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A Policy on Geometric Design of Highways and Streets

A Policy on Geometric Design of Highways and Streets

Thomas Grant 3 years ago 0 15

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 expertis during the development of the seventh edition.

New Framework for Geometric Design

1.1 INTRODUCTION

This seventh edition of the A Policy on Geometric Design of Highways and Streets incorporates recent research that provides insight into the effect of specific geometric design elements of roads and streets for all transportation modes. This edition of the policy also introduces the consideration of five specific context classifications as an element of the geometric design process and emphasizes the consideration of multimodal needs in design. Together, context classification and functional classification constitute a new framework for geometric 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. In the past, designers sought to assure good traffic operational and safety performance for the design of specific projects primarily by meeting the dimensional design criteria in this policy.

This approach was appropriate in the past because the relationship between design dimensions and future performance was poorly understood. Traditional applications of this policy took the approach that, if the geometric design of a project met or exceeded specific dimensional design criteria, it would be likely to perform well. In some cases, this may have led to overdesign, constructing projects that were more costly than they needed to be or were inappropriate for the roadway context.

Recent research has improved our knowledge of the relationship between geometric design features and traffic operations for all modes of transportation and has developed new knowledge about the relationship of geometric design features to crash frequency and severity. Much of the recently developed information about assessing traffic operations for all transportation modes is presented in the TRB Highway Capacity Manual

(25), while the recently developed information about estimating future crash frequen- cies and severities is presented in the AASHTO Highway Safety Manual (4, 7).

This edition of the policy introduces new definitions of project types—new construction, reconstruction, and projects on existing roads—and explains how design flexibility is provided for each project type as part of the project development process.

Project development is broader than just geometric design and should consider many factors for all transportation modes, including:

  • Project purpose and need
  • Existing and expected future traffic operational efficiency
  • Existing and expected future crash frequency and severity
  • Construction cost
  • Future maintenance cost
  • Context classification
  • Service and ease of use for each transportation mode:
  • automobile
  • bicycle
  • pedestrian
  • transit
  • truck
  • Accessibility for persons with disabilities
  • Available right-of-way
  • Existing and potential future development
  • Operational flexibility during future incidents and maintenance activities
  • Stakeholder input
  • Community goals and plans and potential community impacts
  • Historical structures
  • Impacts on the natural environment:
  • air quality
  • noise
  • wetlands preservation
  • wildlife and endangered species
  • Preservation of archeological artifacts

These factors are not necessarily presented in priority order and, indeed, the priorities placed on them vary from project to project. None of these factors is uniquely important and geometric design should complement other aspects of project development in seeking the appropriate bal- ance among their potential effects.

A 2016 resolution of the AASHTO Standing Committee on Highways (8) has directed that geometric design policy and practice should become more flexible and performance-based to more fully address the needs of all transportation modes and the challenges to transportation agencies created by funding and right-of-way constraints. This AASHTO resolution is consistent with the direction set by Federal legislation in the Fixing America’s Surface Transportation (FAST) Act (14). This seventh edition of the policy takes a first step toward implementing a new framework for geometric design to accomplish this goal. There was already substantial flexibility in the geometric design guidance presented in previous editions of this policy, and this seventh edition expands that flexibility. This chapter explains how the flexible, performance-based approach should be applied and describes how Chapters 2 through 10, together with other available resources, can be used in implementing the new framework and the performance-based approach for all transportation modes. The next edition of this policy will more fully incorporate this approach, with full implementation of the new framework and the flexible, performance-based approach in each chapter.

1.2 PROJECT PURPOSE AND NEED

The design of every road or street improvement project should begin with an explicit statement developed by the roadway agency that indicates why the project is being undertaken and what the project is intended to accomplish. This statement may be in the form of the purpose and need statement used in National Environmental Policy Act (NEPA) analyses, a formal statement of objectives for the project, or a combination of the two approaches. Either separately or collectively, these statements set out the purpose and need for the project and the objectives that the project should satisfy in fulfilling that purpose and need. In the remainder of this discussion, these two types of documents collectively will be referred to as the purpose and need statement.

The purpose and need statement informs priorities on what will, and what will not, be undertaken in the project. The designer should refer to this purpose and need statement in determining the scope of geometric design changes to include in the project and assessing whether any geo- metric design changes suggested by others are germane to the project purpose and need.

The purpose and need statement for a project should be built around an assessment of past performance for all transportation modes of the roads and streets within the project limits and a forecast of future performance if no project is undertaken. The purpose and need statement should address the project context and how each transportation mode should be accommodated.

The purpose and need statement should also indicate what aspects of performance should be im- proved and, in some cases, may also set targets for how much performance should be improved.

Thus, improvement needs should be based on specific performance issues that are identified by the agency as in need of improvement. Performance, in this context, is broader than traffic operational or safety performance and potentially addresses any of the performance issues listed in Section 1.1. Both quantitative and qualitative performance measures may be used in defining the purpose and need for projects. The roadway agency should involve other stakeholders in establishing the project purpose and need.

The performance-based approach to establishing the purpose and need for and the objectives of the project enables the designer to focus on addressing the needs of a project without needlessly exceeding them. By limiting a project’s scope to focus only on documented performance improvement needs, more resources are available to be spent on other needs throughout the road and street system.

The scope of projects, based on their purpose and need, may range from simple projects on existing roads (addressing a single issue) to new construction projects that create a new road and complex reconstruction projects that may change the character of an existing road. Construction of roads on new alignment and reconstruction projects that change the basic roadway type (see Section 1.7.2) should utilize the design criteria in Chapters 2 through 10, to the extent practical, while seeking the appropriate balance among transportation modes and among the many factors that affect project development. Less complex projects on existing roads that do not change the basic roadway type are typically undertaken primarily to address specific identified performance issues, such as poor infrastructure condition, current or anticipated traffic congestion, or current or anticipated crash patterns. Such projects should focus on addressing the performance issues that prompted the project, as well as any other known performance issues within the project limits that are identified in the purpose and need statement.

Performance issues identified by the purpose and need statement may address any of the many factors listed in Section 1.1, but need not include them all. In fact, when one aspect of performance needs improvement and other aspects do not, good project management should focus on the performance issues that need improvement. The purpose and need statement serves as a tool for agency management to focus the scope of projects to improvements that are expected to have a specific, anticipated effect on project performance. Performance issues also include assessment of the service provided to each transportation mode.

Performance issues are often identified from existing agency databases or field data, but may also be documented with models such as traffic simulation models, crash prediction or systemic safety models, and air quality or noise models. These same models can be used to quantify the effectiveness of candidate design alternatives in improving performance.

It is important to understand that noncompliance with geometric design criteria is not, by itself, a performance issue for a project on an existing road. Noncompliance with geometric design criteria is not sufficient to be identified as an issue in a project purpose and need statement; such noncompliance with geometric design criteria only becomes an issue to be addressed in the project purpose and need if that noncompliance has resulted in (or is forecast to result in) poor performance that is correctable by a geometric design improvement and that the agency chooses to address. If some aspects of the geometric design for a road or street do not fully comply with the geometric design criteria presented in Chapters 2 through 10, but the road or street is performing satisfactorily, there is no need to change those aspects of the existing geometric design for projects in which the basis roadway type will remain the same. Thus, noncompliance with geometric design criteria should be addressed in projects on existing roads only where it is established that the current design is performing poorly or that a geometric design improvement would be cost-effective. This approach is intended to avoid expenditures that have no impact on performance. Some exceptions, where quantitative performance measures are not available, are addressed below.

It is important to understand that, while great progress has been made in developing performance assessment tools, not every aspect of past and anticipated future performance can be quantified for projects on existing roads. Some issues may need to be assessed qualitatively. For example, there are no performance measures for the effect of pavement cross slope in normal crown sections on safety. Surrogates, such as noting the ponding of water during rainstorms, may need to be applied in the absence of quantitative performance measures; simply applying the applicable design criteria may be the most applicable approach. Similarly, there are no quantitative performance measures for vertical clearance; vertical clearance should generally be addressed by reference to applicable design criteria.

Two types of data may be available for use in establishing the project purpose and need and guiding the design process:

y past performance data y forecasts of future performance

For new construction projects, there are no past performance data, so forecasts of future performance are most relevant to design decisions. For projects on existing roads, past (or present) performance data are available and forecasts of future performance may be developed.

Projects need not address every aspect of poor performance. Designation of the performance issues to be addressed in any given project is an agency management decision, with due consideration of funding availability and the effect that improvements in some aspects of performance may have on other aspects of performance. The purpose and need statement should make clear any limitations on the scope of the project.

1.3 OVERVIEW OF THE NEW FRAMEWORK FOR GEOMETRIC DESIGN

This policy incorporates a framework for geometric design based primarily upon:

y a functional classification system that characterizes roadways by their position in the transportation network and the type of service they provide to motor vehicles y a context classification system that characterizes roadways by their surrounding environment and how the roadway fits into the community

The functional classification system considers four general functional classes: freeways, arterials, collectors, and local roads and streets. These functional classes are defined in Section 1.4. The context classification system considers five context classes: rural, rural town, suburban, urban, and urban core. These context classes are defined in Section 1.5.

The first steps in the design process are to prepare a purpose and need and/or objectives statement for the project (see Section 1.2) and to determine where the project falls in the design framework shown in Figure 1-1. The design framework consists of twenty specific combinations of the four functional classes and the five context classes for roadways. Each of the combinations includes roadways that serve a distinct set of user needs. Nineteen of these twenty roadway types (all but freeways in the rural town context) are commonly found on the road and street network; the exception, representing a freeway in a rural town context, is unlikely to occur often.


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