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Suggested Citation:"Summary." 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:"Summary." 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:"Summary." 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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1   Design vehicles used in AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) generally have larger physical dimensions and larger minimum turning radii than most vehicles in their class. Each design vehicle, thus, represents a larger, but not neces- sarily the largest, vehicle within its class. The applications of such design vehicles to develop roadway and intersection geometrics represent a conservative approach so that if a given design vehicle is used to develop the design, most vehicles in that vehicle class and all smaller vehicles can be accommodated within the roadway geometrics. Vehicle characteristics change over time, and vehicles in many vehicle classes have become larger in recent years. Based on a review of information on the characteristics of specific vehicle makes and models, the research team has recommended changes in the dimensions of 16 of the 20 design vehicles used in the 2018 edition of the Green Book. No change was recommended for the dimensions of the other four design vehicles. Approximately 55% of the passenger vehicles registered in the United States are light trucks, such as sport utility vehicles (SUVs), vans, minivans, and pickup trucks. The largest of these light trucks are pickup trucks, which measure up to 20.8 ft, as compared to the current pas- senger car (P) design vehicle, which measures up to 19 ft. A new pickup truck design vehicle (designated as the PU design vehicle) is recommended for inclusion in the Green Book. This would increase the number of design vehicles in the Green Book from 20 to 21. The recom- mended dimensions for this new design vehicle are presented in this report. Because light trucks constitute such a large percentage of passenger vehicles, the PU design vehicle appears appropriate for application in most cases in which the P design vehicle was previously used. Conventional automobiles, such as sedans and coupes, constitute approximately 45% of passenger vehicles. The lengths of these conventional automobiles measure up to 17.4 ft, i.e., shorter than the current 19-ft P design vehicle. The retention of the P design vehicle is, with a shorter length, recommended for application in situations where a smaller passenger car design vehicle is appropriate. Three of the six bus design vehicles do not appear to have changed appreciably in size from the dimensions used in the current Green Book. However, small changes in the dimensions of the design vehicles appear to be appropriate for both school buses and the articulated buses used by local transit systems. The maximum sizes of the trailers used in most combination trucks in general use on the interstate system and other major roads are limited by state and federal law and cannot change without state or congressional action. However, these limits apply to the lengths of trailers but generally do not limit the length of tractors that pull the trailers of combination trucks. In recent years, the tractors used in over-the-road trucks have become slightly longer. Highway and Street Design Vehicles: An Update S U M M A R Y

2 Highway and Street Design Vehicles: An Update Thus, it appears that most of the combination truck design vehicles should become slightly longer because the size of the tractors has increased. It appears appropriate to increase the trailer length for one of the current combination truck design vehicles that is shorter than, and therefore not constrained by, existing state limits on trailer length. This current design vehicle, the WB-40 truck, has been included in past editions of the Green Book because its trailer represented a common intermodal container size. Intermodal containers have increased in size in recent years, so the most common intermodal container is now 40 ft in length. Therefore, a longer design vehicle, to be designated as the WB-47 design vehicle, now appears appropriate. The two current design vehicles (designated the P/T and P/B design vehicles) representing recreational vehicles (RVs) that are currently pulled by a passenger car should, in the future, be pulled by a pickup truck and should be designated as the PU/T and PU/B design vehicles. The camper trailer included in the P/T design vehicle should be replaced by a travel trailer in the recommended PU/T design vehicle. A review of makes and models currently on the market indicates that motor homes have also increased in length, so it appears appropriate to increase the length of the recommended motor home (MH) design vehicle to 44 ft. This change increases the length of the MH/B design vehicle as well. The turning performance of design vehicles (including turning radius, offtracking, and swept path width) is a function of vehicle lengths (including the lengths of a combination vehicle’s individual units), the spacing between individual vehicle axles, the locations of the hitches that connect the units of combination vehicles relative to the vehicle axles, the capabilities of the vehicle steering system (represented by the steering lock angle), and the maximum angle between the units of a combination vehicle (known as the articulating angle). The changes in vehicle dimensions and characteristics over time have affected the turning performance of the vehicle configurations appropriate for use as design vehicles. New or updated turning path plots have been developed for each of the 21 recommended design vehicles. Recommendations for updating the Green Book to incorporate the revised design vehicles have been developed in the research. Specifically, the section on design vehicles in Chapter 2 of the Green Book should be updated to incorporate the revised design vehicles and to discuss, in greater depth, the selection of design vehicles for the design of specific roadways and intersections and the application of the selected design vehicles in the design process. The guidance in Chapter 3 of the Green Book on traveled way widening on horizontal curves and selection of the width for turning roadways should be updated based on the modified dimensions and turning performance of the recommended design vehicles. The guidance in Chapters 4 and 9 of the Green Book on the design of driveway and railroad- highway grade crossing profiles to accommodate vehicles with low ground clearance should be expanded to indicate how analyses of vehicles with low ground clearance should be per- formed. The discussion in Chapter 4 of the Green Book on the separation of pedestrians and bicyclists from large vehicles should be made more explicit. The discussion in Chapter 4 of the Green Book on transit facilities should specifically mention the six bus design vehicles. The discussion in Chapter 9 of the Green Book on curb radii at intersections should be expanded to further emphasize the consideration of the effect of curb radii on pedestrian crossing distance. The discussion in Chapter 9 of the Green Book on the design of curb radii and left- and right-turn auxiliary lanes should discuss the potential use of a large vehicle as a check vehicle. Some locations may be designed such that a vehicle larger than the selected design vehicle can be accommodated with some encroachment on adjacent or opposing lanes, but without encroachment on curblines. The Green Book table of swept path widths

Summary 3   for left turns and minimum median width for U-turns on divided highways should also be updated. The discussion in Chapter 9 of the Green Book on the design of roundabouts should mention the potential use of a mountable apron on the outside of the central island that may be encroached on by large vehicles. The tables for sight distance at railroad-highway grade crossings in Chapter 9 of the Green Book should be adjusted in response to the small increase in length of the recommended WB-69 design vehicle, as compared to the current WB-67 design vehicle. These recommended changes will be provided for consideration by AASHTO for inclusion in the next revision of the Green Book.

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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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