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Highway and Street Design Vehicles: An Update (2023)

Chapter: Chapter 6 - Turning Performance of Updated Design Vehicles

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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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Suggested Citation:"Chapter 6 - Turning Performance of Updated Design Vehicles." National Research Council. 2023. Highway and Street Design Vehicles: An Update. Washington, DC: The National Academies Press. doi: 10.17226/27236.
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82 This chapter documents the turning performance of the updated design vehicles presented in Chapter 5. The chapter begins by discussing the determination of turning performance. Updated turning templates for each design vehicle are then presented. 6.1 Vehicle Turning Performance Sections 1.1, 2.4, and 2.5 present the fundamentals of the turning performance of vehicles of any size, including the design vehicles presented in the Green Book. As a vehicle turns, its turning performance is characterized by the turning radius and path of the vehicle’s front or steering axle as well as by the paths of other vehicle axles and other points on the vehicle body. The need for special analyses to accommodate larger vehicles in turning maneuvers arises because of the phenomenon known as offtracking, in which the rear wheels of any vehicle making a turn (except for the limited number of vehicles with steerable rear axles) do not follow the path of the front wheels. The amount of offtracking is a function of the spacing of the vehicle axles and, in combination vehicles, the location of the kingpin and hitch connections between the power unit and the trailer and between trailers in double- and triple-trailer vehicles. In particular • Offtracking increases as the wheelbase of each vehicle unit increases. • Offtracking decreases as the kingpin position for a semitrailer is moved farther in front of the rear tractor axle. • Offtracking increases as the kingpin position for a semitrailer is moved farther behind the rear tractor axle. • Offtracking increases as the distance from the kingpin to the next axle on the trailer increases. As a result of offtracking, each vehicle has a swept path width in making a turn that is usually substantially wider than the wheel track width of the front axle. Swept path widths are largest in longer single-unit vehicles and in articulated vehicles such as tractor-trailer combination trucks. The definitions of offtracking and swept path width that result from offtracking are presented in Figure 1. For a vehicle making a turn at low speed (such as 10 mph or 15 mph) on a level pavement surface, the swept path width is a function of vehicle characteristics and the radius (or radii) of the path followed by the front axle of the vehicle. This simple model for level, low-speed turns is typically satisfactory for intersection design applications. In more complex situations, offtracking and swept path width are also influenced by vehicle speed and pavement cross slope. Offtracking develops gradually toward a steady-state value as a vehicle enters a turn. The offtracking amount for steady-state or fully developed offtracking can be estimated with simple equations, known as the SAE equation and the WHI equation (see Section 2.5). However, in many C H A P T E R 6 Turning Performance of Updated Design Vehicles

Turning Performance of Updated Design Vehicles 83 turning maneuvers by larger vehicles, offtracking does not fully develop. In particular, the off- tracking in a 90-degree turn (such as a right or left turn at an intersection) is usually less than the steady-state offtracking. Therefore, swept path plots or turning templates are used because they can illustrate the actual development of offtracking as a vehicle enters and completes a specified turning maneuver. The minimum radius at which a vehicle makes a right or left turn is a function of the following: • Maximum angle(s) at which wheels on the vehicle’s front axle can be turned in response to the driver’s action in rotating the vehicle’s steering wheel. • Driver’s need to avoid turning so sharply that the vehicle’s inside rear wheels leave the traveled way of the roadway and encroach on the inside shoulder or curb during the turn. • Need for the driver of a combination vehicle to avoid turning so sharply that the inside front corner of a trailer makes contact with the rear of the power unit or a preceding trailer. • Need for the driver of a combination vehicle to turn in a manner such that all wheels of the vehicle remain rolling throughout the turning maneuver; turning too sharply can cause some tires to slide, which may lead to loss of vehicle control. The maximum steering angle(s) for the front wheels are a mechanical property of a vehicle’s steering system, but the latter three factors listed previously depend on driver experience and skill in making turning maneuvers. Less experienced drivers may turn at larger radii than highly experienced drivers. Design vehicles and their turning characteristics are used to lay out turning paths for roadway features such as intersections, median openings, and ramps so that their geometrics can be designed accordingly. In past years, turning templates were plotted to match the scale of design plans so that the ability of candidate designs to accommodate a specific design vehicle could be assessed. Plotted turning templates are seldom used now except to illustrate the turning performance of a particular vehicle configuration. Today, commercially available software can be used in conjunction with CADD software to develop designs to accommodate specific design vehicles. In applying these turning path packages, the user specifies the path of the vehicle’s front axle, and the software displays the path of the rear axles, the inside and outside of the vehicle body, and other specified points on the vehicle. The minimum turning radius of a vehicle is represented in turning path software by a maximum steering angle for the wheels of the vehicle’s front or steering axle. The right and left wheels on the steering axle of a vehicle typically have different maximum steering angles, but turning path software uses an equivalent steering angle that combines the effects of both wheels and repro- duces an appropriate turning path. Turning templates like those presented in the Green Book (see Figure 2) were once used directly in roadway design but today are largely for illustrative purposes because roadway design is primarily performed using turning path software in conjunction with the CADD systems used in roadway design. Turning templates in the Green Book show design vehicles making a 180-degree turn to the right. Except at roundabouts and intersections with U-turn roadways, most roadway designs do not involve 180-degree turns, but the initial portion of the turning template for AASHTO design vehicles can represent a minimum-radius turn through any specified angle of turn. Turning path software represents turning paths based on the centerline of the design vehicle. For this reason, left- and right-turn maneuvers are shown with the same turning radius. The actual steering mechanisms of vehicles may result in different turning radii for left and right turns. For example, field tests for a 45-ft bus have shown that the minimum turning path for a left turn is 3.6 ft larger than the minimum turning path for a right turn (T.Y. Lin International 2005).

84 Highway and Street Design Vehicles: An Update This difference has limited implications for intersection design because, except for turns from one one-way street to another, the actual turning path radius used in left-turn maneuvers at intersections is typically larger than the actual turning radius used in right-turn maneuvers at intersections. Furthermore, the 3.6-ft difference in minimum turning paths between left and right turns corresponds to a difference of only 1.8 ft from the centerline turning radius. Turning path software can be set up with a larger radius for left turns than for right turns where appropriate. The turning templates in the Green Book generally show the radii for four turning paths followed by specific points on the vehicle as the design vehicle makes a turning maneuver. These four turning paths, which each represent the path of a specific tire or a specific point on the vehicle body, are summarized in Table 38. Turning template software can also illustrate the turning path of any other tire or specified point on the vehicle body. Other turning paths that may be relevant to roadway design in some cases include the following: • Path of front outside corner of any bicycle rack or other appurtenance that may be mounted on the front of a vehicle. • Path of the outside or inside edge of vehicle mirrors or other appurtenances that may be mounted on the side of a vehicle. • Path of the outside rear corner of the vehicle body that may track outside of the rest of the vehicle body. In turning template software, the steering angle for the vehicle may be entered directly or it may be computed within the software from available data on the FOTR, MDTR, or CTR of the vehicle, as defined in Table 38. The remainder of this chapter defines the turning performance for the 21 vehicle configura- tions recommended in Chapter 5 for consideration by AASHTO as Green Book design vehicles. 6.2 Turning Templates This section presents turning templates for each of the 21 recommended design vehicles for potential use in the Green Book. These turning templates were generated with AutoTURN turning path software for a turning speed equivalent to 10 mph. Table 39 presents a summary of the turning radii for the 21 recommended design vehicles. A preliminary comparison was made between turning paths generated in the AutoTURN software and field-measured turning paths to verify the appropriateness of the software for this application. This comparison involved a typical Description of turning path Symbol Specific point on the vehicle that the turning path represents Front outside turning radius or minimum wall-to-wall turning radius FOTR Front outside corner of vehicle body Minimum design turning radius or minimum curb-to-curb turning radius MDTR Front outside tire of the vehicle Centerline turning radius CTR Center of front axle of the vehicle Minimum inside radius MIR Inside rear tire of the vehicle Table 38. Minimum turning paths for which radii are shown in Green Book turning templates.

Design vehicle type Passenger car (sedan) Pickup truck Single- unit truck (two axles) Single- unit truck (three axles) Intercity bus (motor coach) City transit bus Conven- tional school bus (65 pass.) Largea school bus (84 pass.) Articulated bus Intermediate semitrailer Symbol P PU SU-33 SU-40 BUS-40 BUS-45 CITY-BUS S-BUS-38 S-BUS-40 A-BUS WB-47 Front Outside Turning Radius (ft) 21.5 27.8 49.2 53.0 47.5 47.0 45.0 41.0 43.9 43.4 42.0 Minimum Design Turning Radius (ft) 19.7 25.9 47.5 51.2 41.8 44.0 41.6 39.2 39.6 39.4 41.1 Centerlineb Turning Radius (CTR) (ft) 16.9 23.1 43.7 47.4 37.9 40.2 37.8 35.5 36.1 35.5 37.2 Minimum Inside Radius (ft) 10.3 15.6 33.2 36.3 24.3 24.7 24.5 23.1 22.5 21.3 14.5 Design vehicle type Interstate semitrailer Double bottom combina- tion Rocky Mtn. double Triple semitrailer /trailers Turnpike double semitrailer /trailer Motor home Pickup truck and travel trailer Pickup truck and boat trailer Motor home and boat trailer Symbol WB-64* WB-69** WB-71D WB-96D WB-103T WB-118D* MH PU/T PU/B MH/B Front Outside Turning Radius (ft) 49.4 49.4 48.2 63.0 48.2 64.0 43.9 42.4 27.8 52.7 Minimum Design Turning Radius (ft) 48.0 48.0 47.2 61.8 47.2 62.9 40.4 40.9 25.9 49.8 Centerlineb Turning Radius (CTR) (ft) 44.2 44.2 43.2 58.0 43.2 55.0 36.8 38.0 23.1 46.0 Minimum Inside Radius (ft) 12.2 6.7 21.0 27.0 12.1 15.7 22.6 23.2 9.5 32.4 Numbers in the table have been rounded to the nearest tenth of a foot. * Design vehicle with 48-ft trailer as adopted in the 1982 Surface Transportation Assistance Act (STAA). ** Design vehicle with 53-ft trailer as grandfathered in with the 1982 Surface Transportation Assistance Act (STAA). a School buses are manufactured from 42-passenger to 84-passenger sizes. This corresponds to wheelbase lengths of 11.3 ft to 25.3 ft, respectively. b 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. Table 39. Minimum turning radii of design vehicles (U.S. customary units) (updated).

86 Highway and Street Design Vehicles: An Update light truck and a typical intercity bus and found good agreement between the software-generated and field-measured turning radii for both vehicle types. 6.2.1 Passenger Vehicles Figures 56 and 57 present turning templates for the recommended P and PU design vehicles for potential use in the Green Book. The steering angles on which the turning plots are based were determined from manufacturers’ data on turning radii for vehicles with the dimensions shown in Figures 16 and 17. Specifically, the steering angles for the P and PU design vehicles were computed from the manufacturers’ reported turning radii for common vehicles that matched the overall length of the recommended design vehicles. 6.2.2 Single-Unit Trucks Figures 58 and 59 present turning templates for the recommended SU-33 and SU-40 design vehicles, respectively, for potential use in the Green Book. The steering angles on which the turning plots were based are the same as the steering angles used for the comparable design vehicles in the 2018 edition of the Green Book (AASHTO 2018). 6.2.3 Combination Trucks Figures 60 through 66 present turning templates for the recommended combination truck design vehicles. In generating these turning plots, the steering angles for these design vehicles were based on the steering angles in the 2018 edition of the Green Book for truck tractors equivalent to the tractors from Figure 22 selected for each design vehicle. The smallest steering angles from the 2018 Green Book ranged from 12.6 degrees to 15.6 degrees and represented very short tractors, such as the cab-over-engine tractor used to pull doubles and triples with short 28.5-ft trailers. The recommended steering angles for the updated combination truck design vehicles are all either 20.3 degrees for the city or short-haul tractor shown in Figure 22B or 28.4 degrees for the long-haul tractor shown in Figure 22A. The 20.3-degree steering angle is comparable to the steering angle for the WB-40 design vehicle used in the 2018 Green Book. The 28.4-degree steering angle is comparable to the steering angle for the WB-62 and WB-67 design vehicles used in the 2018 Green Book. The articulating angles are all iden- tical to the articulating angles for trailers presented in the 2018 Green Book, except for the updated intermediate semitrailer design vehicle (WB-47) for which the articulating angle could be increased. 6.2.4 Buses Figures 67 through 72 present turning templates for the six recommended bus design vehicles for potential use in the Green Book. In generating these turning plots, the steering angles on which the turning plots for the BUS-40, BUS-45, and CITY-BUS were based are identical to the steering angles for these same vehicles in the 2018 edition of the Green Book, since these three design vehicles have not changed. The steering angle and turning performance of the BUS-45 is consistent with the turning performance for a bus of this length measured for a California bus operator (T.Y. Lin International 2005). The steering angles for the updated school bus design vehicles, S-BUS-38 and S-BUS-40, were determined from manufacturers’ data on turning radii for vehicles with the dimensions shown in Figures 42 and 43, respectively. The steering and articu- lating angles for the A-BUS design vehicle are identical to those for the A-BUS design vehicle in the 2018 edition of the Green Book; this seems appropriate since the recommended A-BUS design vehicle is nearly identical to the 2018 A-BUS design vehicle, differing only by 2 ft in one dimension.

Turning Performance of Updated Design Vehicles 87 Figure 56. Turning template for recommended passenger car (P) design vehicle.

88 Highway and Street Design Vehicles: An Update Figure 57. Turning template for recommended pickup truck (PU) design vehicle.

Turning Performance of Updated Design Vehicles 89 Figure 58. Turning template for recommended single-unit truck (two-axle) (SU-33) design vehicle.

90 Highway and Street Design Vehicles: An Update Figure 59. Turning template for recommended single-unit truck (three-axle) SU-40 design vehicle.

Figure 60. Turning template for recommended intermediate semitrailer (WB-47) design vehicle.

92 Highway and Street Design Vehicles: An Update Figure 61. Turning template for recommended interstate semitrailer (WB-64) design vehicle.

Turning Performance of Updated Design Vehicles 93 Figure 62. Turning template for recommended interstate semitrailer (WB-69) design vehicle.

94 Highway and Street Design Vehicles: An Update Figure 63. Turning template for recommended double bottom semitrailer/trailer (WB-71D) design vehicle.

Figure 64. Turning template for recommended Rocky Mountain double semitrailer/trailer (WB-96D) design vehicle.

96 Highway and Street Design Vehicles: An Update Figure 65. Turning template for recommended triple semitrailer/trailers (WB-103T) design vehicle.

Turning Performance of Updated Design Vehicles 97 Figure 66. Turning template for recommended turnpike double semitrailer/trailer (WB-118D) design vehicle.

98 Highway and Street Design Vehicles: An Update Figure 67. Turning template for recommended intercity bus (BUS-40) design vehicle.

Turning Performance of Updated Design Vehicles 99 Figure 68. Turning template for recommended intercity bus (BUS-45) design vehicle.

100 Highway and Street Design Vehicles: An Update Figure 69. Turning template for recommended city transit bus (CITY-BUS) design vehicle.

Turning Performance of Updated Design Vehicles 101 Figure 70. Turning template for recommended conventional school bus (65-passenger) (S-BUS-38) design vehicle.

102 Highway and Street Design Vehicles: An Update Figure 71. Turning template for recommended large school bus (84-passenger) (S-BUS-40) design vehicle.

Turning Performance of Updated Design Vehicles 103 Figure 72. Turning template for recommended articulated bus (A-BUS) design vehicle.

104 Highway and Street Design Vehicles: An Update 6.2.5 Recreational Vehicles Figures 73 through 76 present turning templates for the four recommended RV design vehicles (i.e., MH, PU/T, PU/B, and MH/B, respectively) for potential use in the Green Book. In gen- erating these turning plots, the steering angles for the recommended MH and MH/B design vehicles were set equal to 45 degrees. The steering angles for motor homes the size of the recom- mended MH design vehicle appear to range from 40 degrees to 50 degrees. The selected value of 45 degrees is the midpoint of this range. The steering angles for the recommended PU/T and PU/B design vehicles were set equal to the value of 34.3 degrees already used for the PU design vehicle. The articulating angle for the recommended MH/B design vehicle was set to 45 degrees, which appears to be the maximum articulating angle that avoids contact between the motor home and the boat trailer during the turn. The articulating angle for the recommended PU/T and PU/B was set to 61.1 degrees which corresponds to the articulating angle for the current PU/B design vehicle. It was verified that these articulating angles do not result in contact between the trailer and the towing vehicle during a minimum-radius turn. 6.3 Appurtenances Mounted on the Front of a Vehicle Appurtenances mounted on the front of a vehicle may increase the FOTR of a vehicle. A common front-mounted appurtenance is a bicycle rack. Bicycle racks up to 3.5 ft in length are often mounted on the front of buses. The presence of an engaged bicycle rack may increase the FOTR of the bus, as a corner of the bicycle rack or a bicycle in the rack may track outside the front outside corner of the vehicle body. Bicycle racks are incorporated in the BUS-40, BUS-45, and CITY-BUS design vehicles shown in Figures 67 through 69, respectively. However, when using the design vehicles in computerized turning path software, the designer may delete the bicycle racks from these design vehicles, ignore the path of the bicycle rack shown in a turning template, or add a bicycle rack to any other design vehicle, depending on local practices. 6.4 Appurtenances Mounted on the Side of a Vehicle Appurtenances mounted on the side of a vehicle may increase the FOTR of the vehicle, depending on the position of the appurtenance. The most common appurtenance of this type is a side mirror which typically adds up to 10 in. on each side to the body width of a large vehicle. Where a turning vehicle is positioned close to the edge of the traveled way, a mirror on the outside of the vehicle may strike a roadside object. Mirrors are not explicitly shown as part of the design vehicle dimensions, but all vehicles have outside mirrors, so the potential for large vehicles to strike roadside objects should be assessed. This is normally accomplished not by considering mirrors as part of a design vehicle in computerized turning path software, but rather by limiting the presence of roadside objects within the lateral offset of 1.5 ft from the edge of the traveled way, as recommended in the Roadside Design Guide (AASHTO 2011b). Mirrors on the inside of turning vehicles are normally within the swept path defined by the rear inside tire of the vehicle, so a mirror on the inside of a turn is unlikely to strike a roadside object unless the rear of the vehicle also encroaches on the area outside the traveled way. Roadway design to accommodate the swept path of the design vehicle should minimize the likelihood of the inside mirror of a vehicle striking a roadside object. On roadways with narrow lanes and no shoulders, there is a potential for either the left- or right-side mirror of a large vehicle to strike a roadside object even when the vehicle is going straight ahead. This potential can be addressed by providing the 1.5 ft lateral offset from the edge of the traveled way to roadside objects or by widening the right or curb lane on multilane roadways where large vehicles are most likely to travel in that lane.

Turning Performance of Updated Design Vehicles 105 Figure 73. Turning template for recommended motor home (MH) design vehicle.

106 Highway and Street Design Vehicles: An Update Figure 74. Turning template for recommended pickup truck and travel trailer (PU/T) design vehicle.

Turning Performance of Updated Design Vehicles 107 Figure 75. Turning template for recommended pickup truck and boat trailer (PU/B) design vehicle.

108 Highway and Street Design Vehicles: An Update Figure 76. Turning template for recommended motor home and boat trailer (MH/B) design vehicle.

Turning Performance of Updated Design Vehicles 109 6.5 Rear Swingout As a vehicle makes a turning maneuver, the outside rear of the vehicle may briefly track outside the swept path defined by the outside front corner of the vehicle. This is known as rear swingout. Table 40 shows the maximum rear swingout in a minimum-radius turn for each of the 21 design vehicles. The table shows that rear swingout is typically minimal, exceeding 0.5 ft for only four design vehicles and exceeding 1 ft for only one design vehicle (the recommended PU/T design vehicle). The rear swingout is generally too small to be a key concern in design for turning maneuvers. 6.6 Fire Trucks Emergency vehicles should be considered in road design because they need access to all portions of the road network. The largest emergency vehicles that operate frequently are fire trucks. The two most common types of fire trucks are ladder trucks and pumper trucks. Of these, the ladder trucks tend to be larger and need more room to maneuver. Figure 77 shows a turning template for a large (but not the largest) common ladder truck, based on vehicles included in the AutoTURN software library. The figure illustrates that this pumper truck has a turning radius and swept path width less than the recommended SU-33 design vehicle (shown in Figure 58). This indicates that a separate fire truck design vehicle is not needed, but rather the recommended SU-33 design vehicle can be used to check road designs for fire truck access. However, since fire trucks can vary by location, designers should consult with local fire departments to confirm local fire truck characteristics. 6.7 Garbage Collection Trucks Garbage collection trucks also need access to all portions of the road network. Figures 78 and 79 illustrate turning templates for common side-loading and rear-loading garbage collection trucks, respectively. The figures illustrate large (but not the largest) vehicles that would normally be used for garbage collection in residential neighborhoods, based on vehicles included Design vehicle type Symbol Maximum rear swingout (ft) in a minimum-radius turn Passenger car (sedan) P 0.5 Pickup truck PU 0.5 Single-unit truck (two-axle) SU-30 0.5 Single-unit truck (three-axle) SU-40 0.5 Intercity bus (motor coach) BUS-40 0.5BUS-45 0.5 City transit bus CITY-BUS < 0.1 Conventional school bus (65 passengers) S-BUS-38 1.0 Large school bus (84 passengers) S-BUS-40 0.5 Articulated bus A-BUS 0.5 Intermediate semitrailer WB-47 < 0.1 Interstate semitrailer WB-64 < 0.1 Interstate semitrailer WB-69 < 0.1 Double bottom semitrailer/trailer WB-71D < 0.1 Rocky Mountain double semitrailer/trailer WB-96D < 0.1 Triple semitrailer/trailers WB-103T < 0.1 Turnpike double semitrailer/trailer WB-118D < 0.1 Motor home MH 1.0 Pickup truck and travel trailer PU/T 1.5 Pickup truck and boat trailer PU/B 1.0 Motor home and boat trailer MH/B 1.0 Table 40. Maximum rear swingout in a minimum-radius turn for each design vehicle.

110 Highway and Street Design Vehicles: An Update The data source for the fire truck dimensions: Transoft Solutions. Figure 77. Turning template for large re engine (ladder truck).

Turning Performance of Updated Design Vehicles 111 The data source for the garbage collection truck dimensions: Transoft Solutions. Figure 78. Turning template for garbage collection truck (side loader).

112 Highway and Street Design Vehicles: An Update The data source for the garbage collection truck dimensions: Transoft Solutions. Figure 79. Turning template for garbage collection truck (rear loader).

Turning Performance of Updated Design Vehicles 113 in the AutoTURN software library. The figures illustrate that these garbage collection trucks have substantially less turning radius and swept path width than the recommended SU-33 design vehicle (see Figure 58). This indicates that neighborhood streets designed for common single- unit delivery trucks can also accommodate garbage collection trucks. 6.8 Vehicles with Low Ground Clearance The undercarriage of a vehicle with limited ground clearance may drag or hang up on a roadway with a humped profile or any sharp change in grade. This most commonly occurs for drive- way and railroad-highway grade crossing profiles, but any roadway with substantial changes in grade (either crests or sags) at a point or over a short distance can potentially experience contact between the vehicle undercarriage and the roadway surface. Raised crosswalks (e.g., at conven- tional intersections, roundabouts, or midblock crossings) may present similar ground-clearance issues. The vehicle undercarriage may potentially contact the roadway surface within the front overhang, the primary wheelbase, or the rear overhang. Research suggests a ground clearance of 5 in. for the vehicle undercarriage as a conservative value for design (French et al. 2003). Typi- cally, the same ground-clearance dimension is used for the wheelbase and both the front and rear overhangs of a selected design vehicle to check each of the potential manners in which a vehicle might drag or hang up. To check the possibility of contact between a vehicle undercarriage and the roadway surface, the minimum ground-clearance value may be specified with the design vehicle selected for the design of the roadway, but a specialized design vehicle may be used for this check if the roadway, driveway, or railroad-highway grade crossing being designed serves a facility that generates traffic consisting of specialized vehicle types with low ground clearance. The potential for a vehicle to drag or hang up on a specific roadway, driveway, or railroad- highway grade crossing profile design can be checked with any of three methods: • Two-dimensional graphical analysis in a profile view with a profile like that shown in Figure 80 and a drawing of a design vehicle in profile, both prepared at the same scale. • Two-dimensional analysis of the driveway profile and a design vehicle profile in computerized turning path software. • Three-dimensional analysis in the CADD system to develop the roadway design. Two-dimensional analysis is appropriate where vehicles approach the roadway surface dis- continuity at an angle of approximately 90 degrees. Where vehicles approach the discontinuity at an angle other than 90 degrees or where the vehicle’s wheels are at different elevations, three- dimensional analysis is appropriate. The three-dimensional approach is also appropriate to analyze mountable features, such as aprons at roundabouts. If the ground-clearance check shows that the vehicle undercarriage may drag or hang up, potential changes in the vertical alignment of the roadway, driveway, or railroad-highway grade crossing should be assessed. Source: AASHTO 2018. Figure 80. Driveway vertical alignment and profile elements.

Next: Chapter 7 - Development of Design Guidance for the Application of Updated Design Vehicles »
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 Highway and Street Design Vehicles: An Update
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Approximately 55 percent of the passenger vehicles registered in the United States are light trucks, such as sport utility vehicles, vans, minivans, and pickup trucks. Conventional automobiles, such as sedans and coupes, make up the rest of passenger vehicles.

NCHRP Research Report 1061: Highway and Street Design Vehicles: An Update, from TRB's National Cooperative Highway Research Program, proposes revisions to the dimensions of 16 of the 20 design vehicles used in the 2018 edition of AASHTO’s A Policy on Geometric Design of Highways and Streets, commonly known as the Green Book.

Supplemental to the report is a spreadsheet tool.

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