The National Academies

NCHRP 20-07/Task 394 [Final]

Development of Performance-Based Geometric Design Content for the Next Edition of the AASHTO Green Book
[ NCHRP 20-07 (Research for AASHTO Standing Committee on Highways) ]

  Project Data
Funds: $40,000
Research Agency: MRIGlobal
Principal Investigator: Doug Harwood
Effective Date: 5/3/2016
Completion Date: 5/31/2017

The objective of this research was to provide recommended content changes for AASHTO's A Policy on the Geometric Design of Highways and Streets (Green Book) based on the publications mentioned in the background section; the AASHTO Highway Safety Manual; recently completed NCHRP Report Number 785, prepared in NCHRP Project 15-34A, Performance-Based Analysis of Geometric Design of Highways and Streets; and other relevant research. The primary content change should be an introduction to and overview of the concept of performance-based design for incorporation near the beginning of the Green Book.


The contractor's revised final draft Chapter 1 has been received and transmitted to AASHTO Staff for their use in preparing the next edition of the Green Book. 
Tasks anticipated in this project included the following:
  • Review literature, including recently completed NCHRP Report 785
  • Recommend content (text and graphics) changes for the AASHTO Green Book
  • Provide recommended content in draft form prior to the 2016 annual meeting of the AASHTO Technical Committee on Geometric Design 
  • Present and discuss the recommended text at the 2016 and 2017 meetings of the AASHTO Technical Committee on Geometric Design
  • Update the recommended text in response to comments from the Technical Committee at the meeting and submit an updated version
As geometric design professionals encounter physical, fiscal and environmental constraints, guidance is needed to evaluate the performance or impact of alternative geometric design decisions. Gone are the days of simply verifying a roadway’s functional classification and applying a “one-size-fits-all” approach to the application of design standards of a roadway improvement project.
The existing Green Book design criterion provides operational safety, efficiency, and comfort for the traveler, but it is difficult or impossible for the designer to characterize quantitatively how the facility will perform. For both new construction and reconstruction of highways and streets, stakeholders and decision-makers increasingly want reasonable measures of the effect of geometric design decisions on the facility’s performance for all of its users.
Roadway performance can be measured in a number of ways, including mobility, speed, safety and surface condition, as well as by person throughput and the accommodation of multiple transportation modes. Increasingly, the character and context of the environment within which the roadway is located, as well as the expectation of its performance on a number of measures, are driving the design of roadway improvement projects.
The Federal Highway Administration (FHWA) publication Flexibility in Highway Design, AASHTO publication A Guide for Achieving Flexibility in Highway Design and the AASHTO Green Book emphasize the importance of applying “flexibility” in the project development design decision making process. Flexibility in geometric design has been supported for years, and increasingly in recent years, tools like the Interactive Highway Safety and Design Model and publications such as the Highway Safety Manual (HSM) and FHWA’s Speed Concepts: Informational Guide provide the means to consider and measure geometric design performance.

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