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Highway and Street Design Vehicles: An Update (2023)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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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4 1.1 Background The design vehicles presented in Chapter 2 (Design Controls and Criteria) of the AASHTO A Policy on Geometric Design of Highways and Streets, commonly known as the Green Book, are the key to design decisions appropriate to accommodate large vehicles on the road network (AASHTO 2018). Chapter 2 of the Green Book presents the following 20 design vehicles: • One passenger car (P) • Two single-unit trucks (SU-30, SU-40) • Two intercity buses (BUS-40, BUS-45) • One city transit bus (CITY-BUS) • Two school buses (S-BUS-36, S-BUS-40) • One articulated bus (A-BUS) • Three tractor-semitrailer (single-trailer) trucks (WB-40, WB-62, WB-67) • Three tractor-semitrailer/full-trailer (double-trailer) trucks (WB-67D, WB-92D, WB-109D) • One tractor-semitrailer/full-trailer/full-trailer (triple-trailer) truck (WB-100T) • Four RVs [motor home (MH), car and camper trailer (P/T), car and boat trailer (P/B) and motor home and boat trailer (MH/B)] The list of design vehicles and their dimensions has evolved over the years through various edi- tions of the Green Book. The design vehicles were updated during the late 1990s by the AASHTO Technical Committee on Geometric Design, then known as the AASHTO Task Force on Geo- metric Design, and incorporated in the 2001 Green Book (AASHTO 2001). No changes in design vehicles were made in the 2004 Green Book, which was a limited update primarily concerned with changes to superelevation design (AASHTO 2004). NCHRP Report 505: Review of Truck Char- acteristics as Factors in Roadway Design recommended updates to the design vehicles and their dimensions and provided new turning templates developed with AutoTURN software (Harwood et al. 2003). These were implemented in the 2011 Green Book along with revisions based on further work by the AASHTO Technical Committee on Geometric Design (AASHTO 2011a). The design vehicles in the 2018 Green Book were unchanged from those in the 2011 edition (AASHTO 2018). However, several comments received by AASHTO during the review of drafts for the 2018 edition could not be addressed because of a lack of resources. These included concerns that the specified minimum turning radii for some design vehicles were too large and for other design vehicles were too small. Thus, the research in NCHRP Project 07-27 was timely as there was a need to address these issues related to design vehicles, including changes to the vehicle types and composition, dimensions, and turning performance. The selection of an appropriate design vehicle for a highway project should generally be a project-specific design decision. The design vehicle selected should be the largest design vehicle C H A P T E R 1 Introduction

Introduction 5 that is likely to use that facility with considerable frequency or a design vehicle with special char- acteristics appropriate to a particular location. The primary application of design vehicles in the project development process is to help designers decide what geometrics should be used to accommodate larger vehicles at locations where such vehicles turn, including intersections, median openings, ramps, and horizontal curves. 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. As a result, 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. Figure 1 illustrates the definitions of offtracking and swept path width that result from offtracking. For a vehicle making a turn at low speed 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 influ- enced 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 Western Highway Institute (WHI) equation (see Section 2.5). However, in many turning maneuvers by larger vehicles, offtracking does not fully develop. In particular, the offtracking in a 90-degree turn 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. Figure 2 illustrates a typical Green Book figure presenting a turning template for a specific design vehicle (in this case, the WB-67 combination truck). Source: Harwood et al. 2003. Figure 1. Illustration of offtracking and swept path width within a truck turning maneuver.

6 Highway and Street Design Vehicles: An Update Source: Harwood et al. 2003. Figure 2. Typical vehicle dimensions and turning template shown in the Green Book (WB-67 combination truck).

Introduction 7 Information about typical vehicle dimensions has been gathered in this research with careful attention to variations in definitions of the dimensions that may be used in specific sources. For example, a key dimension that influences vehicle offtracking and swept path width for tractor- semitrailer trucks is the kingpin-to-rear-axle (KPRA) distance, as shown by the 45.5-ft dimen- sion in Figure 2. Many trucks have movable rear axles; rear axles can be moved forward, where needed, to reduce offtracking and swept path width. Information on design vehicles used in particular states must be assessed carefully because most sources measure KPRA distance as the distance from the kingpin to the center of the rear-axle group (sometimes even calling this KCRA distance), while others define KPRA distance as the distance from the kingpin to the rearmost axle. Design vehicles and their turning characteristics are used to lay out turning paths for 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 turning path or turning template software can be used in conjunction with computer-aided design and drafting (CADD) software to develop designs to accommodate specific design vehicles. In applying turning path software, 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 edges of the vehicle body, and/or other specified points on the vehicle. Software packages of this type are referred to in this report as vehicle turning path software. Design vehicles are used in other ways as well. Design vehicle characteristics are used in the Green Book in designing traveled way widening on horizontal curves and the widths of turning roadways. Design vehicle lengths are used in determining storage lengths for auxiliary lanes such as left- and right-turn lanes at intersections. Design vehicle heights are considered in establishing vertical clearances for overhead structures. Green Book design vehicles may also be used in the design of truck terminals, loading and unloading facilities, bus terminals, and recreational facilities. Since the types and characteristics of vehicles in the vehicle fleet change over time, there is a need for a periodic reexamination of the vehicles used as Green Book design vehicles and the dimensions used for those vehicles. Because software is used extensively to represent vehicle turning maneuvers in the geometric design process, it is desirable that the Green Book present recommended values for all the vehicle dimensions and other parameters needed as input to such software. Key issues for this research include the following: • Should any design vehicles be added to or dropped from the Green Book? • Should the dimensions of any current design vehicle be modified? • What design vehicle dimensions should be used as inputs to turning path software so that the software can be used correctly and consistently? • Are changes needed in the format or content of the design vehicle tables and turning templates (like Figure 2) presented in the Green Book? Some current design vehicles, such as the WB-67 and the WB-67D, will need only minor adjust- ments because their key dimensions (i.e., trailer lengths and widths) are set by federal laws and regulations, which have not changed substantively since 1982. However, even for these design vehicles, adjustments in tractor or wheelbase lengths might be needed. Design vehicles, other than the WB-67 and WB-67D, are not so strongly influenced by federal regulation and may need more extensive revisions based on recent market trends. Guidance for using design vehicles in project development has been created or refined in this research. This guidance includes design for turning maneuvers and design for low-clearance vehicles at driveways at intersections or railroad crossings with a humped profile. The design guidance is suitable for updating the Green Book.

8 Highway and Street Design Vehicles: An Update 1.2 Objectives and Scope The objective of the research in NCHRP Project 07-27 is to develop design vehicle material for the forthcoming 8th edition of the Green Book that realistically represents the critical vehicles that influence geometric designs. The material should include the following: • Critical dimensions and specifications (or ranges) for design vehicles that can be applied in the intersection design of right- and left-turn maneuvers, roundabouts, and other roadway elements. • Guidance on using this dimensional information in design, including the selection of design vehicle(s) for a project, determination of when to allow large vehicles to encroach on other lanes, and discussion on balancing the needs of different modes (e.g., trucks and pedestrians). • An appropriate number of turning path templates that reflect a range of variability among design vehicles and guidance on when and how they should be applied. The scope of the project is focused on the design of public roads, but the Green Book design vehicles may also be applied in the design of truck and bus terminals, loading/unloading facilities, and recreational roads and facilities. 1.3 Organization of This Report The remainder of this report is organized as follows. Chapter 2 reviews the key vehicle char- acteristics, including an explanation of offtracking and swept path width for single-unit and combination vehicles. Chapter 3 presents the results of a survey of transportation agencies con- ducted as part of the research. Chapter 4 describes the assembly of data on the current vehicle fleet. Chapter 5 presents the Green Book design vehicles and describes recommended updates to those vehicles. Chapter 6 describes the turning performance of the updated design vehicles and presents turning templates for potential use in the next edition of the Green Book. Chapter 7 describes potential updates to the Green Book design guidance. Chapter 8 presents the conclu- sions and recommendations of the research. The report also includes a reference list and two appendices. Appendix A presents the questionnaire for the transportation agency survey that was conducted in the research. Appendix B presents the dimensions of the recommended design vehicles for use as input to turning path software.

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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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