GDOT design policy manual : a Georgia Department of Transportation publication

TABLE OF CONTENTS

Chapter

Revision

1. INTRODUCTION................................................................................ 6/11/2010
1.1 Purpose 1.2 Organization 1.3 Contact 1.4 Manual Updates 1.5 Project Review and Submission Requirements 1.6 Acknowledgements
2. DESIGN POLICIES, GUIDELINES, AND STANDARDS............................. 6/11/2010
2.1 General Design Policy Information 2.2 Exceptions to Design Standards 2.3 Context Sensitive Design
3. DESIGN CONTROLS.......................................................................... 4/22/2011
3.1 Functional Classification 3.2 Design Vehicles 3.3 Design Speed 3.4 Highway Capacity 3.5 Establishment of Access Control 3.6 Frontage Roads and Access Roads 3.7 Fencing 3.8 Right-of-Way Controls 3.9 Value Engineering 3.10 Environmental
4. ELEMENTS OF DESIGN....................................................................... 9/3/2010
4.1 Sight Distance 4.2 Horizontal Alignment 4.3 Vertical Alignment 4.4 Combined Horizontal and Vertical Alignments 4.5 Superelevation
5. ROADSIDE SAFETY AND LATERAL OFFSET TO OBSTRUCTIONS..........02/22/2011
5.1 General Considerations 5.2 Rural Shoulders Lateral Offset to Obstruction 5.3 Urban Shoulders Lateral Offset to Obstruction 5.4 Lateral Offsets for Signs 5.5 Lateral Offsets for Light Standards 5.6 Lateral Offsets for Utility Installations 5.7 Lateral Offsets for Signal Poles and Controller Cabinets 5.8 Lateral Offsets to Trees and Shrubs
6. CROSS SECTION ELEMENTS.............................................................07/22/2011
6.1 Lane Width 6.2 Pavement Type Selection 6.3 Cross Slope

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6.4 Pavement Crowns 6.5 Shoulders 6.6 Side Slopes 6.7 Border Area (urban shoulder) 6.8 Bike Lanes 6.9 Curbs 6.10 Sidewalks 6.11 Barriers 6.12 Medians 6.13 Parking Lanes 6.14 Summary of Design Criteria for Cross Section Elements
7. AT GRADE INTERSECTIONS............................................................... 6/11/2010
7.1 Intersection Design Elements 7.2 Intersection Geometrics 7.3 Median Openings 7.4 Driveways 7.5 Signalization 7.6 Highway-Railroad Grade Crossings
8. ROUNDABOUTS.................................................................................6/27/2011
8.1 Introduction 8.2 Roundabout Validation Process 8.3 Design Guidelines 8.4 References 8.5 Definition of Terms
9. BICYCLE AND PEDESTRIAN ACCOMMODATIONS............................... 3/1/2011
9.1 Overview 9.2 Typical Users & Needs 9.3 Bicycle Route Networks 9.4 Warrants for Accommodation 9.5 Facility Design 9.6 Work Zone Accessibility
10. ROADWAY USER COST UNDER DEVELOPMENT.................................. TBD
11. OTHER PROJECT TYPES.................................................................. 7/21/2011
11.1 Preventative Maintenance (PM), 3R, and Reconstruction Guidelines for Federal Aid Projects
11.2 Special Design Considerations for Other Project Types 11.3 Design Elements for Other Project Types
12. STAGE CONSTRUCTION UNDER DEVELOPMENT.................................. TBD
13. TRAFFIC FORECASTING AND ANALYSIS............................................. 5/21/2009
13.1 Traffic Forcasting Process 13.2 Freeway Traffic Analysis and Design 13.3 Arterial Traffic Analysis and Design 13.4 Trip Generation and Assignment for Traffic Impact Studies

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14. LIGHTING......................................................................................... 2/12/2009

14.1 General Considerations 14.2 Types of Lighting Projects 14.3 Illumination Requirements 14.4 Lighting Calculations 14.5 Design Considerations 14.6 Power Service

RESOURCES Glossaries References Implementation

5/21/2007 5/21/2007 5/21/2007

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1. INTRODUCTION
1.1. Purpose
The GDOT Design Policy Manual is the primary resource for design guidelines and standards adopted by the Georgia Department of Transportation (GDOT) for the preparation of roadway construction plans. This manual is intended to provide guidance to the designer involving "controlling criteria" and "non-controlling criteria" recommended and in some cases stipulated by the Georgia Department of Transportation (GDOT), the American Association of State Highway and Transportation Officials (AASHTO), the Federal Highway Administration (FHWA), and various national research organizations involving the design of roadways and related infrastructure. Designers are encouraged to select design criteria that provide a balance among the design vehicle, other users of the facility, and the context of the surrounding environment.
The GDOT Design Policy Manual was created by committee with representatives from GDOT, FHWA, and the consultant community in Georgia. This manual was written primarily for GDOT personnel, local governments, and consulting engineering firms that design roadway construction plans for Federal-Aid projects and State-Aid projects in accordance with the policies and objectives of Titles 23, 40, and 42 of the United States Code, and Title 32 of the Official Code of Georgia Annotated. Every effort has been made to make this manual as complete and error free as possible.
1.2. Organization
The Georgia Department of Transportation improves, constructs, and maintains the state's roads and bridges and provides planning and financial support for other modes of transportation such as mass transit and airports. GDOT also provides administrative support to the State Road and Tollway Authority (SRTA) and the Georgia Regional Transportation Authority (GRTA). GDOT is managed and operated by the Commissioner of the Georgia Department of Transportation with direct oversight by the State Transportation Board. GDOT's Mission statement is:
The Georgia Department of Transportation provides a safe, seamless and sustainable transportation system that supports Georgia's economy and is sensitive to its citizens and environment.
The Georgia Department of Transportation Organization Chart can be found at: http://www.dot.ga.gov/aboutGeorgiadot/Documents/OrgChart.pdf
The GDOT Division of Engineering has the primary role in the roadway design process. The mission of the Division of Engineering is to develop a quality set of right of way plans, construction plans, and bid documents, through a cooperative effort, that results in a project design and implementation that is the best transportation value for the taxpayers of Georgia.
Offices under the umbrella of the Division of Engineering are:
1. Office of Environmental Services 2. Office of Design Policy & Support
Design Policy/Standards/Hydraulic Engineering and ESPCP Tech Support Statewide Location Bureau Engineering Systems Support 3. Office of Roadway Design 4. Office of Bridge and Structural Design 5. Office of Right-of-Way

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1.3. Contact
The GDOT Design Policy Manual is maintained by the Office of Design Policy & Support. To submit questions or comments specific to the GDOT Design Policy Manual and its contents, please send an e-mail to: designmanual@dot.ga.gov
The current GDOT Design Policy Manual is published on the Department's Repository for Online Access to Documentation and Standards (R.O.A.D.S) homepage at: http://www.dot.ga.gov/doingbusiness/PoliciesManuals/roads/Pages/default.aspx
1.4. Manual Updates
The GDOT Design Policy Manual will be periodically updated so that it appropriately reflects the Department's current design policies and practices. An entire chapter or any portion of one or more chapters of the manual may be re-written and replaced at any time. Revisions to this manual are summarized in the Table of Contents. The version and latest revision date are listed in the manual's Table of Contents, in the Table of Contents for each chapter, and at the bottom of each page of the manual. Implementation dates may be specified for certain revisions.
Subscribers to the R.O.A.D.S. homepage will receive e-mail notices of updates to the GDOT Design Policy Manual.
1.5. Project Review and Submission Requirements
Project review and submission requirements shall be in accordance with the latest edition of the GDOT Plan Development Process (PDP). The current PDP is published online at: http://www.dot.ga.gov/doingbusiness/PoliciesManuals/roads/Pages/default.aspx
The GDOT PDP sets forth the current procedures and steps necessary for GDOT to administer Federal-Aid projects in accordance with the policies and objectives of Titles 23, 40, and 42 United States Code, and to administer State-Aid projects to fulfill the policies and objectives of Title 32, Official Code of Georgia Annotated. The GDOT PDP outlines the current process of project development from project identification through construction award.
All design criteria and design decisions should be documented in a Project Design Data Book, which is to be included with the project files. Procedures and criteria for the Project Design Data Book are provided in the GDOT PDP Chapter 6, Preliminary Design.
1.6. Acknowledgements
The Georgia Department of Transportation Division of Engineering wishes to acknowledge the following for their contribution to the development of the GDOT Design Policy Manual:
GDOT - Brent Story, Eugene Hopkins, Brad Ehrman, Kim Fulbright, Ben Buchan, Darrell Richardson, Darryl Van Meter, Chuck Hasty, Babs Abubakari, Keith Golden, Kathy Bailey, Abby Ebodaghe, Gary Langford, Joel North, Joe Wheeler, Ron Wishon, Del Clippard
FHWA - David Painter and the Georgia Division of FHWA Consultants - Stan Hicks, Bill Pate, Jody Braswell, Jill Hodges, Joe Macrina, Tim Heilmeier, Taylor
Stukes, Julie Doyle, Jeff Dyer, Michael Holt, Mike Reynolds, Harris Robinson, Vern Wilburn

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Chapter 2 Contents

2. Design Policies, Guidelines, and Standards

1

2.1. General Design Policy Information

1

2.1.1. Sources of Design Policy and Practice

2

2.2. Exceptions to Design Standards

4

2.2.1. Design Exception

4

2.2.2. Design Variance

4

2.3. Context Sensitive Design

5

Chapter 2 Index

6

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2. DESIGN POLICIES, GUIDELINES, AND STANDARDS

2.1. General Design Policy Information
Design policy is defined as the basic principles and goals established by GDOT to guide (guidelines) and control (standards) the design of roadways and related infrastructure in Georgia. The intent of design policy is to provide recommended and stipulated values for critical dimensions. Flexibility is permitted to encourage independent design tailored to individual situations. When flexibility is applied to a proposed design, and critical dimensions do not meet GDOT design policy, additional documentation is required to record the decision-making-process.

Criteria within this manual denoted as "standard" or as "controlling criteria" have been identified as a required or mandatory practice with deviation from the controls requiring prior agency approval. All other criteria within this manual are considered to be "non-controlling criteria" and are denoted as "guidelines" intended as recommended practice with deviation allowed if engineering judgment or study indicate the deviation to be appropriate. Designers are encouraged to select design criteria that provide a balance among the design vehicle, other users of the facility, and within the context of the surrounding environment.

Unless stated otherwise, the policies in this manual apply to permanent construction of roadways and related infrastructure. Different criteria may be applicable to temporary facilities. Guidance specific to non-interstate roadway resurfacing, restoration, or rehabilitation (3R) projects is provided in Chapter 11 of this Manual.

The following definitions offer guidance for interpreting policy statements found in this manual:

Standard a required criteria or mandatory practice. Criteria denoted as standard have been identified by the Department as having substantial importance to the operational and safety performance of a roadway such that special agency review and approval (Design Variance or Design Exception) will be required before deviation from the controls can be incorporated into a design. All controlling criteria are denoted as standard. In some cases, GDOT has denoted specific non-controlling criteria as standard.

Controlling Criteria: The FHWA has specifically identified "13 controlling criteria" (listed below) as having substantial importance to the operational and safety performance of a roadway such that special agency attention should be given to the criteria in the design decision making process.

1. Design speed 2. Lane width 3. Shoulder width 4. Bridge width 5. Horizontal alignment 6. Superelevation 7. Vertical alignment

8. Grades 9. Stopping sight distance 10. Cross slope 11. Vertical clearance 12. Lateral offset to obstruction 13. Structural capacity

The conditions of the "13 controlling criteria" are defined by AASHTO and are adopted and

denoted as standard criteria by GDOT. In some cases, GDOT provides more specific and

selective guidelines relating to controlling criteria; however, at a minimum, the conditions defined

by AASHTO control. A decision to use a design value that does not meet the minimum

controlling criteria defined by AASHTO will require the prior approval of a Design Exception from the GDOT Chief Engineer.

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Shall: The use of the word "shall" denotes a required or mandatory condition, and the designer must make every effort to follow the appropriate design criteria or condition.

Guideline recommended practice in typical situations. Deviations from criteria denoted as guidelines are allowed when engineering judgment or study indicates the deviation to be appropriate. Adequate study, justification, and documentation by the GDOT office or consultant responsible for the engineering is required. Decisions to deviate from guidelines are subject to review and scrutiny by GDOT at any time.
Should: The use of the word "should" indicates an advisory condition. Under this condition, it is recommended, although not mandatory, that the designer follow the appropriate design criteria.
May: The use of the word "may" indicates a permissive condition. Under this condition, the designer is encouraged to use sound engineering judgment.
Practical: The use of the word "practical" is intended to indicate that a design decision is effective and applicable with consideration to economic resources; appropriate, adaptable, and balanced.
Where practical: The use of the term "where practical" is intended to indicate that agencies may consider economic resource constraints when making a decision.

2.1.1. Sources of Design Policy and Practice GDOT adopts the AASHTO Green Book, "A Policy on Geometric Design of Highways and Streets," and the AASHTO "A Policy on Design Standards for the Interstate System" as the standard for controlling criteria required and mandatory on State Routes and routes on the National Highway System (NHS) in Georgia.

For additional guidance on the design of roadways and related infrastructure, refer to the most current edition of the following publications, unless a specific version is noted. These publications, including the website addresses (url) for resources available online, are cited in the References section of this Manual:

American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets (Green Book) A Policy on Design Standards---Interstate System Guide for the Development of Bicycle Facilities Guide for High-Occupancy Vehicle (HOV) Facilities Guide for Park-and-Ride Facilities Guide Specifications for Horizontally Curved Steel Girder Highway Bridges Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT 400) Highway-Rail Crossing Elimination and Consolidation Roadside Design Guide Roadway Lighting Design Guide Standard Specifications for Highway Bridges

American Railway Engineering and Maintenance of Way Association (AREMA) Manual for Railway Engineering

Federal Highway Administration (FHWA) Americans with Disabilities Act (ADA) and Transportation Enhancements (TE)

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Flexibility in Highway Design Guidance on Traffic Control Devices at Highway-Rail Grade Crossings Highway-Railroad Grade Crossings: A Guide to Consolidation and Closure Manual on Uniform Traffic Control Devices (MUTCD) Roundabouts: An Informational Guide FHWA-RD-00-67 Value Engineering and The Federal Highway Administration (Website) Roadway Lighting Handbook Mitigation Strategies for Design Exceptions
Georgia Department of Transportation (GDOT) Bridge and Structures Policy Manual Environmental Procedures Manual Manual on Drainage Design for Highways Construction Standards and Details Context Sensitive Design Online Manual Pavement Design Manual Pedestrian and Streetscape Guide Plan Development Process (PDP) Plan Presentation Guide Regulations for Driveway and Encroachment Control Standard Specification Book Traffic Analysis and Design Manual Traffic Signal Design Guidelines Utility Accommodation Policy and Standards Manual
Georgia Soil and Water Conservation Commission (GSWCC) Manual for Erosion and Sediment Control in Georgia

Illuminating Engineering Society of North America (IESNA) Guideline for Security Lighting for People, Property and Public Spaces Lighting Handbook, 9th Edition Lighting For Parking Facilities Recommended Lighting for Walkways Recommended Lighting for Walkways and Class 1 Bikeways Roadway Lighting ANSI Approved Roadway Sign Lighting Tunnel Lighting Roundabout Lighting

Institute of Transportation Engineers (ITE)

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Manual of Uniform Transportation Engineering Studies Trip Generation Handbook
National Cooperative Highway Research Program (NCHRP) Modern Roundabout Practices [Synthesis 264] Design Speed, Operating Speed, and Posted Speed Practices [NCHRP Report 504] Evaluating Intersection Improvements: An Engineering Study Guide [NCHRP Report 457] Impacts of Access Management Techniques [NCHRP Project 3-52] Recommended Procedures for the Safety Performance Evaluation of Highway Features [Report
350]
National Fire Protection Association (NFPA) National Electrical Code [NFPA-70]
Texas Transportation Institute (TTI) Grade Separations - When Do We Separate? Highway-Rail Crossing Conference
Transportation Research Board (TRB) Highway Capacity Manual
2.2. Exceptions to Design Standards
2.2.1. Design Exception If a design feature of a new construction or reconstruction project does not meet the minimum conditions of one of the "13 controlling criteria" defined in the current edition of the AASHTO Green Book and the AASHTO publication, A Policy on Design Standards - Interstate System, then approval to build or retain the feature is required by formal Design Exception. For projects identified as "Full Oversight" such as interstate projects, the FHWA is the agency that grants Design Exceptions. For all other projects, both federally and state funded, the GDOT Chief Engineer grants Design Exceptions.
The requirement of a Design Exception is not meant to impede design flexibility, but to document a very important design decision that is well scrutinized by the Department in a deliberative and thorough manner. To obtain a Design Exception, a comprehensive study and formal request shall be submitted using the format and procedures outlined in the GDOT Plan Development Process (PDP), and in the FHWA publication, Mitigation Strategies for Design Exceptions.

2.2.2. Design Variance
Whenever a "non-controlling" criteria has been denoted by GDOT as a standard, then the approval of a Design Variance must be obtained by the GDOT Chief Engineer before deviation outside the controls can be incorporated into the design.

The requirement of a Design Variance is not meant to impede design flexibility, but to document a very important design decision that is well scrutinized by the Department in a deliberative and thorough manner. To obtain a Design Variance, a comprehensive study and formal request shall be submitted using the format and procedures outlined in the GDOT Plan Development Process (PDP).

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2.3. Context Sensitive Design
Context Sensitive Design (CSD) is a process for achieving design excellence by developing transportation solutions that require continuous, collaborative communication and consensus among transportation agencies, professionals, and stakeholders. A common goal of CSD projects is to develop a facility that is harmonious with the community, and preserves aesthetics, history and the environmental resources, while integrating these innovative approaches with traditional transportation goals for safety and performance.
Refer to the GDOT Context Sensitive Design Online Manual for additional information on communication strategies, design flexibility, environmental sensitivity, and stakeholder involvement for developing successful context-sensitive solutions.

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Chapter 3 Contents

3. Design Controls

3.1. Functional Classification

3.2. Design Vehicles 3.2.1. Design Vehicle Types 3.2.2. Local Input for Selecting a Design Vehicle
3.3. Design Speed

3.3.1. 3.3.2. 3.3.3. 3.3.4.

General Considerations Intersections Approaching a Stopped Condition Freeway Exit and Entrance Ramps Urban Subdivision Streets

3.4. Highway Capacity

3.5. Establishment of Access Control 3.5.1. Definitions 3.5.2. Access Management
3.6. Frontage Roads and Access Roads

3.7. Fencing

3.7.1 Fencing on State Right-of-Way 3.7.2 Fencing on Private Property

3.8. Right-of-Way Controls

3.8.1. 3.8.2. 3.8.3. 3.8.4.

Rural Areas Urban Areas Special Types of Right-of-Way Accommodating Utilities

3.9. Value Engineering

3.10. Environmental

Chapter 3 Index List of Figures

Figure 3.1 Limit of Access Control Interstate/Freeway Interchange

List of Tables Table 3.1 Minimum Design Vehicles

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1 1 5 5 6 7 7 7 7 8 8 8 8 9 11 12 12 13 14 14 15 15 15 16 16 18
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3. DESIGN CONTROLS
This chapter provides information with regard to design controls. Many factors contribute to the roadway design criteria used by designers. These factors are based upon the physical characteristics of the vehicles (vehicle types), the topography in which the road is set, operational safety and speed of traffic on the road, and even driver behavior (speed, turns, following distance, clear zones for emergencies). All of these factors are important and should be balanced when selecting the appropriate design criteria for a particular road or highway design. This chapter addresses:
functional classification; design vehicles; design speed; highway capacity and level of service; access control; frontage and access roads; fencing; right-of-way controls; value engineering; and environmental considerations.
3.1. Functional Classification
Design standards have been developed by the American Association of State Highway and Transportation Officials (AASHTO) for different functional systems of roadways. In order to qualify for federal funding, the Federal Highway Administration (FHWA) requires that each state categorize state routes by functional classification. Detailed discussions on the concept of functional classification and the characteristics of the various functional systems can be found in the AASHTO Green Book1 and FHWA Functional Classification Guidelines2,3. Additional information specific to GDOT policies related to functional classification of roadways is also available in the GDOT Plan Development Process (PDP).
Roadway functional classification serves as the foundation of an access management program. Functional classification systems establish the planned function of different types of roadways and the priority placed on access as opposed to through traffic movement. Functional classification recognizes that design considerations vary for different classes of roads in accordance with the intended use.

1 AASHTO. A Policy on Geometric Design of Highways and Streets (Green Book).
2 FHWA. FHWA Functional Classification Guidelines. 1989 Note: The 1989 version of this publication is available online at http://www.fhwa.dot.gov/planning/fctoc.htm
3 FHWA, FHWA 2008 Updated Guidance for the Functional Classification of Highways, Memorandum from Mary B. Phillips, Associate Administrator for Policy and Governmental Affairs dated October 14, 2008.

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Streets and highways are grouped into major classes based on the type or kind of service they provide. The functional classification process is based on the fact that roads are part of a travel network and that "individual roads and streets do not serve travel independently in any major way" (Functional Classification Guidelines, 1989).
The three major functional systems are:
Freeways; Arterials; and collectors and local streets.
Freeway Classification Freeways can be distinguished from all other roadway systems in that they provide uninterrupted flow. There are no fixed interruptions on freeways. The traffic flow conditions along uninterruptedflow facilities result primarily from the interactions among vehicles in the traffic stream and between vehicles and the geometric and environmental components of the roadway.
Access to the freeway facility is controlled and limited to ramp locations, whereas access to an interrupted flow facility uses at-grade intersections. Categorization of uninterrupted and interrupted flow relates to the type of facility as opposed to the quality of the traffic flow at any given time. A freeway experiencing extreme congestion differs greatly from a non-freeway facility experiencing extreme congestion, in that the conditions creating the congestion are commonly internal to the facility, not external to the facility.
Freeway facilities may have interactions with other freeway facilities as well as other classes of roads in the vicinity. The performance of a freeway may be affected when demand exceeds capacity on these nearby road systems. For example, if the street system cannot accommodate the demand exiting the freeway, over-saturation of the street system may result in queues backing onto the freeway, which adversely affects freeway performance.
Traffic analysts and designers must also recognize that freeway systems have several interacting components, including ramps, and weaving sections. To achieve an effective overall design, the performance of each component must be evaluated separately and the interactions between components must also be considered. For example, the presence of ramp metering affects freeway demand and must be taken into consideration when analyzing a freeway facility.
Arterial Classification Arterials are a functional classification of roadway transportation facilities that are intended to provide for through trips that are generally longer than trips on collector facilities and local streets. While the need to provide access to abutting land is not the primary function, the design of arterials must also balance this important need. To further highlight the often competing demands of urban arterials, other modes of travel such as pedestrians and public transit are also present and must be accommodated.
To assure that an arterial can safely provide an acceptable level of service (LOS) for the design conditions, a number of design elements must be addressed. Since each design element is essentially determined based on separate analyses, the designer should evaluate the entire arterial system and be prepared to refine certain elements to obtain an effective and efficient overall design.

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Arterial systems are often further sub-classified into Principal or Minor arterial functional systems based on the trips served, the areas served, and the operational characteristics of the streets or highways. "Since urban and rural areas have fundamental different characteristics with regards to the density of land use, nature of travel patterns and the number of streets and highway network and the way in which these elements are related, urban and rural functional systems are classified separately as urban principal and minor arterials and rural principal and minor arterials" (FHWA. 1989). These functional systems are therefore discussed individually under the Urban Arterial Classification and Rural Arterial Classification sections below.

Urban Arterial Classification
The AASHTO Green Book defines urban areas as those places within the boundaries set by the responsible State and local officials having a population of 5,000 or more. Urban areas are further subdivided into urbanized areas (population of 50,000 and over) and small urban areas (population between 5000 and 50,000) (AASHTO). For design purposes, the designer should use the population forecast for the design year.

There are four functional systems for urban areas:

Urban Principal Arterials - almost all fully and partially controlled access facilities in urban areas are considered urban principal arterials; however, this system is not restricted to controlled access routes. FHWA further stratifies the principal arterial system as: interstate, other freeways and expressways, and other principal arterials with no control of access (Functional Classification Guidelines, 1989).

Urban Minor Arterials - includes all arterials not classified as a principal. This functional system includes facilities that:
o place greater emphasis on land access than principal arterials and offer a lower level of traffic mobility;

o interconnect with, and augment, the urban principal arterial system;

o provide service to trips of moderate length at a somewhat lower level of travel mobility than principal arterials;

o distribute travel to smaller areas than those of urban principal arterials; and

o may carry local bus routes and provide intra-community continuity, but ideally should

not penetrate identifiable neighborhoods. Note: this system should also include

urban connections to rural collector roads where such connections have not been

classified as urban principal arterials.

(AASHTO Green Book)

Collector Streets Some characteristics of collector streets are that they:
o provide access and traffic circulation within residential neighborhoods, commercial, and industrial areas;

o may penetrate residential neighborhoods, distributing trips from the arterials to destinations; and

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o collect traffic from local streets in residential neighborhoods and channel traffic to

the arterial system.

(AASHTO Green Book)

Local Streets - Some characteristics of local streets are that:
o local streets provide direct access to abutting land and access to higher systems; and

o local street systems offer the lowest level of mobility and usually contain no bus

routes. Service to through traffic movement in this system is usually deliberately

discouraged.

(AASHTO Green Book)

Rural Arterial Classification
The functional systems for urban arterials and rural arterials differ due to factors such as intensity and type of development that occurs on these systems.

Rural Principal Arterials almost all fully and partially controlled access facilities in rural areas are considered rural principal arterials; however, this system is not restricted to controlled access routes. Service characteristics of rural principal arterials include:
o traffic movements with trip length and density suitable for substantial statewide travel or interstate travel;
o traffic movements between urban areas with populations greater than 25,000;
o traffic movements at high speeds;
o divided four-lane roads; and
o desired LOS B.

Rural Minor Arterials have the following service characteristics:

o traffic movements with trip length and density suitable for integrated interstate or inter-county service;

o traffic movements between urban areas or other traffic generators with populations less than 25,000;

o traffic movements at high speeds;
o undivided lane roads;
o striped for one or two lanes in each direction with auxiliary lanes at intersections as required by traffic volumes; and

o desired LOS B.

( AASHTO Green Book)

Refer to the AASHTO Green Book, Chapter 1. Highway Functions, for additional information regarding functional classification.

Mapping of roadway functional classifications for all urban and non-urban areas in Georgia is maintained by the GDOT Office of Transportation Data. Functional Classification Maps for Georgia State roadways may be downloaded from GDOT's website at: http://www.dot.ga.gov/maps/Pages/HighwaySystem.aspx.

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3.2. Design Vehicles
In the design of any roadway facility, the designer should consider the largest design vehicle that is likely to use that facility with considerable frequency or a design vehicle with special characteristics appropriate to a particular location in selecting design vehicles.

3.2.1. Design Vehicle Types

The four general classes of design vehicles as defined by AASHTO are:
Passenger Cars - Passenger automobiles of all sizes, including cars, sport/utility vehicles, minivans, vans, and pick-up trucks;
Buses - Intercity (motor coaches), city transit, school, and articulated buses;
Trucks - Single-unit trucks, truck tractor-semi-trailer combinations, and truck tractors with semi-trailers in combination with full trailers; and
Recreational Vehicles - Motor homes (including those with boat trailers and pulling an automobile), automobiles pulling a camper trailer or a boat trailer.

Table 3.1 Minimum Design Vehicles

Roadway Type
Rural

Minimum Design Vehicle

Typical Design Speed
(mph)

Interstate / Freeway Ramp
Free-Flow Entrance / Exit Loop Primary Arterial Minor Arterial Collector Local Road

WB-67

70

WB-67 WB-67 WB-67 WB-40 or WB-62
SU SU

35 (minimum)(2) 35 (minimum)(2) 35 (minimum)(2)
65 65 55

Paved

S-BUS36

45

Gravel Urban

S-BUS36

35

Refer to the current AASHTO Green Book Chapter 2, Design Controls and Criteria, for further discussion on use of design vehicles and for detailed dimensions of design vehicles.
Table 3.1. lists minimum vehicles which should be considered when selecting an appropriate design vehicle. Design vehicle dimensions are defined in the AASHTO Green Book (2004), Exhibit 2-1. Design Vehicle Dimensions.

Interstate / Freeway Ramp Terminal Ramp
Free-Flow Entrance / Exit Loop Primary Arterial Minor Arterial Collector Residential/Local Road

WB-50

65

WB-67 WB-67 WB-40 or WB-62 WB-40 or WB-62 WB-40 or BUS-40 BUS-40 or SU

35 (minimum)(1) 35 (minimum)(1) 35 (minimum)(1)
55 45 35

SU or P

35

Turning Radii
The minimum turning path of the selected design vehicle is the primary factor in

(1) Refer to Section 3.3.3 Freeway Ramps.
Design Vehicle Type Symbols: BUS=Intercity Bus/Motor Coach, P=Passenger Car, S-BUS=School Bus, SU=Single-Unit Truck, WB=Semi Trailer

designing radii at intersections, radii of

turning roadways, median opening geometry and commercial driveways. The turning radii can

affect the cross-section width of a roadway. In other words, the larger the required turning radii to

accommodate larger design vehicles, the wider the roadway cross-section needs to be. For

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example, a semi-trailer truck would need a much larger turning radii at a median opening to properly access a business or commercial distribution center than a passenger car or van.
Design tools that can be used to determine the turning path for a given design vehicle include:
Published templates which show the wheel paths of a design vehicle, such as the AASHTO Green Book, Exhibits 2-3 through 2-23, which presents the minimum turning path for 19 typical design vehicles; and
Vehicle turn simulation software, such as AutoTURN3, which works within both MicroStation and AutoCAD.
The need to provide turning radii for larger vehicles may sometimes conflict with the need to accommodate pedestrians. The design of an intersection should not prohibit safe pedestrian movements through the intersection. Refer to the GDOT Pedestrian and Streetscape Guide4 for information specific to accommodating pedestrians. Further discussion of GDOT policies relating to intersection design can be found in this Manual in Chapter 7, At-Grade Intersections and Chapter 8, Roundabouts.
3.2.2. Local Input for Selecting a Design Vehicle
The designer should be aware of all potential types of vehicles that will use each part of the facility and larger vehicles should be accommodated, where practical. Input from local personnel should be considered by the designer when determining the proper design vehicle for each local road that intersects the project. Local personnel may include the GDOT Area Engineer, Maintenance Engineer, District Access Engineer, or local government personnel. This determination should be completed during the conceptual design phase.
Scenarios where solicitation of local government input is recommended include: areas where bicycle use is allowed on a roadway - in which case the bicycle should be
considered a design vehicle;
roadways leading to recreational areas like state parks, campgrounds, and marinas - in which case recreational vehicles, such as motor homes or pick-up trucks with boat trailers, may be the appropriate design vehicle;
some areas near timber processing facilities - in which case, "long log" trucks (trucks with logs overhanging the trailer by as much as 12-ft.) may be prevalent, as intersections in these areas may require a design that prevents overhanging logs from striking vehicles in other lanes during turning movements. This can usually be accomplished by physically separating the turning lane from adjacent through lanes; and
school bus routes.

3 AutoTURN is developed by Transoft Solutions. Additional information about this software application is available online at: http://www.transoftsolutions.com/ProductTmpl.aspx
4 GDOT/Otak. Pedestrian and Streetscape Guide. 2003 The 2003 version of this publication is available online at: http://www.dot.ga.gov/travelingingeorgia/bikepedestrian/Pages/PlanningandDesignResources.aspx

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3.3. Design Speed

3.3.1. General Considerations
"Design Speed" has been identified as a "controlling criteria" that has substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the AASHTO Green Book criteria as the standard for design speed options for roadway classifications in Georgia. The designer is encouraged to select a design speed that provides a balance among the design vehicle, other users of the facility, and within the context of the surrounding environment. A decision to use a design speed value that does not meet the controlling criteria defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.

Design speed is different from other controlling criteria in that it is a design control, rather than a specific design element. In other words, the selected design speed is used to establish a range of design values for many of the geometric elements of a roadway. The selected design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use and functional classification of the roadway. Design speed should be consistent with the speeds at which 85 percent of drivers are traveling (referred to as the 85th percentile) and likely to expect on the facility.

In recognition of the wide range of site-specific conditions, constraints, and contexts for roadways AASHTO defines a range of values for design speed. A design speed that is as high as practical that will provide safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and other social or political effects should be selected. Table 3.1. lists typical design speeds which should be considered when selecting an appropriate design speed.

On county roads or city streets, GDOT recommends coordination with the local jurisdictional authority to identify posted speeds on existing roadways and for the selection of the posted speed limit and the design speed for new or reconstructed roadways.

3.3.2. Intersections Approaching a Stopped Condition
To improve the angle of intersection between a local street and major road, a designer may use a lower design speed on the local street for curves approaching an intersection if it is not anticipated that the T-intersection will become a full intersection.

The design speed of the last curve prior to the intersection may be 10 mph less than the design speed of the local street.

3.3.3. Freeway Exit and Entrance Ramps
Typical freeway exit and entrance ramps may have varying design speeds which are based on the operating speed of the vehicle as it decelerates or accelerates on the ramp. A common rule to apply for ramps is that the design speed of the first curve of an exit ramp can be assumed to be 10 mph less than the design speed of the mainline. With each successive curve on the exit ramp, the design speed of the curve can be reduced based on computed vehicle deceleration. The reverse condition applies to the design speed for all entrance ramps.

The design speed for a direct system to system ramp that connects two freeway facilities should be no less than 10 mph below the design speed of the exiting facility.

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On loop ramps, adequate deceleration length should be provided prior to the loop part of the ramp. All areas of deceleration should be separated from the mainline lanes. System to system loop ramps will be evaluated on a case-by-case basis.
3.3.4. Urban Subdivision Streets
In most cases, the design speed for urban subdivision streets should be a minimum of 25 mph.
3.4. Highway Capacity
All portions of roadways that are part of major construction or reconstruction should be designed to accommodate, at a minimum, 20-year forecasted traffic volumes. The design year for the 20-year traffic volumes should be forecasted from the estimated base (or opening) year, which is the year the project is anticipated to be open for traffic use. Refer to Chapter 13, Traffic Forecasting and Analysis Concepts, of this Manual for further discussion on the traffic engineering and analysis.
If a project is not new roadway construction or reconstruction, refer to Chapter 11, Other Project Types for guidance relating to other project types.
3.5. Establishment of Access Control
3.5.1. Definitions
GDOT has adopted the following "Access Control" criteria as standard, having substantial importance to the operational and safety performance of a roadway such that special attention should be given to design decisions. The designer is encouraged to select design elements and features that are consistent with the access control plan established for a roadway. A decision to use a design element or feature that does not meet the standard access control criteria defined by GDOT shall require a comprehensive study by an engineer and the prior approval of a Design Variance from the Department's Chief Engineer.
Roadways serving higher volumes of regional through traffic require greater access control to preserve their traffic function. Frequent and direct property access is more compatible with the function of local and collector roadways. The regulation of access to a roadway is referred to as "access control". It is achieved through the regulation of public access rights to and from properties abutting the roadway facilities. The Official Code of Georgia Annotated (OCGA)5 32-6-111 to 114 give GDOT this authority.
The regulation of public access rights is generally categorized as either full control of access, partial control of access, or control of access by permit (or permitted access).
Full control of access means that preference is given to through traffic by providing access connections by means of ramps with only selected public roads and by prohibiting crossings at grade and direct driveway connections.
Partial control of access means that some preference should be given to through traffic. Access connections, which may be at-grade or grade-separated, are provided with selected public roads

5 Online public access to the Official Code of Georgia Annotated (OCGA) is provided at: http://w3.lexis-nexis.com/hottopics/gacode/Default.asp?loggedIn=done

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and private driveways. In areas with partial control of access, the decision to grant access to private driveways is made at the time of project development, and thereafter, private driveway access should not be added.
Permitted access means that a permit is needed for access. A permit is required prior to performing any construction work or non-routine maintenance within the State highway right-of-way. This includes but is not limited to the following activities: grading, landscaping, drainage work, temporary access to undeveloped land for logging operations, or construction of a development. Any new driveway or revisions to any portion of existing driveways, i.e. widening and/or relocation that are within the State roadway right-of-way shall also require a permit.
3.5.2. Access Management
The following standards shall be used to establish access control:
Full control of access Full control of access shall be established on all Interstates. Full control of access shall be established on principal arterials constructed on new location with grade separated interchanges.
For projects that involve an Interstate interchange, (new construction or reconstruction), access control should be established along the intersecting route for a distance of 600-ft. in urban areas and 1,000-ft. in rural areas, where practical. At a minimum, access control shall not be less than 300-ft. This distance is measured from the radius return of the ramp termini with the intersecting route. (See Figure 3.1, Limit of Access Control Interstate/Freeway Interchange).
Where improved traffic operations and safety warrant, existing driveways may be closed and no access allowed to developed or undeveloped property. Decisions on elimination of access points should be based in part on an economic study of alternate courses of action.
Partial control of access
Partial control of access shall be established on principal and minor arterials that are constructed on a new location with intersections at-grade. Access control should not be established on portions of projects on new location which are less than one mile in length, unless the project connects to a section of roadway were access control has been or will be established or where required to preserve the functional area of an intersection as described below.
Partial control of access should be established on existing principal arterials that are being widened, when it is determined that partial access control is advisable. On this type of project, every attempt shall be made to consolidate existing access to the roadway by developing a supporting roadway network. All undeveloped property frontage should be treated in the same manner as new location construction.
Breaks in access will only be granted for the following conditions:
State or local government public road intersections

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Figure 3.1 Limit of Access Control Interstate/Freeway Interchange

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Where property that is not accessible from existing roadways or has been bisected by the new roadway alignment and no other access is provided and the appraised damages to the remaining property exceeds $50,000 in a rural area or $100,000 in an urban area. Coordination with the District right-of-way office should be performed prior to making a request for a break in access control. All breaks in access control under these conditions must be approved by the Chief Engineer.
Permitted access
On principal and minor arterials and major collector roadways that are being reconstructed, access rights should be acquired so that driveway connections are not allowed within the functional area of any intersection. The functional area of an intersection is the area where motorists are responding to the intersection, decelerating, and maneuvering into the appropriate lane to stop or complete a turn. Access connections too close to intersections can cause serious traffic conflicts that impair the function of the affected facility.
Upstream functional distance is defined as the distance traveled during perception-reaction time, plus the deceleration distance while the driver maneuvers to a stop, plus the queue storage. Downstream functional distance is defined as the stopping sight distance.
Temporary State Routes
For routes that are temporarily placed on the state route system during project development, close coordination to determine the appropriate access control should occur between the Department and the local government responsible for enforcing the access control after the oversight reverts back to the local government. "Permitted Access" should be considered when there is a strong likelihood that access breaks will be requested by potential development along the route. "Full Control of Access" or "Partial Control of Access" should be considered when the project connects to a section of roadway where similar access control has been or will be established, and to preserve the functional classification of the route or corridor. Before Right of Way acquisition begins, it is recommended that the Department receive written confirmation from the local government to enforce the established access control after the oversight reverts back to the local government.
3.6. Frontage Roads and Access Roads
AASHTO defines a frontage road as "a road that segregates local traffic from higher speed throughtraffic and intercepts driveways of residences, commercial establishments, and other individual properties along the highway" (AASHTO Green Book). Frontage roads can serve many functions depending on the type of arterial they serve and the character of the surrounding area. They are commonly used to control access to the arterial, to provide access to adjoining properties, and to maintain traffic circulation on each side of the arterial.
Most existing frontage roads were built along interstate or major arterial routes to control access to these routes and provide access to property that would otherwise be land-locked. Access roads may also be used to provide access to landlocked parcels.
Frontage roads typically run parallel to the mainline route while access roads provide access to individual properties and may not run parallel to the mainline. Access roads and frontage roads should be offset from the mainline route to allow required clear zone and future roadway widening, if anticipated.

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3.7. Fencing
The Georgia Department of Transportation has established the following guidelines for installing fence on state right-of-way and/or private property associated with the design of roadway projects. These guidelines are based on the principles published in the AASHTO An Informational Guide on Fencing Controlled Access Highways (1990).

3.7.1 Fencing on State Right-of-Way
Fencing is provided within the state right-of-way to delineate the boundary of the acquired access control, and as a physical obstacle to deter encroachment onto the roadway right-of-way from children, pedestrians, bicyclist, vehicles, machinery, and animals. Fencing may also be provided to deter access into or across specific features within the right-of-way such as drainage structures, bridges and retaining walls. The following guidelines are provided for the consistent application of fencing on state right-of-way.
Roadways with Full Control of Access are expected to provide a higher level of mobility and operate at higher speeds with protection from all forms of roadside interference. Therefore, fencing should be installed within the state right-of-way on roadways with Full Control of Access, where it is practical to do so. Fencing may not be practical or necessary in areas with steep slopes or natural barriers.
Fencing may be installed within the state right-of-way on roadways with Partial Control of Access or any portion of a state route with an acquired limit-of-access if the Department determines it necessary to deter potential or chronic encroachment.
For roadways with Full Control of Access and parallel frontage roads included within the state right-of-way, fencing should be installed between the mainline traveled-way and the frontage road. In these cases, it may not be necessary to install a duplicate fence along the right-of-way line.
Fence installed within the state right-of-way to delineate the limit-of-access should be offset a minimum of 1-ft inside the right-of-way line to ensure there is adequate space for installation and maintenance.
For non-access grade separations, fence installed along the limit-of-access will be terminated at the points where the state right-of-way intersects the normal right-of-way of the crossing grade separation.
For grade separated interchanges, fence installed along an entrance or exit ramp terminal with a cross road should terminate at the point where the state right-of-way intersects the normal right-of-way along the cross road. Fencing may be extended along the right-of-way of the cross road for the entire length of acquired access control if the Department determines it necessary to deter potential or chronic encroachment (see Figure 3.1, Limit of Access Control Interstate/Freeway Interchange.
Fence installations within the state right-of-way are not intended to control livestock from adjacent private property and should not be installed or permitted for this reason. Where fencing is required to contain livestock within adjacent private property, an independent fence on private property will be required for that purpose (see 3.7.2. Fencing on Private Property).
The installation of 6-ft height Chain Link Wire Fence should be considered on a case-bycase-basis around the perimeter of proposed permanent drainage features that will contain

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water over 24-inches deep for an extended period of time (greater than 48 hours). For example, natural ponds, detention ponds and water quality ponds within the state right-ofway. The fence should be installed with adequate space for routine maintenance and equipped with self-closing and self latching gates.
A fence or handrail should be considered on a case-by-case-basis along the top of a retaining wall with a change in elevation of 30-inches or more above the grade below. These cases should be assessed independent of fencing along a limit-of-access.

For guidance on the design and installation of fence or handrail on bridges, refer to Chapter 3.4.1.2 of the GDOT Bridge and Structures Design Policy Manual.
http://www.dot.ga.gov/doingbusiness/PoliciesManuals/roads/BridgeandStructure/GDOT_Bridge_and_Structur es_Policy_Manual.pdf
For guidance on the construction of fencing, refer to GDOT Construction, Standard Specification, Section 643 Fence. http://www.dot.ga.gov/doingbusiness/theSource/specs/ss643.pdf
The Department has established the following guidelines to determine the type of fencing installation appropriate for the access control along the roadway.
Full Control of Access:
Urban Interstate or Freeway: Ga. Standard Detail 9031-N, Chain Link Wire Fence heavier gage fence typically 6-ft.
height typically used in areas with restricted cross section and limited (narrow) space between the roadway and the right-of-way, such as depressed urban freeways with retaining walls and significant changes in vertical elevation between the roadway and right-of-way may include extension arms with barbed wire strands across the top to enhance security.
Suburban Interstate or Freeway: Ga. Standard Detail 9031-N, Chain Link Wire Fence or, Ga. Construction Detail F-1, Woven Wire Fence 4-ft. height wire mesh with barbed wire
strand along the top and bottom may be used in areas with flatter more rounded sideslopes and wider (more adequate) space between the roadway and the right-of-way.
Rural Interstate or Freeway: Ga. Construction Detail F-1, Woven Wire Fence or, Ga. Construction Detail F-6, Game Fence typically 8-ft height mesh with barbed wire
strands along the top and bottom - may be used on portions of roadways to reduce crash rates related to wild game crossing.
Partial Control of Access or any portion of a roadway with an acquired limit-of-access: Ga. Standard Detail 9031-N, Chain Link Wire Fence or, Ga. Construction Detail F-1, Woven Wire Fence or, Ga. Construction Detail F-6, Game Fence

3.7.2 Fencing on Private Property
In cases where the Department is acquiring additional right-of-way or easement, and displacing fence on private property, the value of "replacement fencing" will be assessed by the right-of-way agent for settlement with the property owner.
Replacement fencing may be installed by the Department's contractor or by the property owner on private property, as determined in the settlement with the property owner and noted on the plans.

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Fence installed on private property should be offset a minimum of 1-ft outside the state right-of-way line. Typically a 5-ft wide temporary "Easement for the Construction of Fence" will be required on private property if the fence is installed by the Department's contractor.
Replacement fencing on private property may consist of chain link wire, woven wire, field fencing/barbed wire, ornamental, or specialty type fencing including gates and associated hardware. In cases where ornamental or specialty fencing is included as a contract item, a special provision with detail drawings will be required in the plans and contract proposal.
A decision to provide replacement fencing on private property will be made during right-of-way acquisition. Designers should coordinate with the Right-of-Way Acquisition Manager for direction on replacement fencing on private property prior to establishing temporary easements or adding notes to the plans.
For additional guidance involving the installation of fence on private property refer to the GDOT Right-Of-Way Manual, currently maintained by the Office of Right-Of-Way in hard-copy format.

3.8. Right-of-Way Controls
Establishing right-of-way widths that adequately accommodate construction, utilities, drainage, and proper roadway maintenance is an important part of the overall design. The border area between the roadway and the right-of-way line should be wide enough to serve several purposes, including provision of a buffer space between pedestrians and vehicular traffic (if applicable), roadway drainage, sidewalk space, lateral offset, clear zone, and an area for both underground and aboveground utilities. A wide right-of-way width allows construction of gentle slopes and also allows for utility poles to be offset further from the road, which in turn results in greater safety for motorists as well as easier and more economical maintenance of the right-of-way.

3.8.1. Rural Areas
In hilly terrain, construction limits vary considerably as the roadway passes through cut and fill sections. In these situations, the required right-of-way will likely vary, so it may be impractical to use a constant right-of-way width.

In flat terrain, it is usually both practical and desirable to establish a minimum right-of-way width that can be used throughout most of the project length. Required right-of-way widths should be set at even offsets from the centerline, typically multiples of 5-ft., unless some physical feature requires otherwise.

As a general rule, the required right-of-way line should be set a minimum of 7-ft. to 10-ft. beyond the proposed limits of construction in cut and 10-ft. to 15-ft. beyond the proposed limits of construction in fill. In areas of high fills a minimum of 20-ft. should be provided beyond the construction limits to provide room for adequate erosion control Best Management Practices (BMPs) that are necessary to minimize sediment transport. Extra right-of-way at the top of cut slopes should be provided for the construction of ditches that will intercept surface drainage and help minimize slope erosion.

If a future project will potentially connect to either end of the proposed project, the required right-ofway line is extended to the nearest property line beyond the extent of construction, if practical. This is done to avoid buying right-of-way from the property owner on two different occasions. In this case, the project limit will correspond to the limit of the required right-of-way.

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3.8.2. Urban Areas
In urban areas, right-of-way widths are governed primarily by economic considerations, physical obstructions, or environmental considerations. Along any route, development and terrain conditions may vary affecting the availability of right-of-way.
It is desirable to set right-of-way in urban areas a minimum of 1-ft beyond the shoulder break point or 2-ft. beyond the greatest required lateral offset specified in Chapter 5. Roadside Safety and Lateral Offset to Obstruction. However, property or environmental impacts may limit the amount of right-of-way that can realistically be acquired. If existing utilities are in conflict within areas of restricted right-of-way, discussions should be held at the Field Plan Reviews to determine how to adequately accommodate utility relocations.
3.8.3. Special Types of Right-of-Way
Construction Easement Construction easement is called for on the plans when an area outside the required right-of-way line is needed only during construction of the project. The most common example of this is for construction of a temporary detour road.
A permanent feature should not be placed in a construction easement. The decision to obtain permanent right-of-way or construction easement is made after considering the circumstances of each project.
The property owner is paid a fee during the time the construction easement is needed. Where applicable, the owner is also paid for damages that may be incurred during the construction process such as for removal of trees or shrubbery.
Permanent Drainage Easement Drainage easement is required when a new lateral outfall ditch is to be constructed beyond the right-of-way or when an existing lateral outfall ditch is to be improved outside of the right-of-way. Drainage easement is obtained when construction of these laterals is critical to proper drainage of the project. As with a construction easement the property owner is paid for use of the drainage easement, and for damages resulting from construction. However, with drainage easements GDOT reserves the right of permanent access to the drainage structure for maintenance purposes.
Control of Access Access rights may be purchased from property owners along major roadways having full or partial access. No roadway access crossing the limited access is allowed and the property owner is compensated for such restrictions. Where limited access is used along a roadway, it typically extends down intersecting roadways to enhance traffic flow at the intersection.
3.8.4. Accommodating Utilities
In addition to primarily serving vehicular traffic, right-of-way for streets and highways may accommodate public utility facilities in accordance with state law or municipal ordinance.
The use of right-of-way by utilities should cause the least interference with traffic using the street. If existing utilities are in conflict within areas of restricted right-of-way, discussions should be held at the Field Plan Review to determine how to adequately accommodate utility relocations. Utility

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features, such as power poles and fire hydrants, should be located as close to the right-of-way line as feasible for safety reasons.

Utilities located within the limits of construction for the roadway and drainage structures of a project may require relocation, adjustment, or encasement. The surveys should identify the utility locations, elevations, types, sizes, and owners. The plans and cross-sections will then be used to inform utility owners of how the project will impact their facilities.

Relocated utilities should normally be accommodated within the required right-of-way. This should be considered in setting required right-of-way limits.

For GDOT policies related to accommodating utilities, the designer should refer to the GDOT Utility Accommodation Policy and Standards Manual, which is available at http://www.dot.state.ga.us/doingbusiness/utilities/Pages/manual.aspx

3.9. Value Engineering
Value Engineering (VE) is defined in Code of Federal Regulations (CFR) Title 23 Part 627as follows:

"the systematic application of recognized techniques by a multi-disciplined team to identify the function of a product or service, establish a worth for that function, generate alternatives through the use of creative thinking, and provide the needed functions to accomplish the original purpose of the project, reliably, and at the lowest life-cycle cost without sacrificing safety, necessary quality, and environmental attributes of the project ().

For GDOT guidelines, policies and further information related to VE studies, the designer should refer to the current GDOT PDP, which is available in the "Other Design Related Links and Resources" section of the GDOT Repository for Online Access to Documentation and Standards (ROADS). Any applicable Design Exceptions and Design Variances shall be obtained prior to the implementation of a VE study recommendation which deviates from design standards adopted or defined by this policy.

3.10. Environmental
To the extent practical, roadways should be designed to fit into the surrounding landscape and environment. This approach helps to minimize potential impacts to the built and natural environment. Some environmental factors to consider in highway design include:
surrounding land uses and landscape elements;

historic and cultural resources;

important community features;

wetlands, streams and other natural resources;

utilities and potentially contaminated sites that are close to the roadway; and

airports and aviation facilities (located within 2 miles of the project).

GDOT encourages proactive coordination with local, and state or federal resource and regulatory agencies to identify important resources that may be of concern on a design project. Various

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techniques can be used to facilitate coordination with local jurisdictions. Several techniques are detailed in the GDOT Context Sensitive Design Online Manual, Section 2.2. Understand Community Input and Values.
Sometimes there are opportunities for a roadway project to enhance the surrounding environment. Refer to the GDOT Environmental Procedures Manual as well as the GDOT Context Sensitive Design Online Manual, Section 2.3. Achieve Sensitivity to Social and Environmental Concerns, for further guidance in this area.
While designing a roadway or major highway alignment so that it complements the surrounding terrain is an important consideration, any deviation from AASHTO or GDOT design policy standards shall require a Design Exception or Design Variance. Care should be exercised to ensure that applicable local, state, and federal environmental regulations are met in accordance with the project environmental document.

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Chapter 3 Index
Arterials Access Control, 9 Rural, 4 Urban, 3
Bicycle Facilities, 6 Buses
Design Vehicle. See Design Vehicle, Buses Classification, Functional
Arterial, 2 Freeway, 2 Rural Arterial, 4 Urban Arterial, 3 Code of Federal Regulations Title 23 Part 627 - Value Engineering, 16 Cross Section Access Roads, 11 Frontage Roads, 11 Right-of-Way Controls, 1416 Utilities, 1516 Design Context Sensitive Design (CSD), 17 Speed. See Design Speed Vehicles. See Design Vehicles Design Speed, 68 10 mph Speed Reduction, 7 Freeway Ramps, 7 Design Vehicles, 56 Buses, 5 Criteria, 4 Passenger Cars, 5 Recreational Vehicles (RVs), 5, 6 Trucks, 5 Types, 5

Easement Construction. See Right-of-Way, Easement Drainage. See Right-of-Way, Easement
Freeways Functional Classification, 2
Interchanges Access Control, 9
Intersections Access Control, 9 Turning Radii, 5
Interstates Access Control, 9
Passenger Cars Design Vehicle. See Design Vehicle, Passenger Cars
Ramps Freeways, 7
Recreational Vehicles Design Vehicle. See Design Vehicle, Recreational Vehicles
Right-of-Way Easement, Construction, 15 Easement, Drainage, 15 Rural Environment, 14 Urban Environment, 15 Utilities, 15 Width, 1416
Traffic Design Year, 8
Trucks Design Vehicle. See Design Vehicle, Trucks
Utilities, 1516 Value Engineering, 16

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Chapter 4 Contents

4. ELEMENTS OF DESIGN

1

4.1. Sight Distance

1

4.1.1. General Considerations

1

4.1.2. Stopping Sight Distance

1

4.1.3. Passing Sight Distance

2

4.1.4. Decision Sight Distance

2

4.1.5. Intersection Sight Distance

2

4.2. Horizontal Alignment

4

4.2.1. General Considerations

4

4.2.2. Types of Curves

5

4.2.3. Pavement Widening on Curves

8

4.2.4. Lane Width Transitions and Shifts

9

4.2.5. Transition in Number of Lanes

11

4.3. Vertical Alignments

14

4.3.1. General Considerations for Vertical Alignments

14

4.3.2. Maximum Vertical Grades

16

4.3.3. Minimum Vertical Grades

17

4.3.4. Vertical Curves

19

4.3.5. Maximum Change in Vertical Grade without Using Vertical Curves

20

4.3.6. Vertical Grade Changes at Intersections

20

4.3.7. Minimum Profile Elevation Above High Water

21

4.3.8. Reporting Changes in Vertical Clearances

22

4.4. Combined Horizontal and Vertical Alignments

23

4.4.1. Aesthetic Considerations

23

4.4.2. Safety Considerations

23

4.4.3. Divided Highways

24

4.5. Superelevation

25

4.5.1. Maximum Superelevation Rates

25

4.5.2. Sharpest Curve without Superelevation

27

4.5.3. Axis of Rotation

27

4.5.4. Superelevation Transitions

28

Chapter 4 Index

33

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List of Figures

Figure 4.1. Crowned Traveled Way Revolved About Centerline

32

List of Tables

Table 4.1. Maximum Horizontal Alignment Deflection without Use of a Curve

8

Table 4.2. Pavement Widening on Curves on Two-Lane Roadways

10

Table 4.3. Miscellaneous Transition Tapers

13

Table 4.4. Turn Lane Transition Tapers

13

Table 4.5. Maximum Vertical Grades

17

Table 4.6. Minimum Vertical Grades for Roadways where Drainage Spread is Considered 19

Table 4.7. Maximum Change in Grade that Does Not Require a Vertical Curve

20

Table 4.8. Vertical Profile Clearances Based on High Water

21

Table 4.9. Maximum Superelevation Rates

26

Table 4.10. Superelevation Rotation Points and Rotation Widths

29

Table 4.11. Maximum Relative Gradients

30

Table 4.12. Adjustment Factor for Number of Rotated Lanes

30

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4. ELEMENTS OF DESIGN
4.1. Sight Distance
4.1.1. General Considerations
A detailed explanation of how to apply the sight distance criteria to a roadway is described in the American Association of State Highway and Transportation Officials (AASHTO) publication, A Policy on the Geometric Design of Highways and Streets (Green Book), Chapter 3, Design Elements. In addition, Chapter 9 of the Green Book (Intersections) discusses special conditions related to sight distance at intersections.
General considerations relating to sight distance noted in the AASHTO Green Book include:
Safe and efficient operation of a vehicle is highly dependent on adequate sight distance.
Two-lane rural highways should generally provide sufficient passing sight distance at frequent intervals and for substantial distances. Conversely, passing sight distance on two-lane urban streets/arterials is normally of little value.
The proportion of a highway's length with sufficient sight distance to pass another vehicle and interval between passing opportunities should be compatible with design criteria pertaining to functional classification, as discussed in this Manual in Chapter 2, Design Policies and Standards.
Special consideration should be given to the sight distance requirements at queue backups over a hill, signals, horizontal curves, turning movements, barriers, guardrails, structures, trees, landscaping, vegetation and other special circumstances.
4.1.2. Stopping Sight Distance
"Stopping Sight Distance" has been identified as a "controlling criteria" that has substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the AASHTO Green Book criteria as the standard for Stopping Sight Distance for roadways in Georgia. A decision to use Stopping Sight Distance values that do not meet the minimum controlling criteria defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.
Designers should note that the values for Stopping Sight Distance listed in the AASHTO Green Book are minimum values based on a 2.5 second brake reaction time. Larger stopping sight distance values may be considered by the designer, within the constraints of economic, environmental, social, and other influences. GDOT encourages designers to consider using greater values for Stopping Sight Distance when practical.
Stopping sight distance across the inside of curves plays a critical role in determining roadway horizontal curvature and applicable shoulder widths. Enough right of way should be purchased to ensure that adequate stopping sight distance is maintained. There should be no obstruction of sight distance on the inside of curves (such as median barriers, walls, cut slopes, buildings, landscaping materials, and longitudinal barriers). If removal of the obstruction is impractical to provide adequate sight distance, a design may require adjustment in the normal highway cross section or a change in the alignment.

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Because of the many variables in alignment, cross section, and in the number, type, and location of potential obstructions, the actual conditions on each curve should be checked and appropriate adjustments made to provide adequate sight distance. The AASHTO Green Book (2004), Exhibit 354 Diagram Illustrating Components for Determining Horizontal Sight Distance, provides additional information on the effects of obstructions located on the inside of horizontal curves.
4.1.3. Passing Sight Distance
Passing sight distance is the sight distance needed for passing other vehicles (applicable only on two-way, two-lane highways at locations where passing lanes are not present).
4.1.4. Decision Sight Distance
Decision Sight Distance is the distance needed for a driver to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete the maneuver safely and efficiently. Examples of locations where Decision Sight Distance should be considered are: multiphase at-grade intersections, interchanges, ramp terminals on through roadways, lane drops, and areas of concentrated traffic demand where there is likely to be more visual demands and heavier weaving maneuvers.
The use of AASHTO Green Book criterion for Decision Sight Distance is encouraged by GDOT and should be considered at appropriate locations along a roadway. In cases where it is not practical to provide Decision Sight Distance, then Stopping Sight Distance shall be provided.
4.1.5. Intersection Sight Distance
Intersection Sight Distance has been identified by the Department as having substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the AASHTO Green Book criteria as the standard for Intersection Sight Distance. Refer to the AASHTO Green Book, Chapter 9, Intersection Sight Distance, for design criteria applicable to the traffic control conditions of an intersection. If it is not practical to provide intersection sight distance values defined by AASHTO, then a decision to select a value or retain an existing condition that does not meet the criteria defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Variance from the GDOT Chief Engineer.
Appropriate calculations and graphical studies verifying intersection sight distance shall be conducted by the engineer and recorded in the design data book. These studies should be performed during preliminary design when both the horizontal and vertical alignments are being finalized. Graphical studies, at a minimum, should include scaled distances on plan and profile sheets and may at times necessitate plotting the sight line location on specific cross-section sheets.
Graphical studies are particularly important for the following conditions:
minor road alignment in skew;
mainline in horizontal curve;
mainline with crest vertical curve on either side of intersection;
mainline and/or minor road within a cut; and
areas where vegetation growth may obstruct the sight triangles (i.e. grass medians).

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Intersection Sight Distance is critical for urban sections with narrow shoulders and limited right-ofway where obstructions on private property may encroach into the sight triangles. Special consideration should be given to obstructions within the right of way such as: bridges, retaining walls, signs, landscaping, signal control boxes, guardrail, etc. The AASHTO Green Book (2004), Exhibit 354 Diagram Illustrating Components for Determining Horizontal Sight Distance, illustrates and provides further discussion of the affects of obstructions located on the inside of horizontal curves.
Angle of Intersection / Skew Angle Ideally, intersecting roadways should meet at or near right angles (90-degrees). This will ensure that the lines-of-sight are optimized for intersection sight distance.
"Skew Angle", also a component of Horizontal Alignment, has been identified as standard criteria, having substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. A decision to use a skew angle less than a 60-degree skew angle defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.
Although a minimum 60-degree intersecting angle is permissible by AASHTO standards, it does not provide the benefits of more perpendicular intersections. Therefore, GDOT has adopted a 70-degree skew angle as the minimum skew angle at intersections. A decision to use a skew angle between 60 and 70-degrees shall require a comprehensive study by an engineer and the prior approval of a Design Variance from the Department's Chief Engineer.
In general, where there is a high percentage of truck traffic, a 90-degree intersection should be provided. The closer an intersection angle is to 90-degrees, the greater the safety and operational benefits because:
exposure time for crossing movements (vehicular and pedestrian) are minimized
sharp angle turns (especially for trucks) are reduced
driver discomfort is reduced, because drivers will not have to turn their head as much to see the intersection. This is especially true for older drivers who tend to have a decline in head and neck mobility. (1)
the bodies of larger vehicles, such as ambulances, motor homes, tractor trailers, etc. tend to interfere with the drivers field of view when at skewed intersection. (2)
signing and pavement markings, channelization, and signalization layouts are simplified
When a "T" intersection becomes a four-way intersection due to extension of an existing side street or construction of a driveway opposite the side street, the new facility will usually be built at or very nearly 90-degrees to the mainline. Cross traffic operations are much safer and more efficient if the existing side street leg is at the same angle. The condition is very likely to occur on divided highways where development is concentrated at established median breaks.
The AASHTO Green Book acknowledges that sharp curves may be as great a hazard as the acuteangle crossing itself. However, rather than omitting the curves and retaining the acute-angle crossing, the effect of the curves should be mitigated. For example, warning signs for reduction of speed in advance of such curves could be provided. This is especially appropriate for "T" intersections and cross roads with low volumes of through traffic.

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(1) FHWA. Highway Design Handbook for Older Drivers and Pedestrians, 2001. The 2001 version of this publication is available online at http://www.tfhrc.gov/humanfac/01103/coverfront.htm
(2) MBTC FR 1073. Intersection Angle Geometry and the Driver's Field of View, 1997. http://ww2.mackblackwell.org/web/research/ALL_RESEARCH_PROJECTS/1000s/1073-gattis/MBTC1073.pdf
Right-of-Way Flares The preferred method to ensure adequate intersection sight distance is to acquire the area(s) within the sight triangles as right of way so the area can be properly managed and kept free of obstructions. These areas are referred to as right-of-way flares.
Right of way Flares should be obtained in order to maintain adequate intersection sight distance at intersections.
4.2. Horizontal Alignment
"Horizontal Alignment" has been identified as a "controlling criteria" that has substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the AASHTO Green Book criteria as the standard for elements of horizontal alignment. A decision to deviate from the minimum controlling criteria defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.
The location and alignment selected for a highway are influenced by factors such as physical controls, environmental considerations, economics, safety, highway classification and design policies. The horizontal alignment cannot be finalized until it is coordinated with the vertical alignment and cross section elements of the roadway.
Horizontal curves should be used for all deflections in a horizontal alignment, with the exception of alignment changes without horizontal curves as discussed in detail in Section 4.2.2. Types of Curves of this Manual. In special situations, such as roadway reconstruction or widening on existing alignment, practicality will dictate when deflection angles (PI without a curve) may be introduced in lieu of horizontal curvature. Spiral curves are generally not utilized on Georgia roadways.
Refer to the AASHTO Green Book Chapter 3, Elements of Design, when determining the radii of horizontal curves and corresponding superelevation (if applicable). Wherever possible, minimum curve radii and maximum superelevation rates should be avoided for any given speed design.
4.2.1. General Considerations
See the Green Book Chapter 3 "General Controls for Horizontal Alignment" section for general considerations when setting a horizontal alignment.
In general, the number of short curves should be kept to a minimum.
Long tangents are needed on two-lane highways such that sufficient passing sight distance is available on as great a percentage of the roadway as possible.

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Sharp curvature should be avoided near the following locations: elevated structures; at or near a crest in grade; at or near a low point in a sag or grade; at or near intersections, transit stops, or points of ingress or egress; and at or near decision points.

The concepts of stopping sight distance, intersection sight distance, decision sight distance and driver expectancy should be considered during the development of horizontal alignments. If possible, the horizontal alignments of roadways should be free of curvature in and around intersections, interchanges, bridges, railroad crossings, toll plazas, drop lanes and roadside hazards.

To facilitate pavement drainage, alignments should be laid out such that the 0% cross slope flat points associated with superelevation transitions on either end of a horizontal curve (if applicable) does not correspond to low points in the roadway vertical profile. Superelevation is discussed in this Manual in Section 4.5.

The horizontal alignment should be coordinated carefully with the vertical profile design. This subject is discussed in further detail in this Manual in Section 4.4.

The design speed of successive horizontal curves on ramps can vary as vehicles are often accelerating or decelerating. A common rule to apply to the speed design of ramps is that the design speed of the first curve of an exit ramp can be assumed to be 10 mph less than the design speed of the mainline. With each successive curve on the exit ramp, the design speed of the curve may be reduced based on computed vehicle deceleration. The process is to be reversed for entrance ramps, i.e., the design speed for curves will successively increase until the design speed of the last curve before the mainline is 10 mph less than that of the mainline.

For additional considerations and guidance in setting horizontal alignments, refer to of the AASHTO Green Book Chapter 3, Elements of Design - General Controls for Horizontal Alignment.

4.2.2. Types of Curves The following types of curves are discussed in this section:

circular curves compound curves reverse curves spiral curves broken back curves curves with small deflection angles minimum length of horizontal curve alignment changes without horizontal curves

Circular Curves
GDOT typically uses the arc definition of the circular curve. Under this definition, the curve is defined by the degree of curve (D ), which is the central angle formed when two radial lines at the
a
center of the curve intersect two points on the curve that are 100-ft. apart, measured along the arc of the curve.

D = 18,000 / ( * R) a

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Where:

D = Degree of Curve (degrees) a
R = Radius of Curve ( ft.)

L = (100 * I)/ D a Where:

L = Length of Curve ( ft.) D = Degree of Curve (degrees)
a
I = Total deflection of curve (degrees)

Compound Curves
Compound curves involve two horizontal curves of different radii sharing a common point for their point of tangent (PT) and point of curve (PC), respectively. For open highways, compound curves between connecting tangents should be used only where existing topographic controls make a single simple curve impractical.

Guidance regarding compound curves falls into two categories:

Roadways (excluding ramps), one-way or two-way - The radius of the flatter curve should not exceed the radius of the sharper curve by more than 50% (a ratio of 1.5:1).
Ramps - A ratio as great as 1.75:1 may be used on one-way interchange ramps, where compound curves are more common. Ratios greater than 2.0:1 are strongly discouraged. The compound radii ratio criteria are only applicable when the curve radii decreases from one curve to the next in the direction of travel.

Reverse Curves
Any abrupt reversal in alignment should be avoided. A reversal in alignment can be suitably designed by including a sufficient length of tangent between the two curves to provide adequate superelevation transitions. See Section 4.4. Combined Horizontal and Vertical Alignments for additional discussion of superelevation transition lengths.

The tangent distance between reverse curves should be the distance (based on the appropriate gradient or ratio) to rotate from 2/3 of the full superelevation rate of the first curve to 2/3 of the full superelevation rate of the second curve. For roadways with design speeds less than or equal to 45
mph, a minimum tangent of 100-ft. should be provided between reverse curves, even if
superelevation is not required.

With or without superelevation, extreme physical constraints may dictate the use of a reverse curve with a 0-ft. length tangent (the PT of the first curve and the PC of the second curve are at the same location). In this case, the 0% cross slope point should be placed at the shared PT/PC and use the best possible superelevation transition ratio.

Where it is impractical to provide a tangent length capable of incorporating the superelevation runoff lengths and the tangent run out lengths of both superelevated curves, the 0% cross slope point should be placed at a point derived from the best possible superelevation transition ratio between the two curves (not necessarily the center of the tangent). For an expanded discussion of superelevation refer to Section 4.4. Combined Horizontal and Vertical Alignments.

On higher-speed roadways (design speeds greater than 45 mph), curves that do not require superelevation are so flat that no tangent between the curves is necessary. However, wherever practical, a 150-ft. minimum tangent should be introduced between reverse curves. On higherspeed roadways with curves requiring superelevation, a tangent length suitable for accommodating

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the necessary superelevation transition should be provided (see Section 4.4. Combined Horizontal and Vertical Alignments). For reverse curves on a roadway with a design speed greater than 45 mph, the use of tangent lengths less than those calculated by AASHTO procedures shall require a design exception.

Spiral Curves
Spiral curves are generally not utilized on Georgia roadways, except in special cases. For overlay or widening projects, existing spiral curves may remain. For roadways to be re-constructed, existing spiral curves may be replaced with simple curves, unless existing property improvements or other controls make this impractical. Refer to the AASHTO Green Book Chapter 3, Elements of Design, for additional information on spiral curves.

Railroads typically utilize spiral curves at the beginning and end of each simple horizontal curve. A project involving a railroad crossing and possibly track relocation may require the use of spiral curves. For additional information related to the design of railroad alignments (including spiral curves), refer to the American Railway Engineering and Maintenance-of-Way Association (AREMA) Manual for Railway Engineering.

Broken-Back Curves
Successive curves in the same direction that are separated by a short tangent are known as broken-back curves. GDOT defines this short tangent as one with a length less than:

15*V for design speeds less than or equal to 45 mph, or 30*V for design speeds greater than 45 mph. In these equations, V is the design speed in mph.

Broken-back curves are very undesirable from both an operational and an appearance standpoint. While it may not be feasible or practical in some situations to completely eliminate broken-back curves, every effort should be made to avoid this type of alignment if possible by separating, combining, or compounding curves in the same direction.

Curves with Small Deflection Angles
A short horizontal curve with a small deflection angle (less than five degrees) may appear as a kink in the roadway. As a minimum, curves should be at least 100-ft. in length for every one degree of central angle.

Minimum Length of Horizontal Curve The minimum length of horizontal curve should be in accordance with the following:

L = 15*V

Where:

L = minimum curve length (ft.) V = design speed (mph)

On high-speed controlled-access facilities that use large-radius curves, the minimum length of horizontal curve should be in accordance with the following:

L = 30*V

Where:

L = minimum curve length (ft.) V = design speed (mph)

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Alignment Changes without Horizontal Curves
There may be instances where existing constraints will make it impractical to utilize horizontal curves which maintain the minimum length criteria specified in the first seven cases cited in Section 4.2.1. Horizontal Alignment General Considerations. Right-of-way, cost, or environmental constraints could be prohibitive on widening, reconstruction, maintenance, safety, and 3R projects in both urban and rural settings.
When situations warrant, slight deflection angles may be introduced to (or maintained on) the roadways horizontal alignment. These angles will be very slight so that they do not adversely affect safety or operations. Acceptable angles of deflection will depend on the design speed of the facility. Table 4.1. lists the maximum angle of horizontal deflection for roadways in Georgia.

Table 4.1. Maximum Horizontal Alignment Deflection without Use of a Curve

Design Speed (MPH)

Maximum Angle of Horizontal Deflection
(minutes)

15

120

The use of horizontal curves is preferable to deflection angles. However, there are cases where small deflections are acceptable. For example, as shown in Table 4.1, an existing deflection angle up to 14 minutes (imperceptible to the eye) on an interstate widening project (design speed 70 mph) may be maintained.

20

90

At intersections with an all-way stop condition (with no

25

60

foreseeable signalization) and some form of constraint,

30

45

there may be a break in the roadway alignment as much

35

40

as five degrees (at the centerline crossing in the

40

35

intersection), provided intersection sight distance is

45

30

maintained in all directions.

50

25

4.2.3. Pavement Widening on Curves

55

20

60

18

On modern highways and streets that feature 12-ft. lanes and high-type alignments, the need for widening on

65

16

curves has decreased considerably in spite of high

70

14

speeds. In many cases, degrees of curvature and

75

12

pavement widths established by policies in this Design

80

10

Manual preclude the necessity of pavement widening on

roadway curves. This is especially true if the alignments

are as directional as practical, consistent with the topography, and developed properties and

community values are preserved (refer to Section 4.2.1. General Considerations). However, for

some conditions of speed, curvature, and width, it may still be necessary to widen pavements. Widening should be evaluated at the following locations:

low speed roadways with near maximum curvature ramps connecting roadways where curves sharper than those specified in this Manual are used

Specific values for pavement widening in curves are shown in Table 4.2. Pavement Widening on Curves on Two-Lane Roadways. For additional discussion and widening values, refer to the AASHTO Green Book, Chapter 3. Elements of Design Traveled Way Widening on Horizontal Curves.

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4.2.4. Lane Width Transitions and Shifts
Lane width transitions can occur at several locations including:
Lane width transitions which are to be developed for curves (see Section 4.2.3. Pavement Widening on Curves)
Connections to existing pavement such as pavement tapers which occur at the back of a turnout on an existing side road
Transitions to a wider lane such as a truck lane or a one-way, one-lane ramp Mainline lane shifts in advance of an intersection Mainline lane shifts in advance of a typical section change such as the addition of a mainline
lane Mainline lane shifts in advance of a typical section change such as a change in median width
There are two methods by which an alignment transition or "shift" may be accomplished:
The first method is to treat the transition or shift as though it were any other required alignment change. With this approach, a transition or shift would be accomplished through the use of a series of reverse curves. Quite often, the use of curve radii which do not require superelevation result in a length of transition greater than that required by providing a taper. Superelevation should be utilized if warranted by normal procedures.
The second method of accomplishing a transition or "shift" involves the use of tapers. Tapers are acceptable provided the following two conditions exist: The alignment shift is consistent with the cross slope of the roadway and does not require "shifting" over the top of an existing pavement crown
The direction of the shift is not counter to the pavement cross-slope (from a superelevation or reverse-crown consideration)

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Table 4.2. Pavement Widening on Curves on Two-Lane Roadways

Degree Curve

Radius (ft.)

30 (mph)

24-ft. Roadway

40

50

60

(mph) (mph) (mph)

70 (mph)

30 (mph)

22-ft. Roadway
40 50 60 (mph) (mph) (mph)

70 (mph)

30 (mph)

20-ft. Roadway
40 50 (mph) (mph)

60 (mph)

1.00 5,729.58 0.0 0.0 0.0 0.0 0.0 0.5 0.5 0.5 1.0 1.0 1.5 1.5 1.5 2.0

2.00 2,864.79 0.0 0.0 0.0 0.5 0.5 1.0 1.0 1.0 1.5 1.5 2.0 2.0 2.0 2.5

3.00 1,909.86 0.0 0.0 0.5 0.5 1.0 1.0 1.0 1.5 1.5 2.0 2.0 2.0 2.5 2.5

4.00 1,432.39 0.0 0.5 0.5 1.0 1.0 1.0 1.5 1.5 2.0 2.0 2.0 2.5 2.5 3.0

5.00 1,145.92 0.5 0.5 1.0 1.0

1.5 1.5 2.0 2.0

2.5 2.5 2.5 3.5

6.00 954.93 0.5 1.0 1.0 1.5

1.5 2.0 2.0 2.5

2.5 3.0 3.0

7.00 818.51 0.5 1.0 1.5

1.5 2.0 2.5

2.5 3.0 3.5

8.00 716.20 1.0 1.0 1.5 9.00 636.62 1.0 1.5 2.0

2.0 2.0 2.5 2.0 2.5 3.0

3.0 3.0 3.5 3.0 3.5 4.0

10.18 562.64 1.0 1.5 12.24 488.04 1.0 2.0

2.0 2.5 2.5 3.0

3.0 3.5 3.5

15.30 374.48 2.0

3.0

4.0

19.35 296.10 2.5

3.5

4.5

22.42 255.59 3.0

4.0

5.0

26.44 216.69 3.5

4.5

5.5

Notes:

1. Values for widening (ft.) for two-lane roadways - one-way or two-way traffic

2. Disregard values less than 2.0-ft. (above heavy line and not within highlighted area) for two-lane (one-way or two-way traffic) pavements

3. For three-lane and four-lane undivided roadways multiply values by 1.5 and 2.0, respectively and round to the nearest 0.5-ft. If values are less than 2.0-ft., disregard

4. Pavement widening is intended for utilization where truck traffic is significant and the increase in pavement width is to be 2.0-ft. or greater

5. Locations for pavement widening shall be shown in the construction plans or specified by the Engineer

6. Pavement widening to occur along the inside of the normal curve. Additional width is to be shared equally by all lanes.

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Taper lengths associated with shifts on Georgia roadways should be calculated as:

Case 1 Design Speed 45 mph: L = W * s

(W * s2 ) Case 2 Design Speed < 45 mph: L
60

Where:

L = distance needed to develop widening (ft)

W = width of lane shift (ft)

s = design speed (mph)

Note: the Case 1 and Case 2 taper lengths described above are applicable to permanent conditions. For a more detailed discussion on temporary conditions associated with construction, refer to Chapter 12. Stage Construction.

4.2.5. Transition in Number of Lanes
Instances where the number of lanes on a roadway is transitioning fall into two categories lane additions and lane drops. Lane drops induce a merge situation. Adequate distance for drivers to perform the merge maneuver should thus be provided. Lane additions that do not involve a shift of the mainline lanes may be accomplished in a much shorter distance.

Lane Drops Lane drops can occur in many situations on all types of roadways, such as:

mainline lane drop due to traffic drop off mainline lane drop to meet lane balance requirements (limited access) mainline lane drop due to transition to non-widened section, etc. end of auxiliary lane end of collector-distributor (cd) system end of climbing lane ramp merges on limited access facilities

With three exceptions, lane drops (or merges) for the situations listed above should be designed based on the minimum convergence tapers provided in Section 4.2.4. Lane Width Transitions and Shifts.

Exception 1 For lane drops and merges on high-speed Limited Access facilities, where design year mainline peak hour traffic rates exceed 1,550 vehicles per lane (LOS C), the convergence ratio should be:

L = 2 * W * s Where:

L = distance needed to develop widening (ft) W = width of lane shift (ft) s = design speed (mph)

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Exception 2 Certain situations require the use of horizontal curves and possibly superelevation in association with lane reductions. An example of this would be tie-ins being constructed on projects between a proposed four-lane section (with 44-ft. median) and a two-lane existing section. In these situations, a lane should first be dropped using the taper rates specified in Section 4.2.4. Lane Width Transitions and Shifts, while still on the fourlane section (in advance of the crossover). Once the lane reduction has been attained, the tie-in to the two-lane section should be accomplished with a tie-in using AASHTO horizontal curves and superelevation rates appropriate for the design speed of the facility. If possible, the curves associated with the tie-in should be no sharper than 1 degree.

Exception 3 If a ramp merge occurs on a significant upgrade, the speed differential of a truck or bus merging into traffic should be evaluated. In general, if the mainline grades exceed 3% (upgrade in merge), the convergence ratio should be:

L = 2 * W * s Where:

L = distance needed to develop widening (ft) W = width of lane shift (ft) s = design speed (mph)

General Rules on Lane Drops lane drops on limited access facilities should occur at exits

lane drops on limited access facilities should occur on the outside lanes

upon departing an intersection, a lane (to be dropped) should be maintained for a minimum of 800-ft. from the intersection before initiating the lane drop

tapers associated with multiple, successive lane drops on the mainline should be separated by a minimum 1,000-ft. tangent section

Lane Additions
Lane additions that are not accompanied by a mainline alignment shift can be performed over a relatively short distance. A minimum 15:1 expansion taper rate should be provided. However, when spatial constraints exist, expansion tapers may be as low as 5:1 (urban) and 8.33:1 (rural).

Required lane addition taper lengths associated with median breaks and intersections can utilize taper rates less than those pertaining to through lanes. GDOT Construction Standards and Details, Construction Detail M-3 depicts turn lane taper lengths associated with Type A, B and C medians.

Table 4.3. Miscellaneous Transition Tapers summarizes taper length and taper ratio requirements as they pertain to the addition of left-turn and right-turn lanes in Georgia. The designer should attempt to meet the values found in this table. However, if constraints such as right-of-way, environmental impacts, utility conflicts and/or driveway/access issues exist, the minimum values may be utilized.
When a lane addition occurs due to the generation of a center lane or a passing lane (i.e., when a two-lane road is to be widened to a three-lane section) the transition tapers must follow the guidelines discussed in Section 4.2.4. Lane Width Transitions and Shifts.

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Table 4.3. Miscellaneous Transition Tapers

Location
Driveways Driveways Parallel Ramps on Limited Access Facilities Entrance Parallel Ramps on Limited Access Facilities Exit

Design Speed (mph)
< 45
> 45
Varies
Varies

Transition Width, W
(ft.)
12
12
12
12

Minimum Taper (ft.)
50
100
250
250

If the widening will be asymmetric or will occur only to one side, the transition width (W) is the width of the additional lane. If the widening will be symmetric, i.e., both directions of travel bifurcate symmetrically to create a center lane, then the transition width (W) can be assumed to be of the width of the additional lane. For instance, if a 14-ft. center turn lane was being generated symmetrically on a 55 mph two-lane roadway, the taper length would be:
L (14.0) *55 385.0 ft. 2
A summary of other special cases for transition tapers is included in Table 4.3. Miscellaneous Transition Tapers.
Turn Lanes in an Intersection, Median or Driveway Refer to Table 4.4. for a general guideline on minimum and desirable turn lane transition taper values.

Table 4.4. Turn Lane Transition Tapers

Design Speed (mph)

Urban or
Rural

Type A Median > 45 Rural > 45 Rural > 45 Rural
Type B Median > 45 Rural > 45 Rural > 45 Rural
Type C Median < 45 Urban*
Flush Median > 45 Rural

Median Width
(ft.)
40 44 64
32 44 64
20
14

Transition Width, W
(ft)
12 12 12
4 16 26
12
14

Minimum

Left Turn

Right Turn

Taper Ratio

Taper Length
(ft.)

Taper Ratio

Taper Length
(ft.)

8.33:1 100 8.33:1 100 8.33:1 100 8.33:1 100 8.33:1 100 8.33:1 100

15:1 60 8.33:1 100 15:1 240 8.33:1 100 15:1 390 8.33:1 100

5:1

60 8.33:1 100

8.33:1 116 8.33:1 100

Desirable

Left Turn

Right Turn

Taper Ratio

Taper Length
(ft.)

Taper Ratio

Taper Length
(ft.)

15:1 180 15:1 180 15:1 180 15:1 180 15:1 180 15:1 180

15:1 60 15:1 180 15:1 240 15:1 180 15:1 390 15:1 180

15:1 180 15:1 180

15:1 210 15:1 180

> 45 Rural Varies

14

8.33:1 116 8.33:1 100 15:1 210 15:1 180

* An urban section with a Type C Median can be used for design speeds of 45 mph

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4.3. Vertical Alignments
"Vertical Alignment", "Grade" and "Vertical Clearance" have been identified as "controlling criteria" that have substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the AASHTO Green Book criteria as the standard for elements of vertical alignment, grade and vertical clearance. A decision to deviate from the minimum controlling criteria defined by AASHTO shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.
GDOT uses design controls for crest and sag vertical curves based on sight distance. The AASHTO Green Book Chapter 3, Elements of Design, provides additional discussion on rates of vertical curvature (K). Maximum allowable vertical grades are dependent on the classification of the facility and are discussed in the following AASHTO Green Book chapters:
Chapter 5. Local Roads and Streets Chapter 6. Collector Roads and Streets Chapter 7. Rural and Urban Arterials Chapter 8. Freeways
GDOT typically uses symmetrical parabolic vertical curves at changes in grade. Exceptions to this include spot locations such as alignment breaks near intersections and overlay transitions where vertical grade breaks can be accomplished without the use of vertical curves. The maximum grade break (%) varies based on the design speed of the facility.
4.3.1. General Considerations for Vertical Alignments
The following are general considerations for vertical alignments:
Maximizing sight distances should be a primary consideration when establishing vertical alignment.
Long, gentle vertical curves should be used wherever possible and appropriate.
"Roller coaster" or "hidden dip" profiles should be avoided by using gradual grades made possible by heavier cuts and fills or by introducing some horizontal curvature in conjunction with vertical curvature. The "roller coaster" may be justified in the interest of economy and may be acceptable in low-speed conditions, but is aesthetically undesirable.
A single long vertical curve is preferred over "broken-back" grade lines (two crest or two sag vertical curves separated by a short tangent).
Use a smooth grade line with gradual changes, consistent with the type of highway and character of terrain, rather than a line with numerous breaks and short lengths of tangent grades.
On a long ascending grade, it is preferable to place the steepest grade at the bottom and flatten the grade near the top.
Moderate grades should be maintained through intersections to facilitate turning movements. Grades should not exceed 6%, and 3% maximum is preferred.

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Sag vertical curves should be avoided in cuts as roadway flooding or ponding conditions may occur at these locations should the drainage system become clogged or overburdened.
Vertical grades should be coordinated with required acceleration and deceleration areas, wherever possible. For instance, at an interchange, it is preferable for the crossing roadway to go over the limited access facility. That way, vehicles are on an upgrade as they decelerate towards a stop condition and are on a downgrade as they are entering the limited access facility.
As much as possible, the vertical alignment should be closely coordinated with the natural topography, available right of way, utilities, roadside development, and existing drainage patterns.
Vertical alignments should be properly coordinated with environmental constraints (e.g., encroachment into wetlands).
When a vertical curve takes place partly or wholly in a horizontal curve, the vertical curvature should be coordinated with the horizontal curvature. See Section 4.4. Combined Horizontal and Vertical Alignments.
When one roadway is in a tangent section and an intersecting roadway has a continuous vertical grade through an intersection, consideration should be given to rotating the pavement cross slope on the tangent roadway to a reverse crown to better match the profile of the intersecting roadway. Standard superelevation transition rates would apply.
Additional considerations for setting vertical alignments are detailed in the AASHTO Green Book Chapter 3, Elements of Design.
Factors That Influence Roadway Grades There are several factors that influence roadway grades:
topography and earthwork control points at the beginning and end of the project vertical clearances for drainage structures intersecting railroads applicable glide slopes for roadways near airports intersecting roads and streets driveway tie-ins existing bridges to remain vertical clearances at grade separations vertical clearances for high water and flood water proposed new bridges driver expectancy at intersections

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4.3.2. Maximum Vertical Grades
The grades selected for vertical alignments should be as flat as practical, and should not exceed the values listed in Table 4.5 Maximum Vertical Grades. Maximum values vary based on types of terrain, facility classification, and design speed. The maximum design grade should be used infrequently; in most cases, grades should be less than the maximum design grade. In Table 4.5. Maximum Vertical Grades, industrial roadways are defined as local and collector streets with significant (15% or more) truck traffic. Exceptions to the maximum vertical grades listed in Table 4.5. are as follows:
For short sections less than 500-ft. and for one-way downgrades, the maximum grade may be 1% steeper than the values listed in Table 4.5.
The maximum vertical grade for local streets, collectors and arterials may be increased by as much as 2% under extreme conditions
Maximum values in Table 4.5. may be reduced when upgrades cause a speed reduction greater than or equal to 10 mph.
For streets and highways requiring long upgrades, the maximum grade should be reduced so that the speed reduction of slow-moving vehicles (i.e., trucks and buses) is not greater than 10 mph. Where reduction of grade is not practical, climbing lanes should be provided to meet these speed reduction limitations. A design exception is required where a climbing lane cannot be provided and grade cannot be reduced.
Climbing lanes, speed reductions on upgrades and the critical lengths of grade associated with speed reductions are concepts that are discussed in detail in the AASHTO Green Book Chapter 3, Elements of Design. These concepts should be considered and appropriate provisions should be incorporated into any facility in which vertical grades will cause a significant (10 mph or more) reduction in the speed of a slow-moving vehicle.
American's with Disabilities Act (ADA) Requirements That Influence Maximum Vertical Grades

Relative to roadway profiles, ADA requirements dictate that:
The running (or longitudinal) slope of sidewalks should not exceed 5%.
In urban and suburban situations where the roadway typical section includes curb and gutter, the sidewalk is normally located behind (and adjacent to or offset from) the roadway. Since topography and practicality often dictate that many curb and gutter roadways have longitudinal slopes in excess of 5%, the running slope of sidewalks often exceed 5%.

Currently, GDOT interprets the ADA requirements for running slope to be applicable only to sidewalk ramps not to longitudinal sidewalks paralleling curb and gutter roadways. However, GDOT recognizes the merit in attempting to limit longitudinal sidewalk slopes wherever possible. With regard to sidewalks, longitudinal slopes and mainline roadway profiles, GDOT offers the following approach:

o On new alignment urban roadways, roadway grades should be limited to 5%, wherever practical. Applicable overriding constraints include environmental, right-ofway, cost, topography, and context-sensitive design areas. The maximum values in Table 4.5. may be used, if necessary.

o When an existing urban roadway is to be reconstructed, the practicality of vertical reconstruction by limiting proposed grades to 5% should be evaluated. If this

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approach is found to be impractical or technically infeasible (see above), the maximum values in Table 4.5. may be used, if necessary.
In either a new location or existing reconstruction situation where roadway grades exceed 5%, the following should be considered:
o Provision of pedestrian signage and connections to an alternate pedestrian route which does not have running slopes greater than 5%. This can be a footpath parallel to the roadway (but not adjacent to the sidewalk) or a detour route.
The cross slope of cross walks at intersections should not exceed 2%. To the extent practical, profile grades and cross slopes in intersections should be minimized ideally to 2%. Where this is not technically feasible, profile grades and cross slopes in urban intersections may exceed 2%.
4.3.3. Minimum Vertical Grades Minimum vertical grades are typically used to facilitate roadway drainage. This is especially true of curbed roadway sections where drainage or gutter spread is a consideration.
Uncurbed Pavements For projects involving uncurbed pavements, longitudinal grades may be flat (0%) in areas where appropriate cross slopes are provided. In areas of superelevation transitions and/or flat cross slopes on those projects, minimum vertical grades should be consistent with those listed in Table 4.6. Minimum Vertical Grades for Roadways where Drainage Spread is a Consideration. However, there are situations with uncurbed pavements where it is prudent that consideration be given to maintaining minimum vertical grades - similar to those for curbed roadway sections. These situations include:
a new location rural section
roadways with high truck percentages that experience appreciable pavement rutting
current rural roadways in urban, suburban or developing areas that have a realistic chance of being converted to a curb and gutter sometime in the foreseeable future
areas containing superelevation transitions and/or flat cross slope areas
interstate or other high speed facilities

Type of Terrain
Industrial Roadways
Level Rolling Mountainous
Local Rural Roads

Table 4.5. Maximum Vertical Grades
Maximum Grade (%) for Specified Design Speed (mph) 15 20 25 30 35 40 45 50 55 60 65 70 75 80
- - 44443333 - - - - - 55554444 - - - - - 66665555 - - - -

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Type of Terrain

Table 4.5. Maximum Vertical Grades
Maximum Grade (%) for Specified Design Speed (mph)
15 20 25 30 35 40 45 50 55 60 65 70 75 80

Level

9877777665 - - - -

Rolling

12 11 11 10 10 10 9 8 7 6 - - - -

Mountainous
Local Urban Streets
Level

17 16 15 14 13 12 11 10 10 - - - - 12 11 11 10 10 9 9 8 8 - - - - -

Rolling

14 13 12 11 11 10 10 9 - - - - - -

Mountainous
Rural Collectors
Level

17 16 15 14 13 12 11 - - - - - - - 777777665 - - - -

Rolling

- 10 10 9 9 8 8 7 7 6 - - - -

Mountainous
Urban Collectors
Level

- 12 11 10 10 10 10 9 9 8 - - - - 999998776 - - - -

Rolling

- 12 12 11 10 10 9 8 8 7 - - - -

Mountainous
Rural Arterials
Level

- 14 13 12 12 12 11 10 10 9 - - - - - - - - 554433333

Rolling

- - - - - 665544444

Mountainous
Urban Arterials
Level

- - - - - 877665555 - - - 8776655 - - - -

Rolling

- - - 9887766 - - - -

Mountainous

- - - 11 10 10 9 9 8 8 - - - -

Rural and Urban Freeways (Limited Access Facilities)

Level

- - - - - - - 4433333

Rolling

- - - - - - - 5544444

Mountainous

- - - - - - - 66655 - -

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Curbed Pavements

For curbed pavements, minimum longitudinal grades are controlled by the values in Table 4.6. This includes

Table 4.6. Minimum Vertical Grades for Roadways where Drainage Spread is a Consideration

roadways with concrete median barriers or side barriers, V-gutter and those roadways adjacent to walls. These values will generally ensure that roadway "spread" is not

Type of Facility
Industrial Roadways with Curb and Gutter Local Urban Streets with Curb and Gutter

Minimum Grade (%)

Desirable Minimum

0.30

0.20

0.30

0.20

excessive and can be contained

Urban Collectors with Curb and Gutter

0.50

0.30

within acceptable ranges by a

Urban Arterials with Curb and Gutter

0.50

0.30

minimum (reasonable) number of

Urban Freeways or Limited Access Facilities

0.50

0.30

roadway drainage catch basins. The

minimum values in Table 4.6 should be used only under extreme conditions.

4.3.4. Vertical Curves
In almost all cases, changes in grade should be connected by a parabolic curve (the vertical offset being proportional to the square of the horizontal distance). Vertical curves are required when the algebraic difference of intersecting grades exceeds a minimum threshold (refer to Section 4.3.5. Maximum Change in Vertical Grade without Using Vertical Curves).

Refer to the AASHTO Green Book Chapter 3, Elements of Design Vertical Curves, for considerations that must be made for vertical curves.

General Considerations
Vertical alignment has significant effect on roadway drainage. Special consideration should be given to the following:

Curbed roadways should have a minimum grade of not less than the values specified in Section 4.3.3. Minimum Vertical Grades in order to avoid excessive gutter spread. This includes roadways with concrete median barriers or side barriers, and V-gutter.

Non-curbed roadways should maintain a minimum grade consistent with the directives of Section 4.2.3. Pavement Widening on Curves and Section 4.3.3. Minimum Vertical Grades

For drainage purposes, the K value should not exceed 167 for curbed roadways (crest or sag verticals). In cases where design speeds are higher than 65 mph, this criteria does not apply.

For curbed roadways in sag vertical curves with low points, a minimum grade of 0.30% should be provided within 50-ft. of the low point. This corresponds to a K value of 167.

The minimum K values as defined by AASHTO are based on the assumption that there are significant tangent sections on either side of the vertical curve. Therefore, when using compound or unsymmetrical vertical curves, sight distances should be checked graphically to ensure that adequate sight distance is provided. Additional information can be found in the National Cooperative Highway Research Program (NCHRP) Report 504 Design Speed, Operating, Speed, and Posted Speed Practices.

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In establishing the vertical alignment, sound engineering practice should be used to strike a reasonable balance between excavation (cut) and embankment (fill). Other overriding factors must also be considered, including:
maintenance of traffic environmental impacts right-of-way impacts pedestrian (ADA) requirements safety considerations sight distance considerations (all types) drainage considerations high water considerations ability to tie the roadway profile into side roads, driveways and at grade railroad crossings. drivability and driver expectancy

4.3.5. Maximum Change in Vertical Grade without Using Vertical Curves
GDOT typically uses vertical curves for changes in vertical grades. However, there are situations where it is either impractical or impossible to utilize a vertical curve. Such situations include:

temporary vertical tie-ins profile tie-ins such as overlay transitions avoidance and/or minimization of an environmental impact point profiles in overlay and widening sections profile reconstruction near fixed objects such as bridges and approach slabs

Table 4.7. lists the maximum vertical grade changes that do not require a vertical curve. Note that these values change per design speed. Grade breaks should only be used when necessary (vertical curves should be used, wherever practical). If two or more of these vertical grade breaks are utilized in succession (i.e., a point profile), the cumulative effect of these grade breaks in the profile shall be evaluated for stopping sight distance and it shall be verified that typical stopping sight distance is always provided. If the cumulative effect of a series of vertical grade breaks violates stopping sight distance criteria, the values in Table 4.7. may need to be reduced.

Maximum Change in Grade (%)

Table 4.7. Maximum Change in Grade that Does Not Require a Vertical Curve
Design Speed (mph) 20 25 30 35 40 45 50 55 60 65 70
1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20

4.3.6. Vertical Grade Changes at Intersections
If it is impractical to match the elevation of an intersecting road, the crossroad should be reconstructed for a suitable distance using adequate vertical geometry to make the grade adjustment. In general, a 2% maximum tangent grade break is allowed at the edges of signalized

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intersections to allow vehicles on the crossroads to pass through an intersection on a green signal safely without significantly adjusting their speed. A 4% maximum grade break is allowed in the center of an intersection corresponding to the 4% crown breakover associated with a crossing road. For the edges of unsignalized or stop condition intersections, a maximum tangent grade break of 4% may be employed.
4.3.7. Minimum Profile Elevation Above High Water
One major factor in establishing a vertical profile for either a roadway or a bridge is clearance over high water or a design flood. For roadways, this is important for two reasons:
Pavement Protection - A major factor in a roadway's durability is minimizing the amount of moisture in the base and pavement. Keeping the roadway base as dry as possible will help prevent or minimize pavement deterioration.
Safety - A roadway with a profile set above the design high water will keep water from overtopping the roadway. Overtopped roadways are a hazard to moving vehicles and can effectively shut down a facility when they are needed most, i.e., a hurricane evacuation route.
For bridges, prescribed low-chord clearances must be maintained to protect the bridge superstructure from unanticipated lateral forces associated with high-velocity flood waters.
Table 4.8. summarizes the required high water clearances for roadways and bridges in Georgia. A vertical profile that satisfies the worst-case situation for either clearance or overtopping shall be established.

Table 4.8. Vertical Profile Clearance Based on High Water

Facility
Interstate Hurricane Evacuation Routes Roads Designed as State Routes Roads Not Designed as State Routes
ADT: 0 99 ADT: 100 399 ADT: 400 1,499 ADT: 1,500 or more
Driveways Temporary Detours Permanent Bridges Temporary Bridges
Local Road with ADT < 400 All Other Roads

Designer's First Priority

Roadway Base

Bridge Low Chord Clearance

Required Clearance
1-ft
1-ft.
1-ft.

Design Flood Frequency 50-year
50-year
50-year

Required Clearance
2-ft.
2-ft.
2-ft.

Design Flood Frequency 50-year
50-year
50-year

Designer Must Check

Shoulder Break Point Clearance or Bridge Low Chord Clearance

Required Clearance

Design Flood Frequency

1-ft. below shoulder break point 1-ft. below shoulder break point 1-ft. below shoulder break point

100-year 100-year 100-year

1-ft.

5-year

2-ft.

1-ft.

10-year

2-ft.

1-ft.

25-year

2-ft.

1-ft.

50-

2-ft.

year

1-ft.

25-year

2-ft.

1-ft.

10-year

2-ft.

1-ft.

50-year

2-ft.

5-year 10-year 25-year 50-year
25-year 10-year 50-year

1-ft. below shoulder break point 1-ft. below shoulder break point 1-ft. below shoulder break point 1-ft. below shoulder break point
Shoulder break point not overtopped Shoulder break point not overtopped
1-ft. low-chord clearance

10-year 25-year 50-year 100-year
50-year 25-year 100-year

1-ft.

2-year

2-ft.

2-year

1-ft.

10-year

2-ft.

10-year

Low-chord not overtopped Low-chord not overtopped

5-year 25-year

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Refer to the most current version of the GDOT Manual on Drainage Design for Highways (also referred to as the Drainage Manual), which may be downloaded from the GDOT Repository for Online Access to Documentation and Standards (R.O.A.D.S.). For roadways, designers should be familiar with the concept of culvert hydraulics and be aware that head losses associated with culverts will generally produce a headwater greater than the design flood elevation of the natural conditions. A vertical profile that provides the prescribed clearances over either the headwater of the natural conditions or the headwater created by a culvert, whichever is greater, shall be developed.
For bridges, designers should be familiar with the concept of riverine hydraulics and coordinate the bridge profile with the results of the bridge hydraulic study. As bridges will tend to generate backwater, a vertical profile that provides the prescribed clearances over the backwater created by the bridge or other nearby influencing structures shall be established. For additional information on Bridge Hydraulic guidelines, please refer to the GDOT Drainage Manual.
4.3.8. Reporting Changes in Vertical Clearances
The GDOT Office of Maintenance (Maintenance Office) has the responsibility of providing the Office of Permits and Enforcement with the height limitation of structures. The Bridge Maintenance Office and the Office of Permits & Enforcement have the responsibility of approving the proposed routing on state routes for vehicle movements which are over the legal vertical clearance.
It is extremely important for these offices to be kept informed of any change in vertical clearance as soon as possible after the change occurs. Persons (Area Engineer, Project Engineer) directly involved with vertical clearance revisions to any structure on a state route shall immediately notify:
The GDOT Maintenance Office - Such a report should be made by telephone to the Routing Engineer at (404) 656-5287 or to the Assistant State Maintenance Engineer, Bridges. The Maintenance Office will handle the reporting of the above changes to the Office of Permits & Enforcement.
The GDOT Bridge Maintenance & Inventory Office - This office should be notified of any changes in vertical clearances on the state system within 24 hours.
In cases where the actual measured minimum vertical clearance must be revised, the person directly involved with the revision (Area Engineer, Project Engineer) shall advise the District Maintenance Office of the actual measured minimum vertical clearances on his/her specific construction project(s). The revised information should then be reported to the Bridge Inventory Office, and this information will be directed to local Bridge Inspection personnel, such that the revisions to the Bridge Information System may be verified at a later date. The Bridge Inventory Office in Atlanta will initiate revisions to the system with notification to units requiring the revised information.
The actual measured minimum vertical clearance should be recorded at both edges of the pavement, the crown point (if present) and at the edges of paved shoulders (if present). In addition, measurements at any other restricting locations caused by the geometrics of the overhead structure or roadway should be recorded. Special attention should be paid to the effects of reconstruction at a restrictive location. For example, to resurface beneath a posted clearance without insuring a correction in posting misinforms the traveling public and thus creates a possible hazardous condition.

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4.4. Combined Horizontal and Vertical Alignments
Horizontal and vertical alignments are permanent design elements that warrant thorough study. Horizontal and vertical alignments should not be designed independently, but should complement each other. Poorly designed combinations can negate the benefits and aggravate the deficiencies of each. A well-designed combination, in which horizontal and vertical alignments work in concert, increases highway usefulness and safety, encourages uniform speed, and improves appearance.
4.4.1. Aesthetic Considerations
Coordination of the horizontal and vertical alignment can result in a highway that is visually pleasing. This can be achieved in several ways:
A sharp horizontal curve should not be introduced at or near the low point of a sag vertical curve, which produces a distorted appearance.
There should be a balance between curvature and grades. The use of steep grades to achieve long tangents and flat curves, or the use of excessive curvature to achieve flat grades, are considered poor design. A logical design is a compromise between the two conditions. Wherever feasible the roadway should "roll with" rather than "buck" the terrain.
If possible, every effort should be made to line up points of vertical intersection (PVI's) with horizontal points of intersections (PI's) and to maintain consistency between the horizontal and vertical curve lengths. Vertical curvature superimposed on the horizontal curvature generally results in a more visually pleasing facility. Successive changes in profile not in combination with horizontal curvature may result in a series of dips not visible to the driver. If PVI's and PI's cannot be made to coincide, the horizontal curvature should "lead" the vertical curve and the horizontal curve should be slightly longer than the vertical curve in both directions.
A balanced design which provides horizontal and vertical alignments in the middle range of values is preferable to allowing either horizontal or vertical to become extreme in order to optimize the other.
Design the alignment to enhance attractive scenic views of the natural and manmade environment, such as rivers, rock formations, parks, and outstanding buildings.
In residential areas, wherever possible, design the alignment to minimize nuisance factors to the neighborhood. Generally, a depressed facility makes a highway less visible and less noisy to adjacent residents. Minor horizontal adjustments can sometimes be made to increase the buffer zone between the highway and clusters of homes.
Refer to the GDOT Context-Sensitive Design Online Manual , for additional information.
4.4.2. Safety Considerations
The superimposed effect of horizontal and vertical alignments can influence both sight distance and driver expectancy which translate directly into safety. As safety should be the designer's primary consideration, the following guidelines are presented:
Sharp horizontal curves should not be introduced at or near the top of a pronounced vertical curve, since the driver cannot perceive the horizontal change in alignment, especially at night.

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Sharp horizontal curves should not be introduced at or near the low point of a sag vertical curve, since vehicles, particularly trucks, are traveling faster at the bottom of grades.

Both horizontal and vertical curvature should be as flat as possible at intersections where vehicles have to decelerate, stop, or accelerate.

To maintain proper pavement drainage, design vertical and horizontal curves so that the flat profile of a vertical curve will not be located near the flat cross slope of the superelevation transition. As a general rule, pavement cross slope should be at least 1.0% near vertical curve sag points and longitudinal roadway grades should be at least 0.30% at locations where the pavement cross slope is flat (0%), for instance at superelevation transitions.

On two-lane roadways, the need for safe passing sections (at frequent intervals and for an appreciable percentage of the length of the roadway) often supersedes the general desirability for combination of horizontal and vertical alignment. The Designer should strive to implement long tangent sections to secure sufficient passing sight distance.

It is generally poor practice to place the superelevation rotation point at a different point than the profile grade line.

Particular attention shall be paid to all forms of sight distance when horizontal and vertical alignments are superimposed on each other. The combination of horizontal and vertical curvature can sometimes result in effectively less sight distance than the individual affect of either horizontal or vertical curvature.

4.4.3. Divided Highways
A well designed roadway will incorporate a litany of considerations including safety, economy, and aesthetics, etc. When terrain is hilly, mountainous or undulating, the profile of the roadway should generally follow the contours of the land (barring overriding considerations). On divided highways and rural interstates, the Designer should recognize where terrain dictates, separate horizontal alignments and vertical profiles can be utilized for opposing traffic.

Independent Profiles and Increasing Median Width
On state and federal divided highways, an increase in the width of the median and the use of independent alignments to derive the design and operational advantages of one-way roadways should be considered. Where right of way is available, a superior design, without significant additional costs, can result from the use of independent alignments and profiles. Bifurcated medians are especially effective where the general fall of the terrain is significant and perpendicular to the roadway.

Increasing the width of the median and/or bifurcating the roadway should be considered in the following situations:

Where right of way is available and where the general fall of the terrain is significant and perpendicular to the roadway

In isolated areas on rural reconstruction projects where the height of vertical reconstruction is significant. This will facilitate efficiency and ease conflicts during an intermediate stage of construction. As a general rule of thumb, standard 44-ft. median width can be maintained with independent profiles until the difference in elevations in opposing PGL's is approximately 5-ft. Consideration should be given to increasing the median width (beyond 44-ft.) a minimum of 2-ft.

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for every 1-ft. of vertical profile reconstruction greater than 5-ft. Obviously, increasing the median width will result in greater right of way impacts. However, in many instances, minor right of way impacts - especially in rural areas where it is plentiful are ultimately less costly than significant vertical reconstructions that require the contractor to utilize earth stabilization techniques or sheet pile to construct.
At intersections to eliminate breakovers.
4.5. Superelevation
When a vehicle travels around a horizontal curve, it is forced radially outward by centrifugal force. When this force becomes too great for a given design speed, the roadway is "superelevated" to counter it. Five methods of counteracting centrifugal forces through curves are discussed in the AASHTO Green Book Chapter 3, Elements of Design.
4.5.1. Maximum Superelevation Rates
"Superelevation" has been identified as a "controlling criteria" that has substantial importance to the operational and safety performance of a roadway such that special attention should be given to the design decision. Therefore, GDOT adopts the Superelevation rates shown in table 4.9 as the standard for superelevation rates in Georgia. The FHWA has stated that a Design Exception is required if the State's superelevation rate cannot met. Therefore, a decision to use a Superelevation rate that does not meet the maximum Superelevation Rate shown in Table 4.9 shall require a comprehensive study by an engineer and the prior approval of a Design Exception from the Department's Chief Engineer.
Horizontal alignments are composed of tangent sections connected by arcs of circular curves (GDOT does not normally use spiral curves). Vehicles traveling in a circular path counter the centrifugal force that would cause them to leave the road through a combination of two factors: lateral friction between the vehicle's tires and the road, and superelevation.

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The maximum rates of superelevation used on highways are controlled by four factors: Climatic conditions (i.e., frequency and amount
of snow and ice)
Terrain conditions (i.e., flat, rolling, or mountainous)
Type of area (i.e., rural or urban)

Table 4.9. Maximum

Superelevation Rates

Setting

Maximum Superelevation Rates
(emax)(1)

Urban (Curb and Gutter) Roads (DS < 45mph)

4%

Suburban / Developing Areas

6%

Frequency of very slow-moving vehicles whose operation might be affected by high superelevation rates

Rural (Non Curb and Gutter) Paved Roads Unpaved Roads

6% Reverse Crown

Interstates, Expressways,

Superelevation requirements for maximum

L/A Facilities

superelevation rates (0.04 to 0.12-ft./ft) for various

Rural

8%

design speeds (15 mph to 80 mph) are provided in

Urban

6%

the AASHTO Green Book Chapter 3, Elements of Design Superelevation Tables. GDOT has designated the values in Table 4.9. as the

System-to-System Ramps Rural

8%

maximum values (emax) for use on Georgia roadways.

Urban Exit-Entrance Ramps

6% 8%

Free Flowing Loop Ramps

10%

It is important for designers to realize that the minimum curve radii and maximum superelevation rates depicted in the AASHTO Green Book are extremes and should be avoided wherever possible.

Long Ramps with STOP

8%

(1) The maximum allowed values (emax) for usage on Georgia roadways, as designated by GDOT.

In general, GDOT does not require superelevation on lowspeed urban roadways or roadways with a design speed of 25 mph or less

The emax values presented in Table 4.9. requires the use of the more moderate design value ranges for curvature and superelevation. In certain situations, such as those described below, the emax values in Table 4.9. may require further reduction:

Wherever practical, consideration should be given to maximizing curve radii and minimizing superelevation rates on curves which include bridges. This is due to the increased potential for icing. Where constraints do not exist, an emax of 4% should be utilized.
Wherever possible, the maximum superelevation rates on roadways within an intersection should be limited to 4% (2% for urban areas with crosswalks). Wherever possible and when applicable in intersections, superelevation cross slopes of one roadway should be coordinated with the mainline profile grade of the intersecting roadway.

Where traffic congestion or extensive development acts to restrict top speeds on a rural roadway, a maximum rate of superelevation of 6% should be used.


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4.5.2. Sharpest Curve without Superelevation
Although superelevation is advantageous for high-speed traffic operations, various factors combine to make its use impractical in many built-up areas. Such factors include:
wide pavement areas need to meet grade of adjacent property surface drainage considerations frequency of cross streets, alleys and driveways at major intersections or other locations where there is a tendency to drive slowly because of
turning and crossing movements, warning devices, and traffic signals

The minimum curve radius is a limiting value of curvature for a given design speed and is determined from the maximum rate of superelevation and the maximum side friction factor selected for design. Very flat curves need no superelevation. In many instances, it is desirable to maintain a normal crown typical section on the roadway. In these cases, implementation of a curve with a radius flat enough as to not require superelevation should be considered.

4.5.3. Axis of Rotation
Roadway alignments are generally defined by a centerline (CL) and a profile grade line (PGL). The roadway may be rotated about various points on the typical section to achieve superelevation. Typically, the point of superelevation rotation (axis of rotation) corresponds to the PGL located on the inside edges of the travel lanes. On two-way roadways with a flush, raised or no median, the axis of rotation typically corresponds to the roadway centerline. Generally, rotation will occur about the centerline on roadways with an urban ty