A Policy on Geometric Design of Highways and Streets 2018 [7th Edition] – Green Book

Green Book

Description:
A Policy on Geometric Design of Highways and Streets, 7th Edition, 2018, commonly referred to as the “Green Book,” contains the current design research and practices for highway and street geometric design. This edition presents an updated framework for geometric design that is more flexible, multimodal, and performance-based than in the past. The document provides guidance to engineers and designers who strive to make unique design solutions that meet the needs of all highway and street users on a project-by-project basis. Not only are the traditional functional classifications for roadways (local roads and streets, collectors, arterials, and freeways) presented, but also an expanded set of context classifications (rural, rural town, suburban, urban, and urban core) to guide geometric design. The completely rewritten Chapter 1: A New Framework for Geometric Design, introduces the updated approach to design, with specific design guidance throughout each chapter.

PREFACE

Public works projects of all scales are more sensitive to funding than ever before. In many cases,
cost magnitude and cost effectiveness play increasingly large roles in scoping projects. Often,
reconstruction projects are limited in scope or available funding, or may be affected by physical
constraints or social or environmental considerations. In some locations, especially constrained
locations, designing to the criteria recommended herein simply is not feasible. Adaptive, flexible,
and cost-effective designs customized to each project context are encouraged. Flexibility in
the application of design criteria herein is recommended to encourage a sustainable approach to
highway design decision making by weighing and balancing choices among the environmental,
economic, and social aspects while meeting the project’s performance objectives.

Designers should recognize the joint use of transportation corridors by motorists, pedestrians,
bicyclists, public transit, and freight vehicles. Designers are encouraged to consider not only
vehicular movement, but also movement of people, distribution of goods, and provision of essential
services. A more comprehensive transportation program is thereby emphasized.
A Policy on Geometric Design of Highways and Streets provides geometric design guidance based
on established practices that are supplemented by recent research. This document is intended as
a comprehensive reference manual to assist in administrative, planning, and educational efforts
pertaining to design formulation. This policy is not intended to be a prescriptive design manual
that supersedes engineering judgment by the knowledgeable design professional.

The design concepts and criteria in this policy are intended for use when designing new construction
projects on new location or designing reconstruction projects on an existing location.
Projects on existing roads particularly call for a flexible, performance-based approach to design.
The policy also encourages flexible design, which emphasizes the role of the planner and designer
in determining appropriate design dimensions based on project-specific conditions and existing
and future roadway performance more than on meeting specific nominal design criteria.
This publication is not intended as a policy for resurfacing, restoration, or rehabilitation (3R);
traffic engineering; safety; and preventive maintenance-type projects that include very minor or
no roadway work. When designing 3R projects, the designer should refer to the design guidelines
presented in NCHRP Report 876, Guidelines for Integrating Safety and Cost-Effectiveness into
Resurfacing, Restoration, and Rehabilitation (3R) Projects, for more information. NCHRP Report
876 was developed as a replacement for TRB Special Report 214, Designing Safer Roads: Practices
for Resurfacing, Restoration, and Rehabilitation.

The fact that new design values and concepts are presented herein does not imply that existing
streets and highways are unsafe, nor does it mandate the initiation of improvement projects. The
highway, vehicle, and individual users are all integral parts of transportation safety and efficiency.
While this document primarily addresses geometric design issues, a properly equipped and
maintained vehicle and reasonable and prudent performance by the user are also needed for safe
and efficient operation of the transportation facility.

Chapter 1 of this edition has been rewritten entirely and provides a new framework for geometric
design. It expands the land use contexts from two (urban or rural) to five (rural, rural
town, suburban, urban, or urban core). It emphasizes design flexibility provided in this policy
and encourages designers to take advantage of that flexibility. Chapter 1 also introduces a performance-
based approach to geometric design which, when used, will allow practitioners to
quantify and convey design tradeoffs in meaningful terms to a broad audience and, ultimately,
for consideration by decision makers.

Design values are presented in this document in both U.S. customary and metric units and were
developed independently within each system. The relationship between the U.S. customary and
metric values is neither an exact (soft) conversion nor a completely rationalized (hard) conversion;
and the use of brackets around metric values does not indicate as in some AASHTO publications
that these are soft conversions. The U.S. customary values are those that would have
been used if the policy had been presented exclusively in U.S. customary units; the metric values
are those that would have been used had the policy been presented exclusively in metric units.
Therefore, the user is advised to work entirely in one system and not attempt to convert directly
between the two.

This publication supersedes the 2011 AASHTO publication of the same name. Because the
concepts presented cannot be completely covered in this one document, references to additional
literature are given at the end of each chapter. These references include works that were cited
or consulted in the development of the chapter or are of interest to the discussion of the subject
matter therein. Of these documents, only those balloted and published by AASHTO represent
AASHTO policy.

The Committee on Design and the Technical Committee on Geometric Design would like to
extend a special thank you to Doug Harwood of MRI Global for his technical editing expertise
during the development of the seventh edition.