The National Academies

NCHRP 15-77 [Final]

Aligning Geometric Design with Roadway Context

  Project Data
Funds: $349,986
Research Agency: Kittelson and Associates
Principal Investigator: Brian Ray
Effective Date: 6/18/2020
Completion Date: 12/17/2021


The objective of this research is to draft Part IV (Facility Design in Context) of the proposed eighth edition of the Green Book (GB8), using a consistent structure for the context chapters and drawing content from the Green Book and research-based sources. This material should be suitable for direct use in a future project to develop the GB8, although the development of Parts I-III may prompt changes.



Since 1984, AASHTO’s A Policy on the Geometric Design of Highways and Streets (the “Green Book”) and other roadway design criteria have been primarily based on a functional classification system of a hierarchical network composed of arterials, collector, and local roads. This classification was further assigned by an urban or rural designation. This system is described in detail in Highway Functional Classification Concepts, Criteria, and Procedures (FHWA-PL-13-026).


In May 2016, the AASHTO Standing Committee on Highways (SCOH) passed a resolution on Direction on Flexibility in Design Standards. This resolution tasked the Committee on Design to develop guidance, for users of the Green Book, on flexibility in design that is multimodal, research-based, and peer-reviewed. That guidance should educate users on the flexibility inherent in the Green Book and provide new and additional guidance on specific design issues. It should also lead other AASHTO publications toward a future set of flexible design standards.


The AASHTO Technical Committee on Geometric Design (Technical Committee) reviewed NCHRP Report 785: Performance-Based Analysis of Geometric Design of Highways and Streets; NCHRP Research Report 839: A Performance-Based Highway Geometric Design Process; NCHRP Research Report 855: An Expanded Functional Classification System for Highways and Streets; and other references to determine how the seventh edition of the Green Book could satisfy, at least in part, the SCOH resolution. Chapter 1, New Framework for Geometric Design, was added to emphasize the importance of explicitly stating the purpose and need of a project; introduce the context classes presented in NCHRP Research Report 855 (i.e., rural, rural town, suburban, urban, and urban core); stress that all transportation modes should be considered in design; point out the flexibility that can be brought to the design of new construction, reconstruction, and projects on existing roads; and recommend a performance-based approach to design. Changes were made in other chapters to reflect the SCOH resolution as resources and time allowed before publication in 2018.


The Technical Committee on Geometric Design also commissioned NCHRP Project 20-07/Task 423, “Planning for a Comprehensive Update and Restructuring of AASHTO’s A Policy on Geometric Design of Highways and Streets (8th Edition),” to help them plan for the eighth edition of the Green Book (GB8). The Technical Committee and Committee on Design envisioned the GB8 as fully satisfying the SCOH resolution and were willing to consider a total restructuring to support a flexible, multimodal, performance-based, and context-sensitive design process. That project reviewed the previous research and conducted significant outreach to develop recommendations. At its July 2019 meeting, the Technical Committee reviewed the recommendations from NCHRP Project 20-07/Task 423 and decided to adopt the following outline. This outline was presented to the AASHTO Council on Highways and Streets in October 2019.




Part I—Introduction




Performance-Based Design Concepts


Design Decision Making

Part II—Performance-Based Evaluations


Performance Metrics


Design Model


Applying a Performance Based Process Framework

Part III—Geometric Elements and Configurations


Design Information and Sources


Elements of Design


Cross-Section Elements


Intersection Fundamentals


Freeways and Controlled Access Fundamentals


Interchange Fundamentals

Part IV—Facility Design In Context


Context and Facility Type Considerations


Rural and Natural Areas


Rural Towns


Suburban Roadways


Urban Roadways


Urban Core Roadways


Industrial, Warehouse, or Port Roads


One of the major changes is Part IV that will build upon the context classes in NCHRP Research Report 855 and Chapter 1 of the Green Book, seventh edition. Some of the reasons for this addition are:

  • The context classes better delineate the needs of all users of the roadway and provide a linkage to the land use.
  • The context class of the road will often influence design decisions more than the functional class of the road.
  • This approach better reflects the state of the practice and emerging design issues, including context-sensitive design, livable communities, practical design, and complete streets.
  • This organization could reduce redundancies in the existing Green Book text while also allowing more specificity in recommendations for design solutions and evaluation of tradeoffs.
  • These chapters should promote consistency in how these context classes are defined and applied across the United States.
  • The reorganization will allow a fresh look at the content of the Green Book and help identify areas for improvement.

Research is needed to determine how the current design guidance in the Green Book and other good sources can be reorganized by context class. Production of the GB8 will be done in a larger, subsequent effort.



The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 320: Aligning Geometric Design with Roadway Context drafts the Facility Design in Context portion of a proposed Green Book 8, using a consistent structure for the context chapters and drawing content from the Green Book and research-based sources to support a flexible, multimodal, performance-based, and context-sensitive design process.

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