Chapter 1: Design General


Section 1: Overview

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Application of Design Guidelines

The criteria contained in the Roadway Design Manual (RDM) are applicable to all classes of highways from freeways to two-lane roads. This RDM 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 are 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 significant transportation infrastructure in place, the intention is to use the most current design techniques on future construction projects.

Various environmental impacts can be mitigated or eliminated using appropriate design practices. The result of the application of this manual should result in projects which provide user safety and operational efficiency while taking into account environmental quality. Whereas “desirable” design criteria are generally preferred, the selection of “minimum” design criteria may be warranted to produce a finished project that is more consistent with surrounding terrain and/or settings.

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Roadway Design Manual Format

The RDM is formatted to incorporate the following categories of highway construction: resurfacing, restoration, rehabilitation, and reconstruction. The following is a brief description of each chapter:

Chapter 1 describes the requirements of Design Exceptions, Design Waivers, Design Variances, and the Texas Highway Freight Network's Design Deviations. The chapter also summarizes the requirements for schematic layouts and approval, preliminary design, access to the interstate, and maintenance considerations during design.

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 involve substantial design since they are either new roadways, added capacity projects or almost totally reconstructed roadway sections. The chapter is broken into the following roadway classifications: urban streets, suburban roadways, two-lane highways, multi-lane 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 non-freeway 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 and roadside safety hardware criteria, fencing, pedestrian separation and ramps, parking, rumble strips, emergency median openings on freeways, and minimum turning designs for trucks and buses. These individual design elements can be selected as needed and incorporated into appropriate project designs.

Chapter 8 describes and provides design guidance on mobility corridors with design speeds of 85 mph to 100 mph. Guidance is given on roadway design criteria, roadside design criteria, ramps, and direct connectors.

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.

Appendix C provides guidance on the location and design of driveway connections.

Appendix D provides guidance on right-turn slip lanes.

Appendix E provides guidance on alternative intersection designs. An overview, design considerations, pedestrian and bicyclist considerations, and access management are discussed for roundabouts, Diverging Diamond Interchanges (DDI), Median U-Turn Intersections (MUT), Restricted Crossing U-Turn Intersections (RCUT), and Displaced Left Turn Intersections (DLT).

NOTE: Guidance on Preventative Maintenance (PM) projects can be found in TxDOT's Maintenance Management Manual and the Maintenance Operations Manual.

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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 the RDM. These listed publications are produced by entities other than the Texas Department of Transportation.

AASHTO has established various policies, standards, and guides relating to transportation design practices. These documents are approved references to be used in conjunction with the RDM. However, the instructions given in the RDM will take precedence over AASHTO documents unless specifically noted otherwise.

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