Chapter 1: General Guidance

1.1 Overview

The criteria contained in the Roadway Design Manual
(RDM) are applicable to all classes of highways. This RDM represents a synthesis of current information and design practices related to highway design.

The guidelines may require modification for local conditions to cover all highway design situations. It is important that significant deviations from the manual be documented and be based on an objective engineering analysis.

Roadway design criteria and technology are rapidly changing. New design values that 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 intent 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 considering environmental quality.

1.1.1 Project Objectives and Goals

Each project should begin with an explicit statement that indicates why the project is being undertaken and what the project is intended to accomplish. This statement should be documented and can be referred to in the Design Summary Report (DSR)
.

The Project Scoping portion of the DSR should contain information including what will, and what will not, be undertaken in the project. This includes the project problems and proposed solutions, and project goals and objectives. The Project Scope should be built around an assessment of past performance of all transportation modes and a forecast of future performance. It should identify aspects of performance and metrics for performance improvement.

The Project Scoping Statement will be documented in the DSR.

1.1.2 Framework for Geometric Design

Historically, a range of “minimum” to “desirable” criteria for design values were provided with the “desirable” design criteria preferred.

American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets

provides a framework by which projects should be assessed to determine appropriate design values rather than defaulting to a “minimum” or “desirable” value. This value is shown as the “target” value
throughout the remainder of the manual.

“Target” values
are meant to serve as a starting point for selecting the specific design criteria. The final selected design criteria value may be more or in some cases less (a “minimum” value) than the “target” value. The designer should use their engineering judgement to select the final design criteria based on the functional classification, context classification and performance metrics of the project established during project scoping

Similarly, the RDM provides the following framework based on five factors:

Functional Classification
– characterizes roadways by their position in the transportation network and the type of service it provides for vehicular traffic. See

Chapter 3

Context Classification System
– characterizes roadways by their surrounding environment and their place in the community. See

Chapter 3

Modal Classification System
– considers the needs of all transportation modes in design of the roadway. See

Chapter 3

Project Type of Construction
– incorporates a design process to choose the appropriate design approach for each project. See

Section 1.1.3

Practical Design
– an approach to finding and balancing the needs of all users for each project. See

Chapter 2

Performance-based Design
– an approach for considering the effects of geometric design decisions. See

Chapter 2

The concept of Performance-Based Practical Design (PBPD)
is a decision-making approach that helps agencies better manage transportation investments and serve system-level needs and performance priorities with limited resources. See

Chapter 2

for a full discussion of PBPD.

1.1.3 Types of Construction

The RDM describes design criteria for the following categories of highway construction: resurfacing, restoration, rehabilitation, and reconstruction.

Table 1-1

gives definitions of the different highway construction categories.

**Table 1-1: Highway Construction Category Definitions**
Highway Construction Category	Definition
Resurfacing	Limited existing pavement surface repairs such as milling, spot patching, seal coat, rut repair, crack sealing, and fog sealing.
Restoration	Existing pavement structure repairs such as patching or rebuilding short sections of road, milling and hot mix overlays, shoulder edge repairs, or minor safety improvements (i.e., guardfence, pipe headwalls, etc.).
Rehabilitation	Partial or full depth pavement reconstruction on existing profile/alignment such as removal and replacement of the existing riding surface and base material, driveway improvements, ditch regrading or safety improvements (i.e., guardfence, pipe headwalls, etc.). If approved, minor lane/shoulder widening, adding medians/curbing, minor improved drainage design or intersection improvements are included.
Reconstruction	Projects that utilize an existing roadway alignment (or make only minor changes to an existing alignment) but involve a change in the basic roadway type. Changes in the basic roadway type include widening a road to provide additional through lanes or adding a raised or depressed median where none currently exists, and where these changes cannot be accomplished within the existing roadway width (including shoulders).
New Construction	Projects on a new alignment or new construction.

TxDOT designates the following as applicable design criteria in

Table 1-2

using the definitions above.

**Table 1-2: TxDOT Construction Types and Applicable Design Criteria**
Roadway Design Criteria	Highway Construction Categories	Definition
2R	Resurfacing and Restoration	Projects consist of non-freeway work on facilities not on the National Highway System (NHS) with a current ADT ≤ 2500 per lane that propose to restore the pavement to its original condition. Includes restoration of pavement structure, ride quality, or other necessary components to its existing cross section configuration. The principal purposes of these projects are surfacing and repair of the pavement structure. The addition of continuous two-way left-turn lanes (TWLTL), acceleration/deceleration lanes, turning lanes, and shoulders are acceptable as restoration work as long as the existing through lane and shoulder widths are maintained as a minimum. Work DOES NOT include the addition of through travel lanes.
3R	Resurfacing, Restoration and Rehabilitation	Projects consist of transportation projects that extend the service life and enhance the safety of a roadway including freeways and NHS roadways. In addition to the work described under resurfacing and restoration, the activities include upgrading the geometric design and safety of the facility. Work may include the upgrading of geometric features such as roadway widening, minor horizontal and vertical realignment (typically less than 10% of the project length) and improving bridges to meet current standards for structural loading and to accommodate the approach roadway width. Work DOES NOT include the addition of through travel lanes.
4R	Resurfacing, Restoration, Rehabilitation, and Reconstruction and New Construction	Projects on a new alignment or new construction. Projects that upgrade an existing roadway to meet geometric criteria for a new facility. Projects that include substantial changes in the geometric character of the highway, such as widening to provide additional through lanes, significant horizontal or vertical realignments, and major improvements to the pavement structure to improve long term service. Bridge replacement projects with ADT > 400.
5R	Mobility Corridors	Projects that are intended to regenerate, or produce new, long-term transportation opportunities including multiple modes such as rail, utilities, freight, and passenger characteristics.
Notes:
Guidance on Preventative Maintenance (PM) projects can be found in TxDOT's Maintenance Management Manual and the Maintenance Operations Manual .

1.1.4 Roadway Design Manual Format

The RDM is formatted to present general guidance, concepts of design presented in

AASHTO’s A Policy on Geometric Design of Highways and Streets

(i.e., the Green Book) and basic design criteria.

Table 1-3

lists the chapters and chapter descriptions of the RDM.

**Table 1-3: RDM Chapter Contents**
Chapter	Chapter Name	Description
1	General Guidance	Describes the design elements that require Design Exceptions, Design Waivers, Design Variances, and the Texas Highway Freight Network’s Design Deviations. Also includes references for schematic design, access control, preliminary engineering, and maintenance in design.
2	Performance-Based Practical Design Concepts	Discusses the concepts and application of performance-based practical design.
3	Context and Facility Type Considerations	Describes the five context classifications along with considerations for all modes of users and traffic networks.
4	Basic Design Criteria	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.
5	Local Roads(4R)	Describes target design criteria for Local Roads.
6	Collectors(4R)	Describes target design criteria for Collector roads and streets.
7	Arterials (4R)	Describes target design criteria for Arterial roads and streets
8	Freeways (4R)	Describes target design criteria for freeways. Freeways include Interstates, Expressways, and Freeways.
9	Mobility Corridor Facilities (5R)	Describes and provides design guidance on mobility corridors with design speeds of 85 mph to 100 mph.
10	3R Design Criteria	Describes 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.
11	2R Design Criteria	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.
12	Special Facilities	Describes special facility design criteria. Special facilities may include offsystem bridge projects, historical roadways or structures, and park roads. 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.
13	Intersections	Discusses types of intersections and design criteria and considerations for the design of intersections.
14	Alternative Intersections and Interchanges	Discuss types of alternative intersections and interchanges and design criteria and considerations for the design of alternative intersections and interchanges.
15	Grade Separations and Interchanges	Discusses grade separations and interchanges. Describes design criteria for ramps.
16	Driveways	Discusses design criteria and considerations for the design of driveways.
17	Roadside Safety Design and Roadside Safety Hardware	Discusses the design of roadside safety and roadside safety hardware.
18	Bicycle Facilities	Describes design criteria and considerations for the design of bicycle facilities
19	Pedestrian Facilities	Describes design criteria and considerations for the design of pedestrian facilities.
20	Motorcyclist Design Considerations	Discusses motorcyclist design considerations.
21	Texas Highway Freight Network, Texas Trunk System, NHS, STRHNET, and Hurricane Evacuation Routes	Describes the policy for highways on the Texas Highway Freight Network.
22	Transit	Discusses the design of transit for buses and trains as it pertains to vehicular roadways.
23	Temporary Traffic Control Design	Discusses design criteria and considerations for the design of temporary traffic control.
24	Other Design Elements	Discusses roadway design considerations for fencing, illumination, signing, pavement markings, intelligent transportation systems (ITS),wildlife crossings, and the Texas Aggregate Quarry and Pit Safety Act
Appendix A	Minimum Values for Projects Requiring a Design Exception	Tables of the minimum design values, that if not met, would require a design exception as described in Chapter 1.
Appendix B	Minimum Values for Projects Requiring a Design Waiver	Tables of the minimum design values, that if not met, would require a design waiver as described in Chapter 1.
Appendix C	List of References	List of references found in the manual with links.