An Update of the Green Book Design Vehicles

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. 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 the following: (1) 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; (2) 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 (3) 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.


  • English


  • Status: Active
  • Funding: $400000
  • Contract Numbers:

    Project 07-27

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Jared, David

  • Performing Organizations:


    425 Volker Boulevard
    Kansas City, MO  United States  64110-2241
  • Principal Investigators:

    Potts, Ingrid

  • Start Date: 20190508
  • Expected Completion Date: 20201107
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01672240
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 07-27
  • Files: TRB, RiP
  • Created Date: Jun 18 2018 3:02PM