The objective of this research is to develop design vehicle material for the 8th Edition of the Green Book that realistically represents the critical vehicles that influence geometric designs. The material should include:
- Critical dimensions and specifications (or ranges thereof) for design vehicles that can be applied to the design of intersection right and left turn lanes, roundabout elements, and other roadway elements;
- Guidance on using this dimensional information in design, including selection of design vehicle(s) for a project, determination of when to allow large vehicles to encroach upon other lanes, and discussion on balancing the needs of different modes (e.g., trucks and pedestrians); and
- An appropriate number of turning path templates that reflect a reasonable range of variability among design vehicles and guidance on when and how they should be applied.
Task 1. Conduct Literature and Database Review. The research team will review published literature and existing databases related to design vehicle characteristics and application.
Task 2. Conduct Surveys and Identify Highway Agency Needs. The research team will identify current usage of the Green Book design vehicles in the design process and document highway agency assessments of the need to update those design vehicles. The survey will be sent to: the design engineers of the 50 state highway agencies, selected local highway agencies, the three FHWA Federal Lands Highway regional offices (which design recreational facilities such as roads in national parks), and every independent toll authority in the U.S.
Task 3. Develop Work Plans for Phase II Research. The research team will develop recommended work plans for the research that will be performed as part of Task 5 in Phase II of the research. A preliminary list of work plans that expected to be developed includes:
· Work Plan A—Finalize Recommendations for Adding or Dropping Design Vehicles from the Current List of Green Book Design Vehicles
· Work Plan B—Revise the Dimensions and Turning Radii of Design Vehicles As Needed
· Work Plan C—Develop Revised Drawings and Turning Templates for Design Vehicles
· Work Plan D—Develop and Refine Key Principles for Design Guidance
Task 4. Prepare Interim Report. The research team will prepare and submit an interim report, presenting the results of Tasks 1 through 3, including the works plans for the Task 5 research developed in Task 3. Approximately one month after submission of the interim report, research team members will meet with NCHRP and the project panel to review the interim report and discuss the Task 5 work plans. The research team will not proceed with research in Phase II of the project until authorized by NCHRP.
Task 5. Execute Approved Work Plans. The research team will execute the work plans developed in Task 3 and presented in the Task 4 interim report, as approved by the NCHRP project panel.
Task 6. Develop Text for Revisions to the AASHTO Green Book. The research team will develop text for recommended revisions to the next edition of the AASHTO Green Book. This will include a complete revision of the material on design vehicles in Section 2.8.1 (General Characteristics) and Section 2.8.2 (Minimum Turning Paths of Design Vehicles) in the 2018 edition of the Green Book. Material for other sections may also be developed.
Task 7. Prepare Final Report. The research team will prepare the final report and other final deliverables for the research. The final report will document the research objectives, methodology, findings, conclusions, and recommendations. The recommended revisions to the AASHTO Green Book will be incorporated as an appendix to the final report. Other final deliverables will include a Microsoft Excel® spreadsheet with the dimensional data for design vehicles and a stand-alone technical memorandum titled, “Implementation of Research Findings and Products.” The implementation memorandum will present recommendations for keeping the vehicle dimensional data current.
Design vehicle classification, dimensions, and turning path templates have been an important part of AASHTO’s A Policy on Geometric Design of Highways and Streets, 7th Edition (Green Book) for over 40 years. Due to limited or nonexistent supporting data and research documentation, these design vehicle dimensions and minimum turning radii are difficult to support and verify.
While AASHTO was balloting the 7th Edition of the Green Book (released in September 2018), several comments from state departments of transportation were related to the turn radii and prevalence of different design vehicles (see Special Note A). Over the past few years, designers have observed that some real world vehicles have significantly different turning paths compared to their counterpart turning path templates in the AASHTO Green Book. Questions have been raised about the steering angles for some of the Green Book design vehicles that appear small for modern vehicle (e.g., 13 degree steering angle for the WB-92D and WB-109D). This can result in overly conservative or large geometric layouts and striping plans. The Golden Gate Bridge’s Highway and Transportation District has noted that their buses require more turning space than the turning radii provided in the Green Book. With the increased value of right-of-ways and increased usage of modern roundabout designs and alternative intersection treatments (e.g., thru-turn, continuous flow, and displaced-left turn intersections), accurate vehicle steering angles and swept paths are of the utmost importance.
The Green Book turning path templates do not provide sufficient data for vehicle turn simulation software and computer-aided design (CAD) software to faithfully reproduce them. Critical dimensions for determining a vehicle’s swept path are its front overhang, rear overhang, wheel base, steering angle, vehicle width, and, in the case of multi-part vehicles, the inter-vehicle angles and kingpin and hitch locations. Rear overhang and mirror widths have also generated safety concerns in the design of bus passenger platforms. Further, with industry movement toward 3D design, loaded ground clearances and heights of the key design vehicles are desirable.