn assumes that the outer front wheel follows the circular arc defining the minimum centerline turning radius as determined by the vehicle steering mechanism. The minimum F-6 US Customary Design Vehicle Type Passenger Car Single Unit Truck Single Unit Truck (Three Axle) Inter-city Bus (Motor Coach) City Transit Bus Conven- tional School Bus (65 pass.) Large2 3 School Bus (84 pass.) Articu- lated Bus Intermed- iate Semi- trailer Symbol P SU SU-25 BUS-40 BUS-45 CITY-BUS S-BUS36 S-BUS40 A-BUS WB-40 Minimum Design Turning Radius (ft) 24 42 51.5 45 45 42.0 38.9 39.4 39.8 40 Center-line1 Turning Radius (CTR) 21 38 47.5 40.8 40.8 37.8 34.9 35.4 35.5 36 Minimum Inside Radius (ft) 14.4 28.3 36.4 27.6 25.5 24.5 23.8 25.4 21.3 19.3 Design Vehicle Type Interstate Semi-trailer “Double Bottom” Combina- tion Rocky Mtn Double Triple Semi- trailer/ trailers Turnpike Double Semi- trailer/ trailer Motor Home Car and Camper Trailer Car and Boat Trailer Motor Home and Boat Trailer Farm Tractor w/One Wagon Symbol WB-62* WB-65** or WB-67 WB-67D WB-92D WB-100T WB-109D* MH P/T P/B MH/B TR/W Minimum Design Turning Radius (ft) 45 45 45 82.0 45 60 40 33 24 50 18 Center-line1 Turning Radius (CTR) 41 41 41 78.0 41 56 36 30 21 46 14 Minimum Inside Radius (ft) 7.9 4.4 19.3 82.4 9.9 14.9 25.9 17.4 8.0 35.1 10.5 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. EXHIBIT 2-2 Minimum turning radii of design vehicles—REVISED (continued)

radii of the outside and inside wheel paths and the centerline turning radii (CTR) for specific design vehicles are given in Exhibit 2-2. Trucks and buses generally require more generous geometric designs than do passenger vehicles. This is largely because trucks and buses are wider and have longer wheelbases and greater minimum turning radii, which are the principal vehicle dimensions affecting horizontal alignment and cross section. Single-unit trucks and buses have smaller minimum turning radii than most combination vehicles, but because of their greater offtracking, the longer combination vehicles need greater turning path widths. Exhibit 2-11 defines the turning characteristics of a typical tractor/semitrailer combination. Exhibit 2-12 defines the lengths of tractors commonly used in tractor/semitrailer combinations. A combination truck is a single-unit truck with a full trailer, a truck tractor with a semitrailer, or a truck tractor with a semitrailer and one or more full trailers. Because combination truck sizes and turning characteristics vary widely, there are several combination truck design vehicles. These combination trucks are identified by the designation WB, together with the wheel base or another length dimension in both metric and U.S. customary units. The combination truck design vehicles are: (1) the WB-12 [WB-40] design vehicle representative of intermediate size tractor-semitrailer combinations, (2) the WB-19 [WB-62] design vehicle representative of larger tractor semitrailer combinations allowed on selected highways by the Surface Transportation Assistance Act of 1982, (3) the WB-20 [WB-67] design vehicle representative of a larger tractor-semitrailer allowed to operate on selected highways by “grandfather” rights under the Surface Transportation Assistance Act of 1982, (4) the WB-20D [WB-67D] design vehicle representative of a tractor-semitrailer/full trailer (doubles or twin trailer) combination commonly in use, (5) the WB-28D [WB-92D] Rocky Mountain double tractor-semitrailer/full trailer combination with one longer and one shorter trailer used extensively in a number of Western states, (6) the WB-30T [WB-100T] design vehicle representative of tractor-semitrailer/full trailer/full trailer combinations (triples) selectively in use, and (7) the WB-33D [WB-109D] design vehicle representative of larger tractor-semitrailer/full trailer combinations (turnpike double) selectively in use. Although Rocky Mountain doubles, turnpike doubles, and triple trailers are not permitted on many highways, their occurrence does warrant inclusion in this publication. The minimum turning radii and transition lengths shown in the exhibits are for turns at less than 15 km/h [10 mph]. Longer transition curves and larger curve radii are needed for roadways with higher speeds. The radii shown are considered appropriate minimum values for use in design, although skilled drivers might be able to turn with a slightly smaller radius. The dimensions of the design vehicles take into account recent trends in motor vehicle sizes manufactured in the United States and F-7

represent a composite of vehicles currently in operation. However, the design vehicle dimensions are intended to represent vehicle sizes that are critical to geometric design and thus are larger than nearly all vehicles belonging to their corresponding vehicle classes. The turning paths shown in Exhibits 2-3 through 2-10 and Exhibits 2-13 through 2-23 were derived by using commercially available computer programs. The P design vehicle, with the dimensions and turning characteristics shown in Exhibit 2-3, represents a larger passenger car. The SU design vehicle represents a larger single-unit truck. The control dimensions indicate the minimum turning path for most single-unit trucks now in operation (see Exhibit 2-4). On long-distance facilities serving large over-the-road truck traffic or inter-city buses (motor coaches), the design vehicle should generally be either a combination truck or an inter-city bus (see Exhibit 2-5 or Exhibit 2-6). For intra-city or city transit buses, a design vehicle designated as CITY-BUS is shown in Exhibit 2-7. This design vehicle has a wheel base of 7.62 m [25 ft] and an overall length of 12.20 m [40 ft]. Buses serving particular urban areas may not conform to the dimensions shown in Exhibit 2-7. For example, articulated buses, which are now used in certain cities, are longer than a conventional bus, with a permanent hinge near the vehicle’s center that allows more maneuverability. Exhibit 2-10 displays the critical dimensions for the A-BUS design vehicle. Also, due to the importance of school buses, two design vehicles designated as S-BUS 11 [S-BUS 36] and S-BUS 12 [S-BUS 40] are shown in Exhibits 2-8 and 2-9, respectively. The larger design vehicle is an 84-passenger bus and the smaller design vehicle is a 65-passenger bus. The highway designer should also be aware that for certain buses the combination of ground clearance, overhang, and vertical curvature of the roadway may present problems in hilly areas. Exhibits 2-13 through 2-19 show dimensions and the minimum turning paths of the design vehicles that represent various combination trucks. For local roads and streets, the WB-12 [WB-40] is often considered an appropriate design vehicle. The larger combination trucks are appropriate for design of facilities that serve over-the-road trucks. Exhibits 2-20 through 2-23 indicate minimum turning paths for typical recreational vehicles. In addition to the vehicles shown in Exhibits 2-3 through 2-10 and Exhibits 2-13 through 2-23, other vehicles may be used for selected design applications, as appropriate. With the advent of computer programs that can F-8

derive turning path plots, the designer can determine the path characteristics of any selected vehicle if it differs from those shown (1). Exhibit 2-1 (Design Vehicle Dimensions) and Exhibit 2-2 (Minimum Turning Radii of Design Vehicles) should be revised as shown to incorporate the recommended SU-8 [SU-25] and WB-28D [WB-92D] design vehicles and to delete the WB-15 [WB-50] design vehicle. In Exhibit 2-1, it is recommended that the rightmost column be changed from KCRA distance to KCRT distance for two reasons. First, most states that regulate the kingpin-to-rear-axle distance regulate the KCRT distance rather than the KCRA distance. Second, the KCRT distance, rather than the KCRA distance, is illustrated in Exhibits 2-13 through 2-19. Exhibit 2-14 (Minimum Turning Path for Intermediate Semitrailer WB-15 [WB-50] Design Vehicle) should be deleted. New minimum turning path exhibits for the recommended SU-8 [SU-25] and WB-28D [WB-92D] design vehicles should be added. The new exhibits to be used are shown in Figures C-15 and C-20 of this report. Exhibit 2-15 (Minimum Turning Path for Intermediate Semitrailer WB-19 [WB-62] Design Vehicle) needs to be updated to change the KCRT distance from 12.3 to 12.5 m [40.5 to 41.0 ft]. The updated exhibit is presented in Figure 10 in this report. Exhibit 2-16 (Minimum Turning Path for Intermediate Semitrailer WB-20 [WB-65 or WB-67] Design Vehicle) should be modified to apply onto a WB-20 [WB-67] design vehicle with a KCRT distance of 13.9 m [45.5 ft]. The applicable truck is shown in Figure C-7 and the applicable turning plot is shown in Figure C-16. Chapter 3—Elements of Design In Exhibit 3-47 (Track Width for Widening of Traveled Way at Horizontal Curves), it is recommended that the WB-15 [WB-50] design vehicle be deleted and the WB-19 [WB-62] design vehicle be added. In Exhibit 3-48 (Front Overhang for Widening of Traveled Way on Curves), delete the reference to the WB-15 [WB-50] in the legend for Line P and add a reference to the WB-28D [WB-92D] in the legend for Line P and a reference to the SU-8 [SU-25] design vehicle in the legend for Line SU. In the text for Design Values for Traveled Way Widening on p. 214, replace the reference to the WB-15 [WB-50] design vehicle with a reference to the WB-19 [WB-62] design vehicle. In addition, replace Exhibit 3-51 (Calculated and Design Values for Traveled Way Widening on Open Highway Curves [Two-Lane Highways, One-Way or Two-Way]) and Exhibit 3-52 (Adjustments for Traveled Way Widening Values on Open Highway Curves [Two-Lane Highways, One-Way or Two-Way]) with the revised versions presented here. These exhibits have been revised to use the WB-19 [WB-62] design vehicle, rather than the WB-15 [WB-50] design vehicle, as the base. In Exhibit 3-54 (Derived Pavement Widths for Turning Roadways for Different Design Vehicles), delete the column for the WB-15 [WB-50] design vehicle and add columns for the SU-8 [SU-25] and WB-28D [WB-92D] design vehicles. F-9

In the text for Widths for Turning Roadways at Intersections on p. 225, in the discussion of design values for Traffic Condition C, delete the reference to the WB-15 [WB-50] truck. In the box at the top of p. 226, replace the references to the WB-15 [WB-50] with the WB-12 [WB-40]. The note in the second to last paragraph on p. 223 addresses the applicability of larger design vehicles to the cases discussed here. The text of the section on Critical Lengths of Grade for Design on pp. 242 to 247 should be modified as follows: Critical Lengths of Grade for Design Maximum grade in itself is not a complete design control. It is also appropriate to consider the length of a particular grade in relation to desirable vehicle operation. The term “critical length of grade” is used to indicate the maximum length of a designated upgrade on which a loaded truck can operate without an unreasonable reduction in speed. For a given grade, lengths less than critical result in acceptable operation in the desired range of speeds. If the desired freedom of operation is to be maintained on grades longer than critical, design adjustments such as changes in location to reduce grades or addition of extra lanes should be considered. The data for critical lengths of grade should be used with other pertinent factors F-10 Metric Roadway width = 7.2 m Roadway width = 6.6 m Roadway width = 6.0 m Design speed (km/h) Design speed (km/h) Design speed (km/h) Radius of curve (m) 50 60 70 80 90 100 50 60 70 80 90 100 50 60 70 80 90 100 3000 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.3 0.3 0.3 0.3 0.5 0.5 0.6 0.6 0.6 0.6 2500 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.3 0.3 0.3 0.3 0.3 0.5 0.6 0.6 0.6 0.6 0.6 2000 0.0 0.0 0.0 0.0 0.0 0.1 0.3 0.3 0.3 0.3 0.3 0.4 0.6 0.6 0.6 0.6 0.6 0.7 1500 0.0 0.0 0.1 0.1 0.1 0.1 0.3 0.3 0.4 0.4 0.4 0.4 0.6 0.6 0.7 0.7 0.7 0.7 1000 0.2 0.2 0.2 0.3 0.3 0.3 0.5 0.5 0.5 0.6 0.6 0.6 0.8 0.8 0.8 0.9 0.9 0.9 900 0.2 0.2 0.3 0.3 0.3 0.4 0.5 0.5 0.6 0.6 0.6 0.7 0.8 0.8 0.9 0.9 0.9 1.0 800 0.2 0.3 0.3 0.3 0.4 0.4 0.5 0.6 0.6 0.6 0.7 0.7 0.8 0.9 0.9 0.9 1.0 1.0 700 0.3 0.3 0.3 0.4 0.4 0.5 0.6 0.6 0.6 0.7 0.7 0.8 0.9 0.9 0.9 1.0 1.0 1.1 600 0.3 0.4 0.4 0.4 0.5 0.5 0.6 0.7 0.7 0.7 0.8 0.8 0.9 1.0 1.0 1.0 1.1 1.1 500 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 1.1 1.1 1.2 1.2 400 0.6 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 1.1 1.1 1.2 1.2 1.3 1.3 1.4 1.4 300 0.7 0.8 0.8 0.9 1.0 1.0 1.0 1.1 1.1 1.2 1.3 1.3 1.3 1.4 1.4 1.5 1.6 1.6 250 0.8 0.9 1.0 1.0 1.1 1.1 1.2 1.3 1.3 1.4 1.4 1.5 1.6 1.6 1.7 200 1.1 1.2 1.3 1.3 1.4 1.5 1.6 1.6 1.7 1.8 1.9 1.9 150 1.5 1.6 1.7 1.7 1.8 1.9 2.0 2.0 2.1 2.2 2.3 2.3 140 1.6 1.7 1.9 2.0 2.2 2.3 130 1.8 1.9 2.1 2.2 2.4 2.5 120 1.9 2.0 2.2 2.3 2.5 2.6 110 2.1 2.2 2.4 2.5 2.7 2.8 100 2.2 2.3 2.5 2.6 2.8 2.9 90 2.5 2.8 3.1 80 2.8 3.1 3.4 70 3.2 3.5 3.8 NOTES: Values shown are for WB-19 design vehicle and represent widening in meters. For other design vehicles, use adjustments in Exhibit 3-52. Values less than 0.6 m may be disregarded. For 3-lane roadways, multiply above values by 1.5. For 4-lane roadways, multiply above values by 2. EXHIBIT 3-51 Calculated and design values for traveled way widening on open highway curves (two-lane highways, one-way, or two-way)—REVISED

(such as traffic volume in relation to capacity) to determine where added lanes are warranted. To establish design values for critical lengths of grade for which gradeability of trucks is the determining factor, data, or assumptions are needed for the following: 1. Size and power of a representative truck or truck combination to be used as a design vehicle along with the gradeability data for this vehicle: Recent data show that the 85th percentile weight/power ratios for trucks on main highways are typically in the range from 102 to 126 kg/kW [170 to 210 lb/hp] NCHRP Report 505. A typical loaded truck, powered so that the weight/power ratio is about 120 kg/kW [200 lb/hp], is representative of the size and type of vehicle normally used as a design control for main highways. Data in Exhibits 3-59 and 3-60 apply to such a vehicle. More powerful trucks with weight/power ratios in the range from 102 to 108 kg/kW may be appropriate in some Western states, while some two-lane highways that are not major intercity routes may have distinctly different truck populations with weight/power ratios higher than 126 kg/kW [210 lb/hp]. F-11 US Customary Roadway width = 24 ft Roadway width = 22 ft Roadway width = 20 ft Design speed (mph) Design speed (mph) Design speed (mph) Radius of curve (ft) 30 35 40 45 50 55 60 30 35 40 45 50 55 60 30 35 40 45 50 55 60 7000 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.7 0.7 0.8 0.8 0.9 1.0 1.0 1.7 1.7 1.8 1.8 1.9 2.0 2.0 6500 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.7 0.8 0.8 0.9 0.9 1.0 1.1 1.7 1.8 1.8 1.9 1.9 2.0 2.1 6000 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.7 0.8 0.9 0.9 1.0 1.1 1.1 1.7 1.8 1.9 1.9 2.0 2.1 2.1 5500 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.8 0.8 0.9 1.0 1.0 1.1 1.2 1.8 1.8 1.9 2.0 2.0 2.1 2.2 5000 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.8 0.9 1.0 1.0 1.1 1.2 1.2 1.8 1.9 2.0 2.0 2.1 2.2 2.2 4500 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.9 0.9 1.0 1.1 1.2 1.2 1.3 1.9 1.9 2.0 2.1 2.2 2.2 2.3 4000 0.2 0.2 0.2 0.3 0.4 0.4 0.5 1.0 1.1 1.2 1.3 1.4 1.4 1.5 2.0 2.1 2.2 2.3 2.4 2.4 2.5 3500 0.2 0.2 0.3 0.4 0.5 0.5 0.6 1.1 1.2 1.3 1.4 1.5 1.5 1.6 2.1 2.2 2.3 2.4 2.5 2.5 2.6 3000 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2500 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.5 2.6 2.7 2.8 2.9 3.0 3.1 2000 0.7 0.8 0.9 1.0 1.1 1.3 1.4 1.7 1.8 1.9 2.0 2.1 2.3 2.4 2.7 2.8 2.9 3.0 3.1 3.3 3.4 1800 0.9 1.0 1.2 1.3 1.4 1.5 1.6 1.9 2.0 2.2 2.3 2.4 2.5 2.6 2.9 3.0 3.2 3.3 3.4 3.5 3.6 1600 1.1 1.2 1.3 1.4 1.6 1.7 1.8 2.1 2.2 2.3 2.4 2.6 2.7 2.8 3.1 3.2 3.3 3.4 3.6 3.7 3.8 1400 1.3 1.5 1.6 1.7 1.9 2.0 2.1 2.3 2.5 2.6 2.7 2.9 3.0 3.1 3.3 3.5 3.6 3.7 3.9 4.0 4.1 1200 1.6 1.7 1.9 2.0 2.2 2.3 2.4 2.6 2.7 2.9 3.0 3.2 3.3 3.4 3.6 3.7 3.9 4.0 4.2 4.3 4.4 1000 2.0 2.2 2.3 2.5 2.6 2.8 3.0 3.0 3.2 3.3 3.5 3.6 3.8 4.0 4.0 4.2 4.3 4.5 4.6 4.8 5.0 900 2.3 2.5 2.7 2.8 3.0 3.2 3.3 3.5 3.7 3.8 4.0 4.2 4.3 4.5 4.7 4.8 5.0 5.2 800 2.7 2.9 3.0 3.2 3.4 3.6 3.7 3.9 4.0 4.2 4.4 4.6 4.7 4.9 5.0 5.2 5.4 5.6 700 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 600 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 500 4.6 4.8 5.0 5.2 5.6 5.8 6.0 6.2 6.6 6.8 7.0 7.2 450 5.1 5.3 5.5 6.1 6.3 6.5 7.1 7.3 7.5 400 5.8 6.0 6.3 6.8 7.0 7.3 7.8 8.0 8.3 350 6.7 7.0 7.2 7.7 8.0 8.2 8.7 9.0 9.2 300 7.8 8.1 8.8 9.1 9.8 10.1 250 9.4 10.4 11.4 200 11.8 12.8 13.8 NOTES: Values shown are for WB-62 design vehicle and represent widening in feet. For other design vehicles, use adjustments in Exhibit 3-52. Values less than 2.0 ft may be disregarded. For 3-lane roadways, multiply above values by 1.5. For 4-lane roadways, multiply above values by 2. EXHIBIT 3-51 Calculated and design values for traveled way widening on open highway curves (two-lane highways, one-way, or two-way)—REVISED (continued)

2. Speed at entrance to critical length of grade: The average running speed as related to design speed can be used to approximate the speed of vehicles beginning an uphill climb. This estimate is, of course, subject to adjustment as approach conditions may determine. Where vehicles approach on nearly level grades, the running speed can be used directly. For a downhill approach it should be increased somewhat, and for an uphill approach it should be decreased. 3. Minimum speed on the grade below in which interference to following vehicles is considered unreasonable: No specific data are available on which to base minimum tolerable speeds of trucks on upgrades. It is logical to assume that such minimum speeds should be in direct relation to the design speed. Minimum truck speeds of about 40 to 60 km/h [25 to 40 mph] for the majority of highways (on which design speeds are about 60 to 100 km/h [40 to 60 mph]) probably are not unreasonably annoying to following drivers unable to pass on two-lane roads, if the time interval during which they are unable to pass is not too long. The time interval is not likely to be annoying on two-lane roads with volumes well below their capacities, whereas it is likely to be F-12 Metric US customary Design vehicle Design vehicle Radius of curve (m) SU WB-12 WB-20 WB-20D WB-30T WB-33D Radius of curve (ft) SU WB-40 WB-67 WB-67D WB-100T WB-109D 3000 –0.3 –0.3 0.0 0.0 0.0 0.1 7000 –1.2 –1.2 0.0 –0.1 –0.1 0.2 2500 –0.3 –0.3 0.0 0.0 0.0 0.1 6500 –1.2 –1.2 0.0 –0.1 0.0 0.2 2000 –0.3 –0.3 0.0 0.0 0.0 0.1 6000 –1.3 –1.2 0.1 –0.1 0.0 0.2 1500 –0.4 –0.3 0.1 0.0 0.0 0.1 5500 –1.3 –1.2 0.1 –0.1 0.0 0.3 1000 –0.5 –0.5 0.0 0.0 –0.1 0.1 5000 –1.3 –1.2 0.1 –0.1 0.0 0.3 900 –0.5 –0.5 0.0 0.0 –0.1 0.1 4500 –1.3 –1.2 0.1 –0.1 0.0 0.4 800 –0.5 –0.5 0.0 0.0 –0.1 0.1 4000 –1.4 –1.4 0.0 –0.3 –0.1 0.3 700 –0.5 –0.5 0.0 0.0 –0.1 0.2 3500 –1.5 –1.4 0.1 –0.3 –0.1 0.4 600 –0.6 –0.5 0.0 0.0 0.0 0.2 3000 –1.5 –1.4 0.1 –0.3 –0.1 0.5 500 –0.6 –0.5 0.1 0.0 0.0 0.3 2500 –1.7 –1.5 0.1 –0.4 –0.2 0.5 400 –0.7 –0.6 0.0 0.0 –0.1 0.3 2000 –1.8 –1.6 0.2 –0.4 –0.1 0.7 300 –0.8 –0.7 0.1 –0.3 –0.1 0.4 1800 –1.9 –1.7 0.1 –0.5 –0.2 0.7 250 –0.9 –0.7 0.1 –0.3 –0.1 0.6 1600 –2.0 –1.8 0.2 –0.5 –0.2 0.9 200 –1.1 –0.9 0.1 –0.4 –0.1 0.7 1400 –2.2 –1.9 0.1 –0.7 –0.3 1.0 150 –1.3 –1.1 0.2 –0.5 –0.2 0.9 1200 –2.3 –2.0 0.3 –0.7 –0.2 1.2 140 –1.3 –1.1 0.2 –0.5 –0.2 1.0 1000 –2.6 –2.2 0.3 –0.8 –0.3 1.4 130 –1.5 –1.2 0.1 –0.7 –0.3 1.0 900 –2.8 –2.4 0.3 –0.9 –0.3 1.6 120 –1.6 –1.3 0.2 –0.7 –0.2 1.1 800 –3.0 –2.6 0.3 –1.1 –0.4 1.8 110 –1.7 –1.4 0.2 –0.8 –0.3 1.1 700 –3.3 –2.8 0.4 –1.2 –0.4 2.0 100 –1.8 –1.5 0.2 –0.8 –0.3 1.3 600 –3.7 –3.1 0.4 –1.5 –0.5 2.3 90 –2.0 –1.6 0.2 –0.9 –0.4 1.4 500 –4.2 –3.5 0.5 –1.7 –0.6 2.8 80 –2.2 –1.8 0.3 –1.0 –0.4 1.6 450 –4.6 –3.8 0.6 –1.9 –0.7 3.2 70 –2.5 –2.0 0.3 –1.2 –0.4 1.9 400 –5.0 –4.1 0.7 –2.1 –0.8 3.5 350 –5.7 –4.7 0.7 –2.5 –0.9 4.0 300 –6.5 –5.2 0.8 –2.9 –1.1 4.7 250 –7.5 –6.1 1.1 –3.5 –1.2 5.7 200 –9.2 –7.4 1.3 –4.4 –1.6 7.2 NOTES: Adjustments are applied by adding to or subtracting from the values in Exhibit 3-51. Adjustments depend only on radius and design vehicle; they are independent of roadway width and design speed. For 3-lane roadways, multiply above values by 1.5. For 4-lane roadways, multiply above values by 2.0. EXHIBIT 3-52 Adjustments for traveled way widening values on open highway curves (two-lane highways, one-way, or two- way)—REVISED

annoying on two-lane roads with volumes near capacity. Lower minimum truck speeds can probably be tolerated on multilane highways rather than on two-lane roads because there is more opportunity for and less difficulty in passing. Highways should be designed so that the speeds of trucks will not be reduced enough to cause intolerable conditions for following drivers. Studies show that, regardless of the average speed on the highway, the more a vehicle deviates from the average speed, the greater its chances of becoming involved in a crash. One such study (41) used the speed distribution of vehicles traveling on highways in one state, and related it to the crash involvement rate to obtain the rate for trucks of four or more axles operating on level grades. The crash involvement rates for truck speed reductions of 10, 15, 25, and 30 km/h [5, 10, 15, and 20 mph] were developed assuming the reduction in the average speed for all vehicles on a grade was 30 percent of the truck speed reduction on the same grade. The results of this analysis are shown in Exhibit 3-62. A common basis for determining critical length of grade is based on a reduction in speed of trucks below the average running speed of traffic. The ideal would be for all traffic to operate at the average speed. This, however, is not practical. In the past, the general practice has been to use a reduction in truck speed of 25 km/h [15 mph] below the average running speed of all traffic to identify the critical length of grade. As shown in Exhibit 3-62, the crash involvement rate increases significantly when the truck speed reduction exceeds 15 km/h [10 mph] with the involvement rate being 2.4 times greater for a 25-km/h [15-mph] reduction than for a 15-km/h [10-mph] reduction. On the basis of these relationships, it is recommended that a 15-km/h [10-mph] reduction criterion be used as the general guide for determining critical lengths of grade. The length of any given grade that will cause the speed of a representative truck (120 kg/kW [200 lb/hp]) entering the grade at 110 km/h [70 mph] to be reduced by various amounts below the average running speed of all traffic is shown graphically in Exhibit 3-63, which is based on the truck performance data presented in Exhibit 3-59. The curve showing a 15-km/h [10-mph] speed reduction is used as the general design guide for determining the critical lengths of grade. Similar information on the critical length of grade for recreational vehicles may be found in Exhibit 3-64, which is based on the recreational vehicle performance data presented in Exhibit 3-61. Where the entering speed is less than 110 km/h [70 mph], as may be the case where the approach is on an upgrade, the speed reductions shown in Exhibits 3-63 and 3-64 will occur over shorter lengths of grade. Conversely, where the approach is on a downgrade, the probable approach speed is greater than 110 km/h [70 mph] and the truck or recreational vehicle will ascend a greater length of grade than shown in the exhibits before the speed is reduced to the values shown. F-13

The method of using Exhibit 3-63 to determine critical lengths of grade is demonstrated in the following examples. Assume that a highway is being designed for 100 km/h [60 mph] and has a fairly level approach to a 4 percent upgrade. The 15-km/h [10-mph] speed reduction curve in Exhibit 3-63 shows the critical length of grade to be 350 m [1,200 ft]. If, instead, the design speed was 60 km/h [40 mph], the initial and minimum tolerable speeds on the grade would be different, but for the same permissible speed reduction the critical length would still be 360 m [1,200 ft]. In another instance, the critical length of a 5 percent upgrade approached by a 500-m [1,650-ft] length of 2 percent upgrade is unknown. Exhibit 3-63 shows that a 2 percent upgrade of 500 m [1,650 ft] in length would result in a speed reduction of about 9 km/h [6 mph]. The chart further shows that the remaining tolerable speed reduction of 6 km/h [4 mph] would occur on 100 m [325 ft] of the 5 percent upgrade. Where an upgrade is approached on a momentum grade, heavy trucks often increase speed, sometimes to a considerable degree in order to make the climb in the upgrade at as high a speed as practical. This factor can be recognized in design by increasing the tolerable speed reduction. It remains for the designer to judge to what extent the speed of trucks would increase at the bottom of the momentum grade above that generally found on level approaches. It appears that a speed increase of about 10 km/h [5 mph] can be considered for moderate downgrades and a speed increase of 15 km/h [10 mph] for steeper grades of moderate length or longer. On this basis, the tolerable speed reduction with momentum grades would be 25 or 30 km/h [15 or 20 mph]. For example, where there is a moderate length of 4 percent downgrade in advance of a 6 percent upgrade, a tolerable speed reduction of 25 km/h [15 mph] can be assumed. For this case, the critical length of the 6 percent upgrade is about 300 m [1,000 ft]. The critical length of grade in Exhibit 3-63 is derived as the length of tangent grade. Where a vertical curve is part of a critical length of grade, an approximate equivalent tangent grade length should be used. Where the condition involves vertical curves of Types II and IV shown later in this chapter in Exhibit 3-73 and the algebraic difference in grades is not too great, the measurement of critical length of grade may be made between the vertical points of intersection (VPI). Where vertical curves of Types I and III in Exhibit 3-73 are involved, about one-quarter of the vertical curve length should be considered as part of the grade under consideration. In many design situations, Exhibit 3-63 may not be directly applicable to the determination of the critical length of grade for one of several reasons. First, the truck population for a given site may be such that a weight/power ratio either less than or greater than the value of 120 kg/kW F-14

assumed in Exhibit 3-63 may be appropriate as a design control. Second, for the reasons described above, the truck speed at the entrance to the grade may differ from the value of 110 km/h [70 mph] assumed in Exhibit 3-63. Third, the profile may not consist of a constant percent grade. In such situations, a spreadsheet program, known as the Truck Speed Profile Model (TSPM), is available and may be used to generate speed truck profiles for any specified truck weight/power ratio, any specified initial truck speed, and any specified sequence of grades. Steep downhill grades can also have a detrimental effect on the capacity and safety of facilities with high traffic volumes and numerous heavy trucks. Some downgrades are long and steep enough that some heavy vehicles travel at crawl speeds to avoid loss of control on the grade. Slow-moving vehicles of this type may impede other vehicles. Therefore, there are instances where consideration should be given to providing a truck lane for downhill traffic. Procedures have been developed in the HCM (14) to analyze this situation. The suggested design criterion for determining the critical length of grade is not intended as a strict control but as a guideline. In some instances, the terrain or other physical controls may preclude shortening or flattening grades to meet these controls. Where a speed reduction greater than the suggested design guide cannot be avoided, undesirable type of operation may result on roads with numerous trucks, particularly on two-lane roads with volumes approaching capacity and in some instances on multilane highways. Where the length of critical grade is exceeded, consideration should be given to providing an added uphill lane for slow-moving vehicles, particularly where volume is at or near capacity and the truck volume is high. Data in Exhibit 3-63 can be used along with other pertinent considerations, particularly volume data in relation to capacity and volume data for trucks, to determine where such added lanes are warranted. Chapter 4—Cross Section Elements No changes recommended. Chapter 5—Local Roads and Streets No changes recommended. Chapter 6—Collector Roads and Streets No changes recommended. F-15

Chapter 7—Rural and Urban Arterials No changes recommended. Chapter 8—Freeways No changes recommended. Chapter 9—Intersections In the discussion on Minimum Edge-of-Traveled-Way Designs on p. 587, eliminate the reference to the WB-50 design vehicle and add references to the SU-8 [SU-25] and WB-28D [WB-92D] design vehicles. Also, change the references to Exhibits 2-3 through 2-23, as appropriate, to reflect the recommended changes in Chapter 2. In Exhibit 9-19 (Edge-of-Traveled-Way Designs for Turns at Intersections) and Exhibit 9-20 (Edge of Traveled Way for Turns at Intersections), delete the rows for the WB-15 [WB-50] design vehicles and add rows for the SU-8 [SU-25] and WB-28D [WB-92D] design vehicles. In the section on Design for Specific Conditions (Right-Angle Turns) on p. 596 to 625, delete references to the WB-15 [WB-50] design vehicle and add references to the SU-8 [SU-25] design vehicle. References to the WB-15 [WB-50] design vehicle should be replaced with the WB-12 [WB-40] design vehicle or the WB-19 [WB-62] design vehicle, as appropriate. Add a new exhibit after Exhibit 9-22 to present minimum traveled way designs for the new SU-8 [SU-25 design vehicle]. Delete Exhibit 9-24 (Minimum Edge-of-Traveled-Way Designs WB-15 [WB-50] Design Vehicle Path). In Exhibit 9-29 (Effect of Curb Radii on Right Turning Paths of Various Design Vehicles) and Exhibit 9-30 (Effect of Curb Radii on Right Turning Paths of Various Design Vehicles), delete the line for the WB-15 [WB-50] design vehicle and use the WB-19 [WB-62] design vehicle instead. In Exhibit 9-31 (Cross Street Width Occupied by Turning Vehicle for Various Angles of Intersection and Curb Radii) and Exhibit 9-32 (Effect of Curb Radii and Parking on Right Turning Paths), delete the rows for the WB-15 [WB-50] design vehicle and add rows for the SU-8 [SU-12] design vehicle. The fourth paragraph on p. 625 should be edited as follows: The WB-19 [WB-62] and larger trucks generally are used principally for “over-the-road” transportation between trucking terminals or industrial or commercial areas. Ideally, such destinations are located near F-16

major highway facilities that are designed to accommodate the larger combination units. Such trucks may be present on urban arterials, but seldom turn into or out of local urban streets. Exhibit 9-41 (Minimum Turning Roadway Designs with Corner Islands at Urban Locations) should be modified to replace the WB-15 [WB-50] with the WB-19 [WB-62] design vehicle. In Exhibit 9-42 (Exhibit 9-42. Typical Designs for Turning Roadways), Design Classification C should be modified to address the WB-19 [WB-62] design vehicle rather than the WB-15 [WB-50] design vehicle. Exhibit 9-76 (Control Radii at Intersections for 90-Degree Left Turns) should be modified to replace the WB-15 [WB-50] and WB-20 [WB-67] design vehicles with the WB-19 [WB-62] design vehicle. The text on p. 697 should be modified accordingly. Exhibit 9-78 (Minimum Design of Median Openings—P Design Vehicle, Control Radius of 12 m [40 ft]), Exhibit 9-81 (Minimum Design of Median Openings—SU Design Vehicle, Control Radius of 15 m [50 ft]), Exhibit 9-82 (Minimum Design of Median Openings—WB-12 [WB-40] Design Vehicle, Control Radius of 23 m [75 ft]), and Exhibit 9-83 (Minimum Design of Median Openings—Radius of 30 m [100 ft]) should be modified to replace the WB-15 [WB-50] design vehicle with the WB-19 [WB-62] design vehicle. In the sections on Median Openings Based on Control Radii for Design Vehicles and Effect of Skew on p. 702 through 706, delete the references to the WB-15 [WB-50] design vehicle. In Exhibit 9-92 (Minimum Designs for U-turns), delete the column for the WB-15 [WB-50] design vehicle. Add a column for the SU-8 [SU-25] design vehicle and replace the WB-18 [WB-60] design vehicle with the WB-19 [WB-62] design vehicle. On p. 726, insert the following new section after the fourth full paragraph: Double or Triple Left-Turn Lanes Offtracking and swept path width are important factors in designing double and triple left-turn lanes. At such locations, vehicles should be able to turn side-by-side without encroaching upon the adjacent turn lane. A desirable turning radius for double or triple left-turn lane is 27 m [90 ft] which will accommodate the P, SU, SU12 [SU40], and WB-12 [WB-40] design vehicles within a swept path width of 3.6 m [12 ft]. Larger vehicles need greater widths to negotiate double or triple left-turn lanes constructed with a 27 m [90 ft] turning radius without encroaching on the paths of vehicles in the adjacent lane. Exhibit 9-## illustrates the swept path widths for specific design vehicles making 90° left turns. Exhibit 9-## can be used to determine width needed at the center of a turn where the maximum vehicle offtracking typically occurs. To help drivers maintain their vehicles within the proper lanes, it is recommended that the longitudinal lane line markings of double or triple left-turn lanes be extended through the F-17

intersection area to provide positive guidance. This type of pavement marking extension provides a visual cue for lateral positioning of the vehicle as the driver makes a turning maneuver. Chapter 10—Grade Separations and Interchanges No changes recommended. It is recommended that minimum acceleration lengths for trucks be considered in future research. F-18 Swept Path Width (m) for Specific Design Vehicles Centerline Turning Radius (m) SU SU-8 WB-19 WB-20D 23 3.3 3.7 6.4 5.1 30 3.0 3.4 5.6 4.4 46 2.8 3.1 4.6 3.8 Swept Path Width (ft) for Specific Design Vehicles Centerline Turning Radius (ft) SU SU-25 WB-62 WB-67D 75 10.7 12.3 21.1 16.6 100 9.8 11.2 18.4 14.7 150 9.1 10.1 15.2 12.5 EXHIBIT 9-## Swept path widths for 90º left turns

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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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