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Highway and Street Design Vehicles: An Update (2023)

Chapter: Chapter 8 - Conclusions and Recommendations

« Previous: Chapter 7 - Development of Design Guidance for the Application of Updated Design Vehicles
Page 117
Suggested Citation:"Chapter 8 - Conclusions and Recommendations." 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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Page 117
Page 118
Suggested Citation:"Chapter 8 - Conclusions and Recommendations." 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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Page 118
Page 119
Suggested Citation:"Chapter 8 - Conclusions and Recommendations." 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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Page 119

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117   8.1 Conclusions The conclusions of the research are as follows: 1. Design vehicles used in the 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 necessarily the largest, vehicle within its class. The application of such design vehicles to develop roadway and intersection geometrics represents 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 should be accommodated within the roadway geometrics. 2. Vehicle characteristics change over time, and vehicles in many vehicle classes have become larger in recent years. Based on a review of information on characteristics for specific vehicle makes and models, a need was identified to change the dimensions of 16 of the 20 design vehicles used in the 2018 edition of the Green Book. The dimensions of the other four design vehicles do not need to be changed. 3. Approximately 55% of the passenger vehicles registered in the United States are light trucks, such as SUVs, vans, minivans, and pickup trucks. The largest of these light trucks are pickup trucks. The overall lengths of pickup trucks measure up to 20.8 ft, as compared to 19 ft for the current passenger car (P) design vehicle. 4. 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 passenger car design vehicle. 5. 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 they 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. 6. 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 vehicle 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 changed the turning performance of the vehicle configurations appropriate for use as design vehicles. No single set of design vehicles can exactly represent the larger vehicles present in the vehicle fleet in every jurisdiction within the United States. The design vehicles recommended for use in the Green Book represent the most appropriate design vehicles for general nationwide use, but they C H A P T E R 8 Conclusions and Recommendations

118 Highway and Street Design Vehicles: An Update should not be expected to be appropriate for every jurisdiction. The Green Book design vehicles should be useful for many, but not necessarily all, projects. Designers are encouraged to investigate the vehicles present in their own jurisdiction and to vary the dimensions of selected design vehicles accordingly. Commercial turning path software provides the capability to readily create custom design vehicles and develop roadway and intersection designs based on the turning performance of those vehicles. 8.2 Recommendations The recommendations of the research are as follows: 1. A new pickup truck design vehicle (designated as the PU design vehicle) is recommended for inclusion in the Green Book. The recommended dimensions for this new design vehicle are presented in Table 37. Since light trucks constitute 55% of passenger vehicles, the PU design vehicle may be appropriate for application in many cases in which the P design vehicle was previously used. The retention of the P design vehicle is recommended for application in situations where a smaller passenger car design vehicle is appropriate. 2. The lengths of tractors used as the power unit in combination trucks have increased, so most of the combination truck design vehicles should become slightly longer. The dimensions of these updated tractors are presented in Figure 21. 3. The WB-40 combination truck design vehicle used should have a longer trailer and should, in the future, be designated as the WB-47 design vehicle. This change is recommended because the size of intermodal containers that are placed on trailers in combination trucks has increased. 4. The two current design vehicles (designated the P/T and P/B design vehicles) representing 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 longer travel trailer in the recom- mended PU/T design vehicle. 5. The dimensions for all 21 recommended design vehicles are shown in Table 37. Where key dimensions of design vehicles have changed, their names/symbols should be updated accordingly. 6. The turning radii for the 21 recommended design vehicles should be updated as shown in Table 39. 7. There is judgment involved in selecting the most appropriate design vehicle(s) for a project and in deciding when it is appropriate to use a design vehicle with dimensions that differ from those presented in the Green Book. Additional training on this issue may be desirable for designers who lack experience in this aspect of design. 8. The section on design vehicles in Chapter 2 of the Green Book should be updated to dis- cuss, in greater depth, the selection of appropriate design vehicles for the design of specific roadways and intersections and the application of the selected design vehicles in the design process. 9. 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. 10. 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 such vehicles should be performed. 11. The discussion in Chapter 4 of the Green Book on the separation of pedestrians and bicyclists from large vehicles should be made more explicit.

Conclusions and Recommendations 119 12. The discussion in Chapter 4 of the Green Book on transit facilities should specifically mention the six bus design vehicles. 13. 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. 14. The discussion in Chapter 9 of the Green Book on the design of curb radii and left- and right-turn auxiliary lanes should address 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 for left turns and minimum median width for U-turns on divided highways should also be updated. 15. 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. 16. 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. 17. Recommended changes to the Green Book address recommendations 8 through 16 and should be considered by AASHTO 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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