The current Green Book design vehicle classifications have been developed over a 40+ year period. These design vehicle dimensions and minimum turning radii, while providing full coverage of the general vehicle fleet, are difficult to support and verify. This difficulty is a result of the limited or nonexistent supporting data and research for these vehicles.
Questions have been raised about the steering angles for some of the Green Book design vehicles which appear small for the 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. Further, with the increased usage of modern roundabout designs, accurate vehicle steering angles and swept paths are of utmost importance.
The Technical Committee on Geometric Design (TCGD) recently received pre-ballot review comments on the Green Book, 7th Edition, from members of the AASHTO Subcommittee on Design. Comments related to the Green Book Design vehicles include:
- Minimum Inside Radius for WB-67 (WB-20 Metric) is not consistent with older versions of Green Book; please verify.
- Why is the CTR of the WB-92D design vehicle so much larger than the WB-109D? The different radii make a huge difference when laying out turn templates. Is one more common than the other?
- Why is the CTR of the WB-109D vehicle so much shorter than the WB-92D? The WB109D has longer trailers but can turn much tighter than the WB-92D? The different radii make a huge difference when laying out turn templates. Is one more common than the other?
The TCGD response to each of these questions was that further research would be needed on these issues and that the need for future revisions will be taken under advisement for the 8th edition of the Green Book.
The Green Book turning path templates provide insufficient data for software developers, such as AutoTURN, to incorporate the turning paths in CAD programs without making assumptions such as the kingpin and hitch locations. As such, the CAD programs provide similar but different turning paths than the Green Book templates. Further, with industry movement towards 3D design, ground clearances or heights of the various design vehicles are needed but not provided by the Green Book.
The proposed research directly supports the Standing Committee on Highways (SCOH) Resolution, passed May 25, 2016 in Des Moines, Iowa, regarding the direction on Flexibility in Design Standards. Specifically, the SCOH resolution included the following elements:
- RESOLVED, AASHTO should provide guidance to state DOTs and other users of the Green Book regarding flexibility in design; and be it further
- RESOLVED, This guidance should follow the AASHTO model of being research-based and peer-reviewed; and be it further
- RESOLVED, This guidance should address designing in and for a multi-modal transportation system; and be it further
- RESOLVED, SCOD should identify gaps in necessary research and develop a plan to fill those gaps.
Some similar research in this area is as follows:
Review of Truck Characteristics as Factors in Roadway Design, NCHRP Report 505. Douglas W. Harwood, Midwest Research Institute, Kansas City, MO, 2003 http://www.trb.org/Publications/Blurbs/153579.aspx
Effects of Turns by Larger Trucks at Urban Intersections, Transportation Research Record 1195, Joseph Hummer, Charles Zeeger, and Fred Hanscom for the FHWA Office of Safety and Traffic Operations Research and Development, 1988 ,http://onlinepubs.trb.org/Onlinepubs/trr/1988/1195/1195‐006.pdf
Austroads Design Vehicles and Turning Path Templates https://www.onlinepublications.austroads.com.au/items/AP‐G34‐13
NAASRA Design Vehicles and Turning Templates http://184.108.40.206/Presto/content/Detail.aspx?ctID=NmQ5ZmY0YWQtNTc4NS00YzZiLTk3 MTItNjEzYWQxZTgyMTRl&rID=MTM2Mw==&qcf=&ph=VHJ1ZQ==&bckToL=VHJ1ZQ==&
|Index Terms:||AASHTO Green Book, Design vehicles, Turning paths, Templates, Radius, Highway design, Design standards, |