BACKGROUND

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. Also, 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, wheelbase, 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.

OBJECTIVE

The objective of this research was to develop design vehicle material for the 8th Edition of the Green Book that realistically represents the critical vehicles that influence geometric designs. The research provided conclusions for:

• 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 a 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.

The final report documents the research objectives, methodology, findings, conclusions, and recommendations. Other final deliverables will include a Microsoft Excel® spreadsheet with the dimensional data for design vehicles.