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Highway and Street Design Vehicles: An Update (2023)

Chapter: Chapter 2 - Key Vehicle Characteristics

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Suggested Citation:"Chapter 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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 2 - Key Vehicle Characteristics." 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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9   This section discusses the design vehicles used in the Green Book, defines the basic vehicle types used as design vehicles, and defines their key dimensions and characteristics, including vehicle length, vehicle height, vehicle width, track width, vehicle wheelbase(s), front overhang, rear overhang, number of axles, and connection types in multi-unit combination vehicles. Key vehicle performance issues relevant to geometric design, including offtracking and swept path width, are also defined. 2.1 Current AASHTO Green Book Design Vehicles Table 1 presents the dimensions of the 20 design vehicles defined in the current Green Book (AASHTO 2018). Diagrams with the dimensions of each Green Book design vehicle, like the diagram that appears at the top of Figure 2, are presented in the discussion of current vehicle dimensions in Chapter 5 of this report. Chapter 2 includes diagrams that define the key longi- tudinal dimensions of each vehicle type that need to be specified for design vehicles. 2.2 Key Dimensions for Specific Vehicle Types This section shows the typical configurations for specific vehicle types that may be of interest in geometric design, with the key dimensions of these vehicle types shown in parametric form. Quantitative values for selected dimensions for these vehicle types from the current Green Book are presented in Table 1 and in Chapter 5 of this report. Figures 3 and 4 show definitions that need to be specified for passenger cars and single-unit trucks, respectively. The figures show dimension lines for the key dimensions, including overall length, wheelbase, front overhang, and rear overhang. Figure 5 shows the key dimensions that need to be specified for a conventional bus; these are comparable to the key dimensions for a single-unit truck. Passenger cars generally have two axles with one tire at the left and right end of each axle. The front axle is the steering axle and, except in front-wheel-drive cars, the rear axle is the drive axle, which is rotated by engine power applied to the axle by the vehicle transmission. Single-unit trucks have a single front steering axle but typically have one or two rear axles. Where the rear axles consist of two closely spaced axles, this axle group is referred to as a tandem axle. The advantage of a tandem axle for truck operators is that it is generally allowed to carry more weight than a single axle. A conventional bus also generally has a single front steering axle and may have either one or two rear axles (see Figure 5). For both the single-unit truck and the conventional bus, the rear axle(s) may have either one or two tires at each end of the axle. C H A P T E R 2 Key Vehicle Characteristics

Source: AASHTO 2018. Design vehicle type Symbol Dimensions (ft) Overall Overhang WB1 WB2 S T WB3 WB4 Typical kingpin-to- center-of-rear tandem axleHeight Width Length Front Rear Passenger car P 4.3 7.0 19.0 3.0 5.0 11.0 — — — — — — Single-unit truck SU-30 11.0–13.5 8.0 30.0 4.0 6.0 20.0 — — — — — — Single-unit truck (three-axle) SU-40 11.0–13.5 8.0 39.5 4.0 10.5 25.0 — — — — — — Buses Intercity bus (motor coaches) BUS-40 12.0 8.5 40.5 6.3 9.0 a 25.3 — — — — — — BUS-45 12.0 8.5 45.5 6.2 9.0a 28.5 — — — — — — City transit bus CITY-BUS 10.5 8.5 40.0 7.0 8.0 25.0 — — — — — — Conventional school bus (65 passengers) S-BUS-36 10.5 8.0 35.8 2.5 12.0 21.3 — — — — — — Large school bus (84 passengers) S-BUS-40 10.5 8.0 40.0 7.0 13.0 20.0 — — — — — — Articulated bus A-BUS 11.0 8.5 60.0 8.6 10.0 22.0 19.4 6.2b 13.2b — — — Combination trucks Intermediate semitrailer WB-40 13.5 8.0 45.5 3.0 4.5a 12.5 25.5 — — — — 25.5 Interstate semitrailer WB-62* 13.5 8.5 69.0 4.0 4.5a 19.5 41.0 — — — — 41.0 Interstate semitrailer WB-67** 13.5 8.5 73.5 4.0 4.5a 19.5 45.5 — — — — 45.5 Double bottom semitrailer/trailer WB-67D 13.5 8.5 72.3 2.3 3.0 11.0 23.0 3.0c 7.0c 22.5 — 23.0 Rocky Mountain double semitrailer/trailer WB-92D 13.5 8.5 97.3 2.3 3.0 17.5 40.0 4.5 7.0 22.5 — 40.5 Triple semitrailer/trailers WB-100T 13.5 8.5 104.8 2.3 3.0 11.0 22.5 3.0d 7.0d 22.5 22.5 23.0 Turnpike double semitrailer/trailer WB-109D* 13.5 8.5 114.0 2.3 4.5a 12.2 40.0 4.5e 10.0e 40.0 — 40.5 Recreational vehicles Motor home MH 12.0 8.0 30.0 4.0 6.0 20.0 — — — — — — Car and camper trailer P/T 10.0 8.0 48.7 3.0 12.0 11.0 — 5.0 17.7 — — — Car and boat trailer P/B — 8.0 42.0 3.0 8.0 11.0 — 5.0 15.0 — — — Motor home and boat trailer MH/B 12.0 8.0 53.0 4.0 8.0 20.0 — 6.0 15.0 — — — * Design vehicle with 48.0-ft trailer as adopted in the 1982 Surface Transportation Assistance Act (STAA). ** Design vehicle with 53.0-ft trailer as grandfathered in with the 1982 Surface Transportation Assistance Act (STAA). a This is the length of the overhang from the back axle of the tandem axle assembly. b Combined dimension is 19.4 ft and articulating section is 4.0 ft wide. c Combined dimension is typically 10.0 ft. d Combined dimension is typically 10.0 ft. e Combined dimension is typically 12.5 ft. WB1, WB2, WB3, and WB4 are the effective vehicle wheelbases, or distances between axle groups, starting at the front and working toward 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 the center of the tandem axle assembly. Table 1. Current design vehicle dimensions.

Key Vehicle Characteristics 11 Figure 3. Dimension terminology for a passenger car. Figure 4. Dimension terminology for a single-unit truck. Figure 5. Dimension terminology for a conventional bus.

12 Highway and Street Design Vehicles: An Update Figure 6 shows the key dimensions that need to be specified for a tractor-trailer combination truck with a single trailer, including overall vehicle length, front overhang, tractor wheelbase, trailer length, trailer wheelbase, location of the connection between tractor and trailer, KPRA distance, and rear trailer overhang. A combination vehicle that consists of two or more separate units that can rotate with respect to one another is called an articulated vehicle. In the combination truck shown in Figure 6, the tractor is the power unit for the vehicle and carries no cargo. All cargo is carried in the trailer. The trailer in this combination vehicle has no front axle, but instead, the front of the trailer rests on and is supported by the rear of the tractor. The trailer is connected to the tractor by a steel pin mounted on the trailer hitch around which the trailer can rotate with respect to the tractor; this pin is known as a kingpin and it fits into a round, horizontal disc, known as the fifth wheel, attached to the tractor frame, which is greased so that the trailer can rotate smoothly. This type of connection between the tractor and trailer allows a portion of the trailer weight (vehicle and cargo) to be transferred through the kingpin and fifth-wheel connection to the tractor axles. The position of the kingpin with respect to the tractor axles determines the distribution of the transferred weight on the front and rear tractor axles. A trailer of this sort that has no front axle(s), but rather rests on the rear portion of the towing unit, is known as a semitrailer. Single-trailer trucks generally have one front steering axle on the tractor, either one or two drive axles at the rear of the tractor, and either one, two, or three axles at the rear of the semitrailer. As in the case of a single-unit truck, two closely spaced axles at the rear of the tractor or trailer are referred to as an axle group, known as a tandem axle. Three closely spaced axles also constitute an axle group, known as a tridem axle. Tandem and tridem axles have an advantage for truck operators in that they are generally allowed to carry more weight than a single axle. Figure 7 shows the key dimensions that need to be specified for an articulated bus that incor- porates a semitrailer in a manner distinctly different from the tractor-semitrailer truck shown in Figure 6. In an articulated bus, the articulation point occurs within the passenger compart- ment of the bus, and the rear semitrailer portion of the bus can rotate with respect to the front powered unit. Figure 6. Dimension terminology for a single-trailer combination truck.

Key Vehicle Characteristics 13 Figure 7. Dimension terminology for an articulated bus. Figure 8. Dimension terminology for a double-trailer combination truck. A double-trailer combination truck typically consists of a tractor, a semitrailer, and a full trailer that is towed by the semitrailer. The first trailer in this combination vehicle is a semitrailer that rests on the rear of the tractor, just as does the semitrailer in Figure 6. The second trailer in this combination is a full trailer that has two sets of axles or axle groups (near the front and rear of the trailer) and is connected to the first trailer by a pintle hook and is towed by the first trailer. The second trailer can rotate with respect to the first trailer at the pintle hook. Unlike a semi- trailer, a full trailer does not transfer any weight to the towing vehicle; the entire weight of the second trailer (vehicle plus cargo) is carried by the axles of that trailer. The full trailer typically has either single or tandem axles at both the front and rear of the full trailer. Figure 8 shows the key dimensions that need to be specified for a double-trailer combination truck. A triple-trailer combination truck typically includes a tractor and three trailers: a semitrailer followed by two full trailers. The first trailer in this combination is a semitrailer that rests on the rear of the tractor, just as does the semitrailer in Figure 5. The second and third trailers are full trailers, each connected to the preceding unit by a pintle hook. Each full trailer typically has the same number of axles at both the front and rear of each trailer. Figure 9 shows the key dimensions that need to be specified for a triple-trailer combination truck.

14 Highway and Street Design Vehicles: An Update RVs presented in the current Green Book include motor homes, a travel or camping trailer pulled by a passenger car, a boat trailer pulled by a passenger car, and a boat trailer pulled by a motor home. The key dimensions of motor homes are comparable to those for a single-unit truck, as shown in Figure 4, and a conventional bus, as shown in Figure 5. Most of the RV trailers are semitrailers pulled by a powered vehicle. The key dimensions of vehicles illustrated in the preceding figures are as follows: • Overall vehicle length: The overall length of the vehicle from the front of the vehicle body to the rear of the vehicle body. For combination vehicles, this includes the power unit and any trailers present. Overall vehicle length does not include any appurtenances that may be present such as a bicycle rack on the front of a bus or a tailgate loading apparatus on the rear of a truck. • Front overhang: The front overhang of a vehicle, tractor, or trailer is the distance from the front of the vehicle body (not including appurtenances) to the center of the front axle. • Wheelbase: The wheelbase of a vehicle, tractor, or trailer is the distance from the center of the front axle or axle group to the center of the rear axle or axle group. The distances between individual axles or axle groups are referred to as axle spacings. For pairs of axles or axle groups on a specific vehicle unit, the axle spacing may be referred to as the wheelbase for that unit. • Trailer length: The trailer length is the length from the front of the trailer body to the rear of the trailer body. • Location of connection: The location of the connection between a vehicle’s power unit and a trailer or between two trailers is defined by the distance between the kingpin or pintle hook and the adjacent axles. • Rear overhang: The rear overhang of a vehicle, tractor, or trailer is the distance from the center of the rear axle or axle group to the rear of the vehicle body (not including appurtenances). Other key dimensions of vehicles include the following: • Overall vehicle height: The overall height of the vehicle is the distance from the top of the vehicle body to the pavement surface (not including any appurtenances attached to the top of the vehicle). • Overall vehicle width: The overall width of the vehicle is the width of the vehicle body at its widest point (not including any appurtenances such as vehicle mirrors). Figure 9. Dimension terminology for a triple-trailer combination truck.

Key Vehicle Characteristics 15 • Track width: The track width of the vehicle is the distance between the left edge of the left tire and the right edge of the right tire of a vehicle axle. On many vehicles, the track width for various axles may differ. The track width of the front axle is most relevant to the vehicle’s offtracking performance because it defines the path of the front of the vehicle. 2.3 Vehicle and Axle Weight Limits The maximum weight carried by a vehicle or an axle or axle group is regulated by federal and state legislation. The 1982 Surface Transportation Assistance Act (STAA) required all states to allow vehicles with weights up to the following gross vehicle weights (GVWs) or axle weights to operate on the interstate system: • GVW 80,000 lb • Weight on a single axle 20,000 lb • Weight on a tandem axle 34,000 lb The 1982 STAA required states to allow at least these weights listed but did not restrict states from allowing greater vehicle weights. The 1982 STAA also established a system of designated highways, known as the National Network (NN), on which states were required to permit trucks of specific sizes to operate (see discussion in Sections 5.5 through 5.7). The 1991 Intermodal Surface Transportation Efficiency Act (ISTEA) restricted states from allowing double- and triple-trailer combination trucks with GVW over 80,000 lb from operating on the interstate system and from allowing double- and triple-trailer combination trucks with trailers more than 28.5 ft in length from operating on the interstate system and the rest of the NN, except where such trucks could legally operate before the passage of the ISTEA. The weights carried by specific axles are also influenced by axle spacings. Federal regulations specify an equation, known as Bridge Formula B, which limits the weight that may be carried by any axle group. The maximum weight that may be carried by any group of two or more axles is determined as follows: W N LN N 1 12 36= - + + J L KK N P OO (1) Where: W = maximum weight carried on any group of two or more axles (lb). L = distance between the outer axles in any group of two or more axles (ft). N = number of axles in the axle group. The gross weight on two or more consecutive axles may not exceed the weight computed with the bridge formula, except that two consecutive sets of tandem axles may each carry a gross load of 34,000 lb if the overall distance between the first and last axle is 36 ft or more and the total gross weight of the vehicle does not exceed 80,000 lb. This restriction, based on Bridge Formula B, applies to the interstate system. Many states also regulate the load that can be carried by any tire to a specified maximum load per inch of tire width. Federal regulations applicable to the interstate system prohibit any state from limiting tire loads to less than 500 lb per inch of tire or tread width, except that such limits may not be applied to tires on the steering axle. Within these federal constraints, states are free to regulate truck sizes and weights.

16 Highway and Street Design Vehicles: An Update 2.4 Vehicle Turning Radius Table 2 shows the turning radii of the 20 design vehicles presented in the Green Book. For each design vehicle, the table includes the following: • Minimum design turning radius • Centerline turning radius • Minimum inside radius The turning radii shown in Table 2 are defined as follows: • Minimum design turning radius (MDTR) or minimum curb-to-curb turning radius: The circular arc formed by the turning path radius of the front outside tire of a vehicle. • Centerline turning radius (CTR): The radius of the circular arc formed by the turning path of the centerline of the front axle of a vehicle with its steering wheels at the steering lock position. • Minimum inside radius (MIR): The radius of the circular arc formed by the inside rear tire of a vehicle. Figure 2 also illustrates the minimum wall-to-wall turning radius or front outside turning radius. This is the radius of the circular arc formed by the turning path of the front outside corner of a vehicle (overhang) with its steering wheels at the steering lock position. * 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 11.0 ft to 20.0 ft, respectively. For these different sizes, the minimum design turning radii vary from 28.1 ft to 39.1 ft and the minimum inside radii vary from 17.7 ft to 25.3 ft. 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. Design vehicle type Passenger car Single- unit truck Single- unit truck (three axles) Intercity bus (motor coach) City transit bus Convention al school bus (65 pass.) Largea school bus (84 pass.) Articulated bus Intermediate semitrailer Symbol P SU-30 SU-40 BUS-40 BUS-45 CITY-BUS S-BUS-36 S-BUS-40 A-BUS WB-40 Minimum Design Turning Radius (ft) 23.8 41.8 51.2 41.7 44.0 41.6 38.6 39.1 39.4 39.9 Centerlineb Turning Radius (CTR) (ft) 21.0 38.0 47.4 37.8 40.2 37.8 34.9 35.4 35.5 36.0 Minimum Inside Radius (ft) 14.4 28.4 36.4 24.3 24.7 24.5 23.8 25.3 21.3 19.3 Design vehicle type Interstate semitrailer Double bottom combina- tion Rocky Mtn double Triple semitrailer/ trailers Turnpike double semitrailer /trailer Motor home Car and camper trailer Car and boat trailer Motor home and boat trailer Symbol WB-62* WB-67** WB-67D WB-92D WB-100T WB-109D* MH P/T P/B MH/B Minimum Design Turning Radius (ft) 44.8 44.8 44.8 82.0 44.8 59.9 39.7 32.9 23.8 49.8 Centerlineb Turning Radius (CTR) (ft) 41.0 41.0 40.9 78.0 40.9 55.9 36.0 30.0 21.0 46.0 Minimum Inside Radius (ft) 7.4 1.9 19.1 55.6 9.7 13.8 26.0 18.3 8.0 35.0 Source: AASHTO 2018. Table 2. Minimum design turning radii, centerline turning radii, and minimum inside radii of design vehicles.

Key Vehicle Characteristics 17 These turning radii are illustrated in the turning template presented in Figure 2. Each of the turning radii defined has a corresponding turning diameter that is twice the turning radius. Vehicle literature must be read carefully because the terms turning radius and turning diameter are often used interchangeably when they differ by a factor of two. Vehicle specifications often use the term turning circle as an alternative to turning diameter to make clear that the full diameter of the turning circle is being presented. The current research has investigated the need for modifications to the turning radius data shown in Table 2. 2.5 Vehicle Offtracking and Swept Path Width Offtracking is the technical term for the phenomenon 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. As a result, each vehicle has a swept path width in making a turn that is often 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. Offtracking and swept path width are key considerations in the design of intersections and other locations where vehicles make turning maneuvers because the paved area is generally designed to accommodate the swept path width of a selected design vehicle. For a vehicle making a turn at low speed on a level pavement surface, the swept path width is typically treated as a function of vehicle characteristics and the radius (or radii) of the path followed by the front axle of the vehicle. This simple level, low-speed model of offtracking is typically satisfactory for design applications. 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 for two-axle vehicles and the WHI equation for multi- axle vehicles (Heald 1986). The SAE equation for a two-axle vehicle in a steady turn of constant radius at low speed is as follows: OT WB R HT WB HT R HT WB HTmax 2 2 2 2 2 2= + + - - - + - +` `j j (2) Where: OTmax = maximum offtracking at low speed of the rear-axle centerline from the front-axle centerline (ft). WB = vehicle wheelbase (ft). R = radius of curvature of the path of the centerline of the front axle (ft). HT = half of the front-axle track width (ft). The WHI equation generalizes the SAE formula for multi-axle vehicles as follows, with a sequence of three terms for each connection between tractors and/or trailers: R R LOTmax 2 2= - -a k/ (3) . . .L WB j ac ca WB j ac caac ac2 12 1 12 12 22 2 22 22= + + + + + +/ (4) Where: OTmax = maximum offtracking at low speed of the rear-axle centerline from the front-axle centerline (ft).

18 Highway and Street Design Vehicles: An Update R = radius of curvature of the path of the centerline of the front axle (ft). L = distance representing vehicle unit wheelbase length, axle-connection distance, or connection-to-axle distance. WBn = wheelbase length for nth vehicle component (n = 1 for tractor, n = 2 for the first trailer, etc.) (NOTE: WBn = wheelbase for tractor or full trailer; 0 for semitrailer because there is only one axle group). jacn = −1 if the connector between units n and n + 1 is in front of the rear axle of unit n; 0 if the connector between units n and n + 1 is directly above the rear axle of unit n; 1 if the connector between units n and n + 1 is behind the rear axle of unit n; this latter case ( jacn = 1) may occur for articulated buses, but rarely for combination trucks). acn = axle-to-connector distance between connector of units n and n + 1 and first trailer axle group of unit n + 1. can = connector-to-axle distance between the rear-axle group of unit n and connector of units n and n + 1. The WHI equation illustrates the following general relationships concerning offtracking: • Offtracking increases as the wheelbase of each vehicle unit increases. • Offtracking decreases as the kingpin position for a semitrailer is moved farther forward 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. Figure 10 illustrates the terms of the WHI equation as applied to a combination truck with two trailers (one semitrailer and one full trailer). The WHI equation is not suitable as a design tool because, in many turning maneuvers, off- tracking does not fully develop. Therefore, swept path plots or turning templates, like the one shown in Figure 2, are used because they can illustrate the actual development of offtracking as a vehicle enters and completes a turn. Printed turning templates represent only one specific turning path. Vehicle turning path software provides a realistic spatial representation of any specified turning path. Figure 10. Illustration of the terms for the WHI equation for a double-trailer truck with one semitrailer and one full trailer.

Key Vehicle Characteristics 19 Turning templates for intersection design, like those presented in Chapter 6 are based on low- speed turning maneuvers (10 mph) on a level pavement. However, at higher speeds, offtracking and swept path width are also influenced by vehicle speed and pavement cross slope. The SAE and WHI equations are accurate at low speeds. Since turning maneuvers are made at low speeds, the geometric design of intersections based on low-speed offtracking, in which the rear axles track inside the front axles, as shown in Figure 2, is appropriate. As the speed of a vehicle increases, offtracking decreases until a speed is reached at which the rear axles of the vehicle track directly behind the front axles. At higher speeds, the rear axles track outside the front axles. However, this phenomenon of high-speed offtracking is not particularly relevant to geometric design. At inter- sections, the relevant turning maneuvers are conducted at low speeds. Widening on horizontal curves is also typically based on low-speed offtracking. While vehicles may traverse some curves at higher speeds, the more critical situation is at low speeds, which may occur due to stalled traffic or on an intersection approach. The offtracking to the inside of the turn in these low-speed situa- tions exceeds any offtracking likely to the outside of the turn at higher speeds. Pavement cross slope, including superelevation on horizontal curves, also affects offtracking. Typical superelevation on horizontal curves can increase fully developed offtracking at low speeds by 10% to 20% in comparison to offtracking on a level pavement surface (Glauz and Harwood 1991). However, the length and radius of horizontal curves are such that offtracking will only partially develop, and superelevation is not likely to contribute much to offtracking on horizontal curves. Therefore, the Green Book model for traveled way widening on horizontal curves does not include consideration of superelevation. 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. 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/or other specified points on the vehicle.

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Highway and Street Design Vehicles: An Update Get This Book
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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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