Chapter 1: Design General


Section 1: Overview

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Application of Design Guidelines

The criteria contained in this Roadway Design Manual (manual) are applicable to all classes of highways from freeways to two-lane roads. This manual represents a synthesis of current information and design practices related to highway design.

Since no document can be expected to cover every highway design situation, the guidelines may require modification for local conditions. It is important that significant deviations from the manual be documented and be based on an objective engineering analysis.

It should be noted that roadway design criteria and technology is a rapidly changing field of study. The fact that new design values are presented or updated herein does not imply that existing highway conditions are less safe. Also, continually enhanced design practices do not mandate the need for improvement projects. With a significant transportation infrastructure in place, the intention is to use the most current design techniques on projects scheduled for future construction. The manual is intended to result in projects, which provide user safety and operational efficiency while taking into account environmental quality. Various environmental impacts can be mitigated or eliminated by the use of appropriate design practices. To the extent practical, the selection of cost effective design criteria can allow the finished project to be more consistent with surrounding terrain and/or settings.

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Roadway Design Manual Format

The manual is formatted to follow the traditional resurfacing, restoration, rehabilitation, and reconstruction (the four R’s) of highway construction. The individual sections are briefly described in the following paragraphs.

Chapter 2 presents basic design criteria. Portions of this section will have application to all projects to varying degrees. The chapter discusses traffic characteristics, sight distance, horizontal and vertical alignment, and cross sectional elements. The dimensions given in this chapter will be referenced for most of the roadway classifications.

Chapter 3 describes new location and reconstruction (4R) project design criteria. These projects usually represent the highest type design since these are either new roadways or almost totally reconstructed roadway sections. This chapter of the manual is broken into roadway classifications such as urban streets, suburban roadways, two-lane highways, multilane rural highways, and freeways.

Chapter 4 describes non-freeway rehabilitation (3R) project design criteria. Rehabilitation projects are intended to preserve and extend the service life of the existing roadway and to enhance safety. The chapter presents criteria for improvements and enhancements within the context of acceptable rehabilitation project design.

Chapter 5 describes nonfreeway restoration (2R) project design criteria. Restoration projects are intended to restore the pavement structure, riding quality, or other necessary components to their existing cross section configuration. The chapter makes a special note that the addition of through travel lanes is not permitted under a restoration project.

Chapter 6 describes special facility design criteria. Special facilities may include off-system bridge projects, historical roadways or structures, park roads, and bicycle facilities. For these projects, the roadway may have preservation or economic considerations which have equal weight with the user access and mobility characteristics of the roadway, bridge, or other facility.

Chapter 7 describes miscellaneous design elements. These elements may not be a part of all highway projects. Guidance is given concerning longitudinal barriers, attenuators, fencing, parking, emergency median openings, and minimum turning designs. These individual design elements can be selected as needed and incorporated into appropriate project designs.

Appendix A describes the components of guardrail installations and the methodology for determining appropriate lengths of need.

Appendix B describes the treatment of pavement drop-offs in work zones.

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External Reference Documents

It is recommended that the following publications, in their current editions, be available for reference in conjunction with this manual. All these listed publications are produced by entities other than the Texas Department of Transportation.

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  • A Policy of Geometric Design of Highway and Streets (Green Book), American Association of State Highway and Transportation Officials (AASHTO).
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  • Roadside Design Guide, American Association of State Highway and Transportation Officials (AASHTO).
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  • Highway Capacity Manual, Transportation Research Board (TRB).
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  • Guide for the Development of Bicycle Facilities, American Association of State Highway and Transportation Officials (AASHTO).
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  • Guide for the Design of High Occupancy Vehicle Facilities, American Association of State Highway and Transportation Officials (AASHTO)

The American Association of State Highway and Transportation Officials (AASHTO) has established various policies, standards, and guides relating to transportation design practices. These documents are approved references to be used in conjunction with this manual. However, the instructions given in this manual will take precedence over AASHTO documents unless specifically noted otherwise.

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