DIVERGING DIAMOND

INTERCHANGE

Informational Guide 

August 2014 
Diverging Diamond Interchange Informational Guide

FOREWORD

The Federal Highway Administration (FHWA) Every Day Counts (EDC) initiative is designed to 

identify and deploy innovation aimed at reducing project delivery time, enhancing safety and 

protecting the environment. In 2012, FHWA chose Intersection & Interchange Geometrics (IIG) 

to feature as one of the innovative technologies in EDC-2. Specifically, IIG consists of a family 

of alternative intersection designs that improve intersection safety while also reducing delay, and 

at lower cost and with fewer impacts than comparable traditional solutions.

As part of the effort to mainstream these intersections, FHWA has produced a series of guides to 

help transportation professionals routinely consider and implement these designs. Concurrent 

with this Diverging Diamond Interchange (DDI) Informational Guide, FHWA developed and 

published guides for three other designs: Median U-turn (MUT), Displaced Left Turn (DLT), 

and Restricted Crossing U-turn (RCUT). These guides represent summaries of the current state 

of knowledge and practice, and are intended to inform project planning, scoping, design and 

implementation decisions.

An electronic version of this document is available on the Office of Safety website at 

http://safety.fhwa.dot.gov/. Additionally, limited quantities of hard copies are available from the 

Report Center; inquiries may be directed to report.center@dot.gov or 814-239-1160. 

Michael S. Griffith

Director

Office of Safety Technologies

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard, 

specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
Diverging Diamond Interchange Informational Guide

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Technical Report Documentation Page

1. Report No.

FHWA-SA-14-067

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Diverging Diamond Interchange Informational Guide

5. Report Date

August 2014

6. Performing Organization Code

7. Authors

Bastian Schroeder; Chris Cunningham; Brian Ray, Andy Daleiden, Pete 

Jenior, Julia Knudsen, Kittelson & Associates, Inc.

8. Performing Organization Report No.

Project 13517

9. Performing Organization Name and Address

Kittelson & Associates, Inc.

610 SW Alder Street, Suite 700 

Portland, OR 97205

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

TO DTFH61-00023-T-13002

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration Office of Safety 

1200 New Jersey Ave., SE

Washington, DC 20590

13. Type of Report and Period 

Technical Report

Informational Report

September 2013 to August 2014

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

Jeffrey Shaw (jeffrey.shaw@dot.gov), Office of Safety Technologies (http://safety.fhwa.dot.gov/), served as the Technical

Manager for the Federal Highway Administration (FHWA). The following FHWA staff contributed as technical working group 

members, reviewers and/or provided input or feedback to the project at various stages: Joe Bared, Mark Doctor, Brian Fouch, 

Elizabeth Hilton, Jim McCarthy, George Merritt, Will Stein, Jim Sturrock and Wei Zhang.

16. Abstract

This document provides information and guidance on the Diverging Diamond Interchange (DDI). To the extent possible, the guide 

addresses a variety of conditions found in the United States, to achieve designs suitable for a wide array of potential users. This 

guide provides general information, planning techniques, evaluation procedures for assessing safety and operational performance, 

design guidelines, and principles to be considered for selecting and designing Diverging Diamond Interchanges. 

17. Key Words

DDI, DCD, Diverging diamond interchange, Double crossover diamond 

interchange, Alternative intersections, Alternative interchanges, Innovative 

interchanges, Innovative intersections

18. Distribution Statement

No restrictions. 

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

226

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
Diverging Diamond Interchange Informational Guide

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Diverging Diamond Interchange Informational Guide

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TABLE OF CONTENTS

CHAPTER 1— INTRODUCTION ............................................................................................. 1

OVERVIEW OF ALTERNATIVE INTERSECTIONS AND INTERCHANGES ................... 1

INTERSECTION CONTROL EVALUATIONS AND CONSIDERATIONS ......................... 1

ORGANIZATION OF THE GUIDELINES .............................................................................. 2

SCOPE OF THE GUIDE ............................................................................................................ 3

DDI OVERVIEW ....................................................................................................................... 3

APPLICATION ........................................................................................................................ 4

GEOMETRIC DESIGN CONSIDERATIONS ........................................................................ 10

RESOURCE DOCUMENTS .................................................................................................... 12

CHAPTER 2— POLICY AND PLANNING ........................................................................... 13

PLANNING CONSIDERATIONS FOR ALTERNATIVE INTERSECTIONS AND 

INTERCHANGES .................................................................................................................... 13

STAKEHOLDER OUTREACH ............................................................................................... 15

POLICY CONSIDERATIONS ................................................................................................ 20

PLANNING CONSIDERATIONS .......................................................................................... 21

PLANNING CHALLENGES ................................................................................................... 22

PROJECT PERFORMANCE CONSIDERATIONS ............................................................... 23

PROJECT DEVELOPMENT PROCESS ................................................................................. 24

SUMMARY OF DDI ADVANTAGES AND DISADVANTAGES ....................................... 25

CHAPTER 3— MULTIMODAL CONSIDERATIONS ......................................................... 29

DESIGN PRINCIPLES AND APPROACH ............................................................................ 30

PEDESTRIANS ...................................................................................................................... 32

BICYCLISTS ...................................................................................................................... 49

TRANSIT ...................................................................................................................... 54

HEAVY VEHICLE CONSIDERATIONS ............................................................................... 56

CHAPTER 4— SAFETY ........................................................................................................... 57

SAFETY PRINCIPLES ............................................................................................................ 57

GENERAL SAFETY CONCERNS ......................................................................................... 59

OBSERVED SAFETY PERFORMANCE ............................................................................... 61

INCIDENT RESPONSE CONSIDERATIONS ....................................................................... 68

SAFETY EVALUATION CONSIDERATIONS ..................................................................... 68

CHAPTER 5— OPERATIONAL CHARACTERISTICS ..................................................... 69

INTRODUCTION .................................................................................................................... 69

OPERATIONAL CONSIDERATIONS ................................................................................... 73
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SIGNAL TIMING AND COORDINATION ........................................................................... 82

COMPARATIVE PERFORMANCE STUDIES ...................................................................... 88

CHAPTER 6— OPERATIONAL ANALYSIS ........................................................................ 91

OPERATIONAL ANALYSIS TOOL OVERVIEW ................................................................ 92

PLANNING-LEVEL ANALYSIS ........................................................................................... 92

HIGHWAY CAPACITY MANUAL (HCM) ANALYSIS ...................................................... 95

MICROSIMULATION ANALYSIS ...................................................................................... 101

CHAPTER 7— GEOMETRIC DESIGN ............................................................................... 107

DESIGN APPROACH............................................................................................................ 107

GEOMETRIC DESIGN PARAMETERS .............................................................................. 108

GEOMETRIC DESIGN PRINCIPLES .................................................................................. 114

ALIGNMENT ALTERNATIVES .......................................................................................... 117

DESIGN GUIDANCE ............................................................................................................ 126

CHAPTER 8— SIGNAL, SIGNING, MARKING, AND LIGHTING ................................ 147

DESIGN PRINCIPLES AND APPROACH .......................................................................... 147

SIGNALS .................................................................................................................... 148

SIGNING .................................................................................................................... 159

PAVEMENT MARKINGS .................................................................................................... 166

LIGHTING .................................................................................................................... 175

CHAPTER 9— CONSTRUCTION AND MAINTENANCE ............................................... 181

CONSTRUCTION .................................................................................................................. 181

COSTS .................................................................................................................... 185

MAINTENANCE ................................................................................................................... 186

LAW ENFORCEMENT NEEDS ........................................................................................... 187

REFERENCES ...................................................................................................................... 189

Appendix A CATALOG OF ALL KNOWN INSTALLATIONS IN THE UNITED 

STATES ....................................................................................................... 193

Appendix B SUPPLEMENTAL OPERATIONAL AND SAFETY DETAILS .......... 197

Appendix C MARKETING AND OUTREACH MATERIALS .................................. 199
Diverging Diamond Interchange Informational Guide

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List of Exhibits

Exhibit 1-1. Key characteristics of a DDI. ...................................................................................... 4

Exhibit 1-2. Locations of DDIs. ...................................................................................................... 5

Exhibit 1-3. First constructed DDI in Utah at Pioneer Crossing and Interstate 15 (American Fork, 

UT).(1) .............................................................................................................................................. 6

Exhibit 1-4. First constructed DDI in Georgia at Ashford Dunwoody Road near Perimeter 

Market.(2) ......................................................................................................................................... 7

Exhibit 1-5. First constructed DDI in Minnesota at Highway 15.(3) ............................................... 7

Exhibit 1-6. First constructed DDI in Idaho at Interstate 86 and US 91/Yellowstone Highway 

(Pocatello-Chubbuck, ID).(4) ........................................................................................................... 8

Exhibit 1-7. Crossover location at the DDI located at SR 92 and Interstate 15 (Utah County, 

UT).(5) .............................................................................................................................................. 8

Exhibit 1-8. Overhead signing at the DDI located at SR 92 and Interstate 15 (Utah County, 

UT).(5) .............................................................................................................................................. 9

Exhibit 1-9. Pedestrian crossing at the right-turn lane of the exit located at Pioneer Crossing and 

Interstate 15 DDI (American Fork, UT).(6) ..................................................................................... 9

Exhibit 1-10. Sidewalk and recessed lighting located on the bridge at the Pioneer Crossing and 

Interstate 15 DDI (American Fork, UT).(6) ................................................................................... 10

Exhibit 1-11. DDI (overpass) at 500 East American Fork (American Fork, UT).(1) .................... 11

Exhibit 1-12. DDI (underpass) at Dorsett Road and Interstate 270 (St Louis, MO).(7) ................ 11

Exhibit 2-1. Relationship between volume and interchange type. ................................................ 14

Exhibit 2-2. Example video screen captures from UDOT of how to travel through a DDI.(1) ..... 16

Exhibit 2-3. Example video screen capture from Nevada DOT of how to travel through a DDI.(20)

....................................................................................................................................................... 16

Exhibit 2-4. Fact sheet from Minnesota DOT on how various users travel through a DDI.(3) ..... 17

Exhibit 2-5. Public outreach brochure used by MoDOT.(7) .......................................................... 18

Exhibit 2-6. Example public outreach brochure used by GDOT.(2) .............................................. 19

Exhibit 2-7. Project development process. ................................................................................... 24

Exhibit 2-8. Summary of DDI advantages and disadvantages. .................................................... 27

Exhibit 3-1. Schematic of DDI right-of-way availability for multimodal facilities. .................... 30

Exhibit 3-2. Center walkway at MO 13 (Springfield, MO).(14) .................................................... 31

Exhibit 3-3. Outside walkway at Dorsett Road (Maryland Heights, MO).(14) .............................. 32

Exhibit 3-4. Shared-use path on outside at Harrodsburg Road (Lexington, KY).(14) ................... 32

Exhibit 3-5. Pedestrian facilities in center of DDI (Springfield, MO).(26) .................................... 34

Exhibit 3-6. Pedestrian facilities on outside of DDI (Maryland Heights, MO).(26) ...................... 35

Exhibit 3-7. Center walkway pedestrian safety and comfort. ....................................................... 36

Exhibit 3-8. Outside path/sidewalk pedestrian safety and comfort. ............................................. 37

Exhibit 3-9. Pedestrian-focused DDI – center walkway. .............................................................. 38

Exhibit 3-10. Pedestrian-focused DDI – outside walkway. .......................................................... 39

Exhibit 3-11. Pedestrian-vehicle conflict points at DDI with center walkway............................. 40

Exhibit 3-12. Pedestrian-vehicle conflict points at DDI with outside walkways. ........................ 41

Exhibit 3-13. Pedestrian-vehicle conflict points at conventional diamond. ................................. 42

Exhibit 3-14. Pedestrian-vehicle conflict points at conventional diamond with pedestrian 

crossing of arterial......................................................................................................................... 42

Exhibit 3-15. Pedestrian-vehicle conflict point comparison. ........................................................ 43

Exhibit 3-16. Example of pedestrian crossing at free-flow left onto freeway.(6) .......................... 43
Diverging Diamond Interchange Informational Guide

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Exhibit 3-17. Pedestrian markings to indicate directionality of traffic (Maryland Heights, 

MO).(14) ......................................................................................................................................... 44

Exhibit 3-18. Channelization toward center crosswalk.(26) ........................................................... 46

Exhibit 3-19. Channelization toward outside crosswalk.(26) ......................................................... 46

Exhibit 3-20. Example of pedestrian wayfinding provision at DDI via curbing.(26) ..................... 47

Exhibit 3-21. Example of pedestrian wayfinding provision at DDI using an urban fence.(26) ..... 48

Exhibit 3-22. Bicycle lane maintained through DDI between crossovers.(20) ............................... 50

Exhibit 3-23. Bicycle provisions on shared-use path.(14) .............................................................. 50

Exhibit 3-24. Bicyclist in travel lane at DDI crossover.(14)........................................................... 51

Exhibit 3-25. Visualization of bicycle lane, approach of first crossover into DDI.(20) ................. 52

Exhibit 3-26. Visualization of bicycle lane, approach of second crossover out of DDI.(20) ......... 52

Exhibit 3-27. Schematic for bicycle lane placement on right side of vehicular traffic. ............... 53

Exhibit 3-28. Aerial view of DDI with center-running light rail at I-494 and 34th Avenue in 

Bloomington, MN.(29).................................................................................................................... 55

Exhibit 4-1. Conflict point diagram for DDI. ............................................................................... 57

Exhibit 4-2. Conflict point diagram for conventional diamond. ................................................... 58

Exhibit 4-3. Conflict point comparison. ....................................................................................... 58

Exhibit 4-4. Oncoming traffic for the right turn at the exit ramp. ................................................ 60

Exhibit 4-5. Oncoming traffic for the left turn at the exit ramp. .................................................. 60

Exhibit 4-6. Conflict zones at ramp terminal intersection. ........................................................... 64

Exhibit 4-7. Wrong-way maneuvers at five DDI sites along with crossover angle and crossroad 

AADT. .......................................................................................................................................... 66

Exhibit 4-8. Southbound outer-most lane closed and marked with pavement markings.(26) ........ 66

Exhibit 4-9. Pavement marking used to channel vehicles through crossovers.(26)........................ 67

Exhibit 5-1. Key operational characteristics of a DDI. ................................................................. 70

Exhibit 5-2. Naming convention of movements at a DDI (east-west cross street). ...................... 71

Exhibit 5-3. Operational consideration zones for DDIs................................................................ 72

Exhibit 5-4. Operational considerations at DDIs. ......................................................................... 73

Exhibit 5-5. Queue spillback into DDI from downstream adjacent signal.(14) .............................. 74

Exhibit 5-6. Different lane configurations at three-lane DDIs. .................................................... 75

Exhibit 5-7. Left lane utilization prediction for two-lane DDI crossovers.(14) .............................. 75

Exhibit 5-8. Field-measured speed parameters for DDI sites.(14).................................................. 76

Exhibit 5-9. Queue spillback caused by heavy traffic volumes and a no RTOR condition.(14) .... 77

Exhibit 5-10. Ramp meter with added lane for queue storage at a DDI.(6) ................................... 78

Exhibit 5-11. Weaving maneuver and conflict points for DDI right turn from freeway. ............. 79

Exhibit 5-12. Lane separation for heavy and emergency vehicle accommodation.(26) ................. 79

Exhibit 5-13. Phasing schematic with exit ramp queue clearance phase. ..................................... 81

Exhibit 5-14. Simplified DDI timing sequence. ........................................................................... 83

Exhibit 5-15. Example use of overlap phases in DDI timing sequence. ....................................... 84

Exhibit 5-16. DDI timing favoring cross street versus turns. ....................................................... 85

Exhibit 5-17. Time-space diagram for corridor of conventional intersections. ............................ 86

Exhibit 5-18. Time-space diagram for corridor with DDI. ........................................................... 87

Exhibit 5-19. Example of percent demand starvation. .................................................................. 88

Exhibit 5-20. Before-after interchange delay comparison at two DDIs.(14) .................................. 90

Exhibit 6-1. Cap-X Planning Level Tool Screen Capture.(39) ....................................................... 94

Exhibit 6-2. LOS criteria for DDIs (based on HCM Exhibit 22-11).(9) ........................................ 96
Diverging Diamond Interchange Informational Guide

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Exhibit 6-3. HCM methodology for DDI evaluation (adapted from HCM Exhibit 22-14).(9) ..... 98

Exhibit 6-4. Input data for HCM DDI evaluation (based on HCM Exhibit 22-15).(9) .................. 99

Exhibit 6-5. Concept of look-back distance in simulation. ......................................................... 103

Exhibit 6-6. Simulation speed settings........................................................................................ 104

Exhibit 7-1 DDI characteristics. ................................................................................................. 107

Exhibit 7-2. Examples of various cross sections utilized at overpasses in the U.S.(26) ............... 109

Exhibit 7-3. Examples of various cross sections utilized at underpasses in the U.S.(26) ............. 111

Exhibit 7-4. Conventional diamond interchange overlaid with a DDI design.(13) ...................... 112

Exhibit 7-5. An oversized load making a left turn onto I-44 at MO 13 (Springfield, MO).(16) .. 115

Exhibit 7-6. Vehicle paths through crossover. ............................................................................ 117

Exhibit 7-7. Alignment alternatives that minimize cross-sections over or under a bridge.(14) ... 119

Exhibit 7-8. Alignment alternatives to minimize the distance between crossovers and amount of 

reverse curvature.(14) ................................................................................................................... 121

Exhibit 7-9. Example of horizontal alignment and cross-sections for under and overpass designs 

that minimize the distance between crossovers.(14) ..................................................................... 123

Exhibit 7-10. Symmetrical alignment with no reverse curves between crossovers (Dorsett Rd., 

Maryland Heights, MO).

(26) ........................................................................................................ 124

Exhibit 7-11. Cross-sectional distances for a symmetrical alignment with no reverse curves 

between crossovers (Dorsett Rd., Maryland Heights, MO).

(26) .................................................. 125

Exhibit 7-12. Shifted alignment south of centerline (red) with no reverse curves between 

crossovers (Pioneer Crossing, American Fork, UT).

(26) ............................................................. 125

Exhibit 7-13. Cross-sectional distances for a shifted alignment with no reverse curves between 

crossovers (Pioneer Crossing, American Fork, UT).(26) ............................................................. 126

Exhibit 7-14. Speed-radius relationship.(42) ................................................................................ 127

Exhibit 7-15. Barriers and traffic islands to discourage wrong-way movements.(26) ................. 129

Exhibit 7-16. Tangent length approaching and departing the crossover. ................................... 130

Exhibit 7-17. Auxiliary left turn lane between crossovers. ........................................................ 132

Exhibit 7-18. Auxiliary left turn lane developed prior to the first crossover. ............................. 132

Exhibit 7-19. Auxiliary lane, shared left and through, developed prior to the first crossover. .. 133

Exhibit 7-20. Auxiliary through lane developed prior to the first crossover. ............................. 133

Exhibit 7-21. Entry ramp right turn yield control - no acceleration lane. ................................... 135

Exhibit 7-22. Entry ramp free right and left turns with acceleration lanes. ................................ 136

Exhibit 7-23. Entry ramp left turn yield control - no acceleration lane. ..................................... 137

Exhibit 7-24. Exit ramp left turn with yield control. .................................................................. 138

Exhibit 7-25. Exit ramp left turn with signal control. ................................................................. 138

Exhibit 7-26. Alignment of exit ramp right and left turn movements. ....................................... 140

Exhibit 7-27. Supplemental advanced signal at crossover.(26) .................................................... 142

Exhibit 7-28. Plan view of supplemental signal. ........................................................................ 142

Exhibit 7-29. Time gap - Case C2, left and right turn from minor approach. ............................ 143

Exhibit 7-30. SSD and ISD calculations (Case C2). ................................................................... 144

Exhibit 7-31. Stopping and intersection sight distance for yield controlled left and right turns. 144

Exhibit 7-32. Median barriers blocking ISD for exit ramp right turns. ...................................... 145

Exhibit 8-1. DDI standard phasing scheme. ............................................................................... 149

Exhibit 8-2. Considerations for one versus two controllers at a DDI. ........................................ 150

Exhibit 8-3. Example of a single controller with four rings servicing protected pedestrian and 

vehicle movements on all approaches......................................................................................... 151
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Exhibit 8-4. Example of two controllers with two rings servicing protected pedestrian and 

vehicle movements on all approaches......................................................................................... 151

Exhibit 8-5. Restricted signal head visibility from opposing movement mast arm.(26) .............. 153

Exhibit 8-6. Supplemental signal on right for an inbound movement at a DDI.(26) .................... 153

Exhibit 8-7. Supplemental signal on left for an outbound movement at a DDI.(26) .................... 154

Exhibit 8-8. Supplemental signal for a DDI exit ramp from freeway.(26) ................................... 154

Exhibit 8-9. Type 2 crossover signals with a type 1 supplemental signal on left.(26) ................. 155

Exhibit 8-10. Undesirable use of single pole with two pedestrian push-buttons, no APS, and 

insufficient separation of the two detectable warning surfaces.(26) ............................................. 156

Exhibit 8-11. DDI splitter island with pedestrian signals on same side. .................................... 157

Exhibit 8-12. DDI splitter island with diagonal pedestrian signals. ........................................... 157

Exhibit 8-13. Pedestrian signal and push-button on outside of crossover (pedestrian facilities in 

center).(14) .................................................................................................................................... 158

Exhibit 8-14. Pedestrian signal for left-turn onto freeway (pedestrian facilities on outside).(14) 159

Exhibit 8-15. No Left and No Right turn signs.(26) ..................................................................... 160

Exhibit 8-16. Stay Right and Stay Left signs.(26) ........................................................................ 161

Exhibit 8-17. Do not Enter and Wrong Way signs.(26) ................................................................ 162

Exhibit 8-18. One Way signs.(26)................................................................................................. 163

Exhibit 8-19. Yield Ahead, Advanced Signal Ahead and Reverse Curve Ahead signs.(26) ........ 164

Exhibit 8-20. Double Arrow signs.(26)......................................................................................... 165

Exhibit 8-21. Lane control signing for signalized movements.(26) .............................................. 166

Exhibit 8-22. Example centerline and edge line pavement markings at a DDI.(26) .................... 168

Exhibit 8-23. Lane line markings at crossover and ramp terminal approaches.(26) ..................... 169

Exhibit 8-24. Use of short skip lines to delineate an exclusive left turn lane onto a limited access 

facility.(26) .................................................................................................................................... 170

Exhibit 8-25. Channelizing lines and vane islands at the exit ramp.(26) ...................................... 171

Exhibit 8-26. Three variations of lane use arrows at the crossover intersection.(26) ................... 171

Exhibit 8-27. Stop bar locations at the intersection crossover and exit ramp terminal.(26) ......... 172

Exhibit 8-28. Yield line locations.(26) .......................................................................................... 173

Exhibit 8-29. Transverse (A) and longitudinal (B) crosswalk markings at a DDI.(26) ................ 174

Exhibit 8-30. Engraved pedestrian pavement marking – “Look Left.”(26) .................................. 175

Exhibit 8-31. Continuous interchange lighting in an urban setting.(26) ....................................... 177

Exhibit 8-32. Partial interchange lighting in a suburban setting. Lighting on the ramps is 

provided as a transition.(26) .......................................................................................................... 178

Exhibit 8-33. Partial interchange lighting in a suburban setting. No lighting is provided on the 

ramps for transition.(26)................................................................................................................ 179

Exhibit 8-34. Recessed lighting in DDI center walkway.(14) ...................................................... 180

Exhibit 9-1. Example of construction staging using an existing structure.(47) ............................ 183

Exhibit 9-2. Self-propelled modular transporter (SPMT) brings superstructure to DDI location.(48)

..................................................................................................................................................... 184

Exhibit 9-3. Construction staging using pre-cast construction methods.(48) ............................... 185

Exhibit 9-4. Construction cost estimates.(14) ............................................................................... 186
Diverging Diamond Interchange Informational Guide

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CHAPTER 1— INTRODUCTION

OVERVIEW OF ALTERNATIVE INTERSECTIONS AND INTERCHANGES

Alternative intersections and interchanges offer the potential to improve safety and reduce delay 

at a lower cost and with fewer impacts than traditional solutions. However, transportation 

professionals are generally unfamiliar with many alternative intersection and interchange forms, 

partially because some forms have only a few installations in operation or because installations 

are concentrated in a few states. Furthermore, at the national level, well-documented and 

substantive resources needed for planning, analysis, design, and public outreach and education, 

were limited. 

Concurrent with this Diverging Diamond Interchange (DDI) Informational Guide, the Federal 

Highway Administration (FHWA) developed and published informational guides for three other 

alternative intersection forms: Median U-Turn (MUT), Restricted Crossing U-Turn (RCUT), and 

Displaced Left Turn (DLT). These guides are intended to increase awareness of these specific 

alternative intersections and interchanges and provide guidance on how to plan, design, 

construct, and operate them. These guides supply transportation professionals with summaries of 

the current state of knowledge, which can be used to support decisions when considering and 

potentially selecting alternative intersection and interchange forms for appropriate applications. 

INTERSECTION CONTROL EVALUATIONS AND CONSIDERATIONS

The term “intersection” means the junction of two or more street facilities. In some cases, this 

may specifically mean an “at-grade” intersection form. In others, it may include the junction of 

two or more streets requiring partial or complete grade separation (“interchanges”). A number of 

state and city transporation agencies have or are implementing intersection control evaluation 

processes or policies as a means of integrating the widest range of intersection forms as project 

solutions. For example, California, Indiana, Minnesota, and Wisconsin have policies or processes 

to objectively consider and select the most appropriate intersection form for a given project 

context. 

Many of the policies or processes include common objectives in selecting the optimal or 

preferred intersection control alternative for a given project context. The common elements 

generally include but are not limited to the following:

• Understanding the intended context, and how operations, safety, and geometry fit that 

context for each intersection or corridor including intended users (pedestrians, bicyclists, 

passenger cars, transit vehicles, freight, emergency responders, and over size/over weight 

[OSOW] vehicles)

• Identifying and documenting the overall corridor or intersection context including the 

built, natural, and community environment and the intended performance outcomes of the 

intersection form 

• Considering and assessing a wide range of traffic control strategies and other practical 

improvement concepts to identify worthy project-level technical evaluation
Diverging Diamond Interchange Informational Guide

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• Comparing engineering and economic analysis results of practical alternatives that 

consider implementation costs, performance benefits and impacts (safety, multimodal, 

operations, environment, etc.), and the estimated service life of alternatives

ORGANIZATION OF THE GUIDELINES

This guide has been structured to address the needs of a variety of readers, including the general 

public, policy makers, transportation planners, operations and safety analysts, and conceptual 

and detailed designers. This chapter distinguishes DDIs from conventional interchanges and 

provides an overview of each chapter in this guide. The remaining chapters in this guide increase 

in the level of detail provided. 

Chapter 2: Policy and Planning—This chapter provides guidance on when to consider 

alternative interchanges in general and DDIs in particular. The transportation professional should 

consider policies, project challenges, and performance measures, as well as the project 

development process throughout the duration of the project to balance trade-offs.

Chapter 3: Multimodal Considerations—This chapter provides an overview of multimodal 

facilities at DDIs and how various types of users can be safely integrated into the design. 

Guidance for pedestrian and bicycle facilities is also discussed in this chapter.

Chapter 4: Safety—This chapter summarizes the safety performance at DDIs based on studies 

completed by state agencies and recent research efforts. Although the documented safety 

performance of DDIs is limited, information about conflict points, wrong-way maneuvers, and 

emergency services at DDIs are discussed in this chapter.

Chapter 5: Operational Characteristics—This chapter provides information on the unique 

operational characteristics of DDIs and how they affect elements such as traffic signal phasing 

and coordination. The chapter also provides guidance for practitioners related to design elements 

such as driveways that may affect the operational performance of DDIs. It is intended to prepare 

readers for conducting operational analysis as described in Chapter 6.

Chapter 6: Operational Analysis—This chapter presents an overview of the approach and tools 

available for conducting a traffic operations analysis of a DDI. 

Chapter 7: Geometric Design—This chapter describes the typical DDI design approach and 

provides guidance for geometric features. It requires input from the chapters on multimodal 

considerations (Chapter 3), safety assessment (Chapter 4), and traffic operational analysis 

(Chapters 5 and 6). 

Chapter 8: Signal, Signing, Marking, and Lighting—This chapter presents information 

relating to the design and placement of traffic control devices at DDIs, including traffic signals, 

signs, pavement markings, and intersection lighting. 

Chapter 9: Construction and Maintenance—This chapter focuses on the constructability and 

maintenance of a DDI. 
Diverging Diamond Interchange Informational Guide

3 

An Appendix is included at the end of this guide for the purpose of providing more detailed 

information about many of the resources and best practices presented in the guide. The Appendix 

contains the following information: 

• A - Catalog of All Known Installations in the United States

• B - Supplemental Operational and Safety Details

• C - Marketing and Outreach Materials

• D - Supplemental Construction and Design Details 

SCOPE OF THE GUIDE

This document provides information and guidance on planning and designing a DDI for a variety 

of typical conditions commonly found in the United States. To the extent possible, the guide 

provides information on the wide array of potential users as it relates to the interchange form.

The scope of this guide is to provide general information, planning techniques, evaluation 

procedures for assessing safety and operational performance, design guidelines, and principles to 

be considered for selecting and designing a DDI. This guide does not include specific legal or 

policy requirements; however, Chapter 2 provides information on planning topics and 

considerations when investigating intersection control forms. This first edition of the Diverging 

Diamond Interchange Informational Guide has been developed from best practices and prior 

research. As more DDIs are built, the opportunity to conduct research that refines and develops 

better methods will result in improved future editions of this guide. 

DDI OVERVIEW

The diverging diamond interchange (DDI) is also known as a double crossover diamond (DCD) 

and is an alternative to the conventional diamond interchange or other alternative interchange 

forms. The primary difference between a DDI and a conventional diamond interchange is the 

design of directional crossovers on either side of the interchange. This eliminates the need for 

left-turning vehicles to cross the paths of approaching through vehicles. By shifting cross street 

traffic to the left side of the street between the signalized crossover intersections, vehicles on the 

crossroad making a left turn on to or off of ramps do not conflict with vehicles approaching from 

other directions. 

The DDI design has shown to improve the operations of turning movements to and from the 

freeway facility and significantly reduces the number of vehicle-to-vehicle conflict points 

compared to a conventional diamond interchange. The DDI also reduces the severity of conflicts, 

as conflicts between left-turning movements and the opposing through movement are eliminated. 

The remaining conflicts are reduced to merge conflicts for turning movements, and the reducedspeed crossover conflict of the two through movements. Chapter 4 provides additional discussion 

of these conflict points and DDI safety benefits. 

Exhibit 1-1 illustrates an example of a DDI and highlights the key features of this interchange 

design. 
Diverging Diamond Interchange Informational Guide

4 

Exhibit 1-1. Key characteristics of a DDI.

The street segment between the crossovers can be designed as an underpass or overpass 

depending on the site characteristics. The interchange design will be directly affected by whether 

or not the arterial passes over or under the limited access facility. In most cases, DDIs designed 

with a cross road as an overpass offer the most design flexibility in serving pedestrians. The 

majority of DDIs evaluated have reconstructed existing diamond interchanges, and the decision 

to go over or under the limited access facility had already been determined. 

APPLICATION

DDIs have been implemented in many different locations with a variety of design features. This 

section includes photos of several of these locations and some of the different environments and 

design features of the constructed DDI. Exhibit 1-2 shows the location of existing and planned 

DDIs in the United States, as of the publication of this guide. The Appendix documents 

installations of DDIs in locations in the United States.
Diverging Diamond Interchange Informational Guide

5 

Exhibit 1-2. Locations of DDIs.

Exhibits 1-3 to 1-6 show several of the DDIs recently constructed in the U.S. Exhibits 1-7 to 1-

10 show some of the unique features of a DDI such as the crossover location, overhead signing, 

pedestrian crossing location and markings, sidewalk location, and recessed lighting on the bridge 

for pedestrians. 
Diverging Diamond Interchange Informational Guide

6 

Exhibit 1-3. First constructed DDI in Utah at Pioneer Crossing and Interstate 15 

(American Fork, UT).

(1)
Diverging Diamond Interchange Informational Guide

7 

Exhibit 1-4. First constructed DDI in Georgia at Ashford Dunwoody Road near Perimeter 

Market.

(2)

Exhibit 1-5. First constructed DDI in Minnesota at Highway 15.(3)
Diverging Diamond Interchange Informational Guide

8 

Exhibit 1-6. First constructed DDI in Idaho at Interstate 86 and US 91/Yellowstone 

Highway (Pocatello-Chubbuck, ID).

(4)

Exhibit 1-7. Crossover location at the DDI located at SR 92 and Interstate 15 (Utah 

County, UT).

(5)
Diverging Diamond Interchange Informational Guide

9 

Exhibit 1-8. Overhead signing at the DDI located at SR 92 and Interstate 15 (Utah County, 

UT).

(5)

Exhibit 1-9. Pedestrian crossing at the right-turn lane of the exit located at Pioneer 

Crossing and Interstate 15 DDI (American Fork, UT).

(6)
Diverging Diamond Interchange Informational Guide

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Exhibit 1-10. Sidewalk and recessed lighting located on the bridge at the Pioneer Crossing 

and Interstate 15 DDI (American Fork, UT).

(6)

GEOMETRIC DESIGN CONSIDERATIONS

The fundamental design features of the DDI are the directional crossovers on either side of the 

interchange, which ultimately improve the operations of turning movements to and from the 

freeway facility. The geometric design necessary to allow for these crossover movements results 

in the use of reverse curvatures in advance of the interchange as vehicular traffic is directed to 

the right before it can cross to the left.

Several overarching principles guide users in conceptualizing and designing DDIs. The 

following principles may support DDI concept development, considering the context of the 

interchange and nearby adjacent intersections:

• Accommodate design vehicles at the crossover ramp terminal junctions

• Promote reduced and consistent design speeds through the interchange 

• Channelize inbound and outbound movements in the crossover design at each 

intersection to guide drivers to use the intended lanes and discourage wrong-way 

movements 

• Create a vehicle path alignment directing vehicles into appropriate receiving lanes

DDI concept design involves balancing and optimizing trade-offs associated with user 

performance, capacity, costs, maintenance, and construction staging, among other items. For 

instance, considering heavy vehicle design at the crossover may lead designers to contemplate 

larger design radii or wider lanes; however, this could promote higher speeds through the 
Diverging Diamond Interchange Informational Guide

11 

crossover for other vehicle types. Instead, to provide adequate facilities for the design vehicle 

while maintaining safe speeds for other motorists, designers may want to consider designs that 

offset one or more of the approaches to the DDI. This method may increase street alignment 

radii, resulting in comparatively narrower lanes to serve design vehicle over tracking. These and 

other tradeoffs of DDI geometric design are discussed in detail in Chapter 7. 

Exhibits 1-11 and 1-12 illustrate typical designs for an overpass and underpass at a DDI. 

Exhibit 1-11. DDI (overpass) at 500 East American Fork (American Fork, UT).

(1)

Exhibit 1-12. DDI (underpass) at Dorsett Road and Interstate 270 (St Louis, MO).

(7)
Diverging Diamond Interchange Informational Guide

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RESOURCE DOCUMENTS

This DDI guide is supplemental to major resource documents including but not limited to: 

• A Policy on Geometric Design of Highways and Streets (American Association of State 

Highway and Transportation Officials [AASHTO] Green Book)(8)

• Highway Capacity Manual (HCM)(9)

• Manual on Uniform Traffic Control Devices (MUTCD)(10)

• Highway Safety Manual (HSM)(11)

• Other research documents that appear in this guide and are more specialized to specific 

areas of the guide include various National Cooperative Highway Research Program 

(NCHRP) reports, Transportation Research Board (TRB) papers, and FHWA

publications 

The following supplemental resource documents related to the DDI are available: 

• FHWA Tech Brief on Double Crossover Diamonds (FHWA-HRT-09-054)(12)

• FHWA Alternative Intersection/Interchanges: Informational Report (AIIR) (FHWAHRT-09-060)(13)

• FHWA Project DTFH61‐10‐C‐00029: Operational and Safety Evaluation of Double 

Crossover Diamonds. Year 1 and Year 2 Project reports(14)

• FHWA Saxton Transportation Lab Project 13-002: HCM Chapters/Guidance for 

Alternative Intersections/Interchanges. New HCM methodologies for DDI

(15)

• MODOT Report: Missouri’s Experience with the Diverging Diamond Interchange(16)

• MODOT Diverging Diamond Interchange Performance Evaluation (I-44 & Route 13)(17)

• Kentucky Transportation Cabinet DDI Evaluation Study(18)

• FHWA Driver Evaluation of the Diverging Diamond Interchange (FHWA-HRT-07-

048)(19)
Diverging Diamond Interchange Informational Guide

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CHAPTER 2— POLICY AND PLANNING

This chapter contains guidance on how to consider alternative intersections and interchanges in 

general and a DDI in particular. This chapter summarizes policy and planning considerations 

related to a DDI. The remaining chapters of this guide will provide specific details of the 

multimodal, safety, operations, geometric design, and traffic control features of a DDI.

Alternative intersections are often initially considered for operational or safety needs, and other 

key factors may include spatial requirements and multimodal needs. This chapter provides 

approximate footprints for different types of DDIs to allow for planning-level screening and 

feasibility analysis. 

PLANNING CONSIDERATIONS FOR ALTERNATIVE INTERSECTIONS AND 

INTERCHANGES

Alternative intersection evaluations may vary depending on the stage of the project development 

process. Each project stage can affect how each of the policy and technical considerations is 

assessed. While the operational, design, safety, human factors, and signing controls should be 

considered at every stage of the development process, a planning-level design evaluation may 

not require the same level of analysis or detailed evaluation as projects in later development 

stages. Evaluations should be as comprehensive as needed to answer key project questions for 

each unique project context. 

Serving Pedestrians and Bicycles

A DDI offers an excellent opportunity to integrate multimodal facilities into an interchange. 

Almost all of the DDIs constructed to date included some combination of pedestrians, bicyclists, 

or transit facilities. 

The reduced number of signal phases can make it easier to serve non-motorized movements 

compared to a multi-phase signal. The two-phase DDI signal in most cases provides sufficient 

time per phase to serve pedestrians. Any pedestrian clearance phases are further minimized, as 

crossing distances are shortened to only cross one direction of traffic at a time. Through the 

separation and channelization of the two directions of vehicular traffic, pedestrians only have to 

interact with one direction of traffic at a time. This simplifies the pedestrian gap acceptance 

process and reduces the risk for pedestrian-vehicle conflicts provided pedestrians understand 

which direction traffic is coming from. 

The reduced crossing distances can also benefit bicyclists by reducing exposure time within the 

intersection (crossover) and minimizing the chance for vehicular conflicts. Some DDIs to date 

have been constructed with bicycle lanes through the crossovers, providing dedicated right-ofway for those road users. Several others have been constructed with bicycle facilities in the form 

of shared-use paths on the outside of the interchange. 

While there are many opportunities for multimodal accommodations at a DDI, these design 

elements are not without challenges. Chapter 3 of this guide discusses challenges and 
Diverging Diamond Interchange Informational Guide

14 

considerations, and provides recommendations for how to achieve safe and efficient provisions 

for multimodal users of a DDI. 

Traffic Volume Relationships

Exhibit 2-1 conceptually depicts the relationship of conventional intersections, alternative 

intersections, and grade separations in their ability to serve increasing traffic volumes. 

Exhibit 2-1. Relationship between volume and interchange type.
Diverging Diamond Interchange Informational Guide

15 

The DDI is an alternative to the conventional diamond interchange, as well as other interchange 

forms like a single-point interchange or a partial cloverleaf. The primary difference between a 

DDI and a conventional diamond interchange is the design of directional crossovers on either 

side of the interchange. This eliminates the need for left-turning vehicles to cross the paths of 

approaching through vehicles. Cross street traffic is shifted to the left side of the street between 

the signalized ramp intersections. Drivers on the cross street who are making a left turn onto the 

ramps are allowed to continue to the ramps without conflicting with opposing through traffic and 

without stopping. The DDI design has shown to improve the operations of turning movements to 

and from the freeway facility, as well as significantly reduce the number of vehicle-to-vehicle 

conflict points compared to a conventional diamond interchange. 

STAKEHOLDER OUTREACH

Similar to other transportation projects, stakeholder outreach is a critical part of the overall 

planning process. Successfully implementing the first DDI in a community may benefit from 

explicit and proactive outreach and education to affected stakeholders and the general public.

This would create opportunities to familiarize others with how the intersections work while 

creating opportunities to hear of general project and DDI specific issues and considerations. 

Special considerations may include minimizing the likelihood of a wrong-way maneuver into 

opposing traffic. The greater the crossing angle, the more the intersection will appear to intersect 

in a familiar manner. Public information and educational campaigns prior to opening a DDI 

intersection can help promote an understanding of unique features. Creating multiple forums to 

engage the public (including presentations at local council or board meetings, briefs at 

community organization functions, and project-specific open house meetings) results in 

opportunities to listen to community interests and share objective information about the 

interchange form.

Exhibit 2-2 and Exhibit 2-3 are two examples of using video animation to describe how to travel 

through a DDI. The video clip includes animation and narration to provide the general public 

with a clear message of how a DDI functions. Both videos were included on the Utah 

Department of Transportation (UDOT) and Nevada DOT project websites. Exhibit 2-4 is an 

example of a fact sheet of how to travel through a DDI with an emphasis on all users of the 

system. The fact sheet highlights how a pedestrian, bicyclist, and motorist would travel through a 

DDI in Minnesota. 
Diverging Diamond Interchange Informational Guide

Exhibit 2-2. Example video screen captures from UDOT of how to travel through a DDI.(1)

16 

Exhibit 2-3. Example video screen capture from Nevada DOT of how to travel through a 

DDI.

(20)
Diverging Diamond Interchange Informational Guide

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Exhibit 2-4. Fact sheet from Minnesota DOT on how various users travel through a DDI.(3)

Exhibit 2-5 is an example of a DDI explanation brochure used by Missouri Department of 

Transportation (MoDOT) for a DDI. Once the interchange is open to the public, monitoring 

driver behavior and using law enforcement as necessary to promote proper use of the new form 

can aid driver acclimation. 
Diverging Diamond Interchange Informational Guide

18 

Exhibit 2-5. Public outreach brochure used by MoDOT.

(7)

Exhibit 2-6 is an example of a DDI branding campaign used by Georgia Department of 

Transportation (GDOT) and local improvement district. The branding campaign included a 

website, a unique logo, and slogan titled, “Can You DDI? Arrive-Crossover-Drive.” This 

branding effort provides the public with an easy, identifiable look to this planned urban DDI. 
Diverging Diamond Interchange Informational Guide

19 

Exhibit 2-6. Example public outreach brochure used by GDOT.(2)

FHWA has created alternative intersection and interchange informational videos and video case 

studies, which can be viewed on the FHWA YouTube channel 

(https://www.youtube.com/user/USDOTFHWA).(21) In addition, FHWA has developed 

alternative intersection brochures that can be found on the FHWA website 

(http://safety.fhwa.dot.gov).(22) Examples of this information are shown in the appendix. 
Diverging Diamond Interchange Informational Guide

20 

POLICY CONSIDERATIONS

Designing, operating, and managing a street and its intersections should align with the 

appropriate jurisdictional policies associated with that facility. The facility location and type can 

often dictate the appropriateness of the right-of-way and access management needs associated 

with alternative intersections. The degree to which motor vehicle throughput should or should 

not be prioritized over other modes also plays a role in determining the appropriateness of 

alternative intersections at specific locations.

Some of the considerations that should be addressed before construction of a DDI include the 

following: 

• Access management considerations

• Operational measures of effectiveness

• Pedestrian facilities with access and wayfinding for persons with disabilities, including

the requirements of the Americans with Disabilities Act (ADA) and Section 504 (the 

Rehabilitation Act)(23)

• Bicycle facilities

• Managed lane scenarios, including ramp metering

• Snow removal and storage 

• Design vehicle

• Incident management

• Emergency response needs

Access Management

The subject of adjacent intersections has been one of the biggest concerns noted by practitioners 

building and operating DDIs as well as by academics who study the effects of DDIs on 

operations and safety. Adjacent intersections are problematic for many interchange 

configurations that may be considered as a replacement to conventional diamond interchanges; 

the specific issues relative to the DDI are discussed here.

From an operational perspective, the DDI’s efficient two-phase signals provide much higher 

throughput than nearby adjacent signals that often allow many more signal phases. This, 

combined with the close proximity of the adjacent intersection, causes limited queue storage and 

spillback into the DDI. To the motoring public, the DDI design will often appear to be a wasted 

effort when in fact the DDI is operating as intended. Safety concerns arise for motorist turning 

right from an exit ramp and weaving across traffic to make a left at the adjacent intersection, 

especially if right turn on red (RTOR) operations are allowed at the exit ramps.
Diverging Diamond Interchange Informational Guide

21 

Transportation agencies considering the DDI with nearby signalized intersections and congested 

cross roads have had to make geometric and signal design modifications to nearby intersections. 

Some potential geometric treatments that could improve operations and/or safety include: 

• Relocating an intersection to the next closest signalized intersection if nearby. This 

treatment was used at Dorsett Road in Maryland Heights, MO. 

• Using grade separation to eliminate one or more signal phases at the intersection. This 

treatment was used at National Avenue in Springfield, MO where a left turn into the 

hospital was modified to take a right, followed by another immediate right turn that

looped around and under the cross road using an underpass. 

• Alternative intersection designs could be used to reduce the number of necessary signal 

phases at adjacent intersections along the corridor. This treatment has yet to be used in 

practice; however, an entire corridor of alternative intersection designs is being designed 

along Poplar Tent Road in Charlotte, NC. 

PLANNING CONSIDERATIONS

Transportation professionals should address the following planning considerations when 

developing an alternative intersection design: 

• Community goals – Outside of formalized land use policies, cities and communities 

often have general goals that provide insights about the nature and character of their 

community. These goals can range from concepts that preserve a historic character or 

identified heritage. Some goals may be to create walkable communities or complete 

streets. Other goals can be to encourage economic development by preserving existing 

business or residential areas while encouraging thoughtful development. Regardless of 

the specific goals or vision, these considerations may influence street and intersection 

design. 

• Surrounding land uses and zoning – DDI intersections are well suited for suburban and 

urban environments. They are more challenging to implement on streets with nearby 

adjacent traffic signals or numerous driveways. 

• Project context – Key questions that help to identify key stakeholders for a particular 

project might include:

o What is the purpose and function of the existing or planned road facilities? 

o What are the existing and planned land uses adjacent to and in the vicinity of the 

road facilities?

o Who will likely desire to use the road facilities given the existing and planned 

land uses? 

o What are the existing and anticipated future socio-demographic characteristics of 

the populations adjacent to and in the vicinity of the existing or planned road 

facilities?
Diverging Diamond Interchange Informational Guide

22 

o What are the perceived or actual shortcomings of the existing road facilities? 

o Who has jurisdiction over the facility? 

o Where is capital funding for the project originating (or expected to originate)? 

o Who will operate and maintain the facility?

• Multimodal considerations – As with any street segment or intersection, each 

configuration must consider and serve the various users who currently or may be 

expected to use the facilities. This includes pedestrians and bicyclists and can also 

include users with special needs such as the visually impaired, elderly users, or young 

users.

• Access management – Access near a DDI needs to be restricted based on local, state, 

and federal requirements for intersection spacing. 

• Design vehicles – The interchange geometry will need to accommodate transit, 

emergency vehicles, freight, and potentially oversize and overweight (OSOW) vehicles.

PLANNING CHALLENGES

The following are several challenges associated with planning DDIs:

• Overpass or underpass – The decision to build over or under the limited access 

facility can have adverse effects on the operation and safety of various transportation 

modes. 

• Driver education – Successful implementations of DDIs are often preceded by 

public outreach and education campaigns, which are typically not conducted for 

conventional intersection improvements. 

• Driver expectation – DDIs relocate through and left-turn movements at the 

crossovers from their conventional location. This is different from what most drivers 

would expect and must be accounted for in the intersection planning and design.

• Multimodal accommodation – As with any street segment or intersection, each 

configuration must consider and serve the various users who currently or may be 

expected to use the facilities. This should always include pedestrians and bicycles, 

understanding that the exact provisions may necessarily vary from site to site. 

However, pedestrian facilities must always be made accessible. DDI intersections are 

generally compatible with transit as well. 

• Sufficient right-of-way – Right-of-way constraints may limit a designer’s ability to 

provide safe movement of vehicles through the crossover or limit the use of 

alternative design configurations. 
Diverging Diamond Interchange Informational Guide

23 

• Proximity to adjacent intersections – Nearby adjacent intersections have been 

found to hinder the ability of the DDI to process traffic as efficiently as it was 

intended. 

PROJECT PERFORMANCE CONSIDERATIONS

Measuring the effectiveness of a project’s overall performance depends on the nature or catalyst 

for the project. Understanding the intended specific operational, safety, and geometric 

performance context for each intersection or corridor including intended users can help 

determine project-specific performance measures. The project performance may be directly 

linked to the specific design choices and the specific performance of the alternatives considered. 

The project performance categories described below can influence and are influenced by the 

specific DDI design elements and their characteristics.(24) 

Accessibility

Chapter 3 of this guide describes accessibility as it relates to special consideration given to 

pedestrians with disabilities including accommodating pedestrians with vision or mobility 

impairments. However, for the purposes of considering a project’s general context and the 

performance considerations, the term “accessibility” goes beyond the conversation of policy 

related to ADA and Public Rights-of-Way Accessibility Guidelines (PROWAG) and is meant to 

be considered in broader terms.(23) With respect to considering applicable intersection forms for 

a given project context, accessibility is defined broadly as the ability to approach a desired 

destination or potential opportunity for activity using highways and streets (including the 

sidewalks and/or bicycle lanes provided within those rights-of-way). This could include the 

ability for a large design vehicle to navigate an intersection as much as it might pertain to the 

application of snow mobiles or equestrian uses in some environments or conditions. 

Mobility

Mobility is defined as the ability to move various users efficiently from one place to another 

using highways and streets. The term “mobility” can sometimes be associated with motorized 

vehicular movement and capacity. For the purposes of this guide, “mobility” is meant to be 

independent of any particular travel mode.

Quality of Service

Quality of service is defined as the perceived quality of travel by a road user. It is used in the 

2010 HCM to assess multimodal level of service (MMLOS) for motorists, pedestrians, bicyclists, 

and transit riders. Quality of service may also include the perceived quality of travel by design 

vehicle users such as truck or bus drivers. 

Reliability

Reliability is defined as the consistency of performance over a series of time periods (e.g., hourto-hour, day-to-day, year-to-year).
Diverging Diamond Interchange Informational Guide

24 

Safety

Safety is defined as the expected frequency and severity of crashes occurring on highways and 

streets. Expected crash frequencies and severities are often disaggregated by type, including 

whether or not a crash involves a non-motorized user or a specific vehicle type (e.g., heavy 

vehicle, transit vehicle, motorcycle). In cases where certain crash types or severities are small in 

number, as is often the case with pedestrian- or bicycle-involved, it may be necessary to review a 

longer period of time to gain a more accurate understanding. 

PROJECT DEVELOPMENT PROCESS

For the purposes of this report, the project development process is defined as consisting of the 

stages described below. Federal, state, and local agencies may have different names or other 

nomenclature with the overall intent of advancing from planning to implementation. Exhibit 2-7 

illustrates the overall project development process.

Exhibit 2-7. Project development process.

Planning Studies

Planning studies often include exercises such as problem identification and other similar steps to 

ensure there is a connection between the project purpose and need and the geometric concepts 

being considered. Planning studies could include limited geometric concepts on the general type 

or magnitude of project solutions to support programming. 

Alternatives Identification and Evaluation 

The project needs identified in prior planning studies inform concept identification, 

development, and evaluation. At this stage, it is critical to understand the project context and 

intended outcomes so potential solutions may be tailored to meet project needs within the 

opportunities and constraints of a given effort. FHWA describes context sensitive solutions as 

“... a collaborative, interdisciplinary approach that involves all stakeholders in providing a 

transportation facility that fits its setting.” (25) In considering the concept of “context sensitive 

design/solutions,” this stage calls for the meaningful and continuous stakeholder engagement to 

progress through the project development process. 
Diverging Diamond Interchange Informational Guide

25 

Preliminary Design

Concepts advancing from the previous stage are further refined and screened during preliminary 

design. For more complex, detailed, or impactful projects, the preliminary design (typically 30-

percent design level plans) and subsequent documentation are used to support more complex 

state or federal environmental clearance activities. The corresponding increased geometric 

design detail allows for refined technical evaluations and analyses that inform environmental 

clearance activities. Preliminary design builds upon geometric evaluations conducted as part of 

the previous stage (alternatives identification and evaluation). Some of the common components 

of preliminary design include: 

• Horizontal and vertical alignment design 

• Typical sections

• Grading plans 

• Structures

• Traffic/intelligent transportation systems (ITS)

• Signing and pavement markings 

• Illumination

• Utilities

Final Design

The design elements are advanced and refined in final design. Typical review periods include 60-

percent, 90-percent, and 100-percent plans before completing the final PS&E. During this stage, 

there is relatively little variation in design decisions as the plan advances to 100-percent. 

Functionally, in this stage of the project development process, the targeted performance 

measures have a lesser degree of influence on the form of the project.

Construction

Construction activities could include geometric design decisions related to temporary streets, 

connections, or conditions that facilitate construction. Project performance measures may relate 

to project context elements. 

SUMMARY OF DDI ADVANTAGES AND DISADVANTAGES

As described in Chapter 1 and the previous sections of this chapter, DDIs have unique features 

and characteristics, including multimodal considerations, safety performance, operations, 

geometric design, spatial requirements, constructability, and maintenance. 
Diverging Diamond Interchange Informational Guide

26 

Exhibit 2-8 provides an overview of the primary advantages and disadvantages of DDIs for

users, policy makers, designers, and planners should to understand when considering this type of 

alternative intersection form.
Diverging Diamond Interchange Informational Guide

27 

Exhibit 2-8. Summary of DDI advantages and disadvantages. 

Advantages Disadvantages

Non-motorized users

• Reduces conflicts between vehicles and pedestrians 

for most crossing movements

• Opportunity to safely accommodate pedestrians 

and bicyclists through interchange

• Creates shorter pedestrian crossing distance for 

some movements 

• Opportunity for bicycle lanes and multi-use paths 

through additional right-of way (no left turn lanes 

needed)

• Provides two-stage crossing opportunities

• Pedestrians may have to cross unsignalized, 

channelized right and left turns onto freeway

• Pedestrians may be unaware what direction traffic is 

coming from

• Center walkway may be unfamiliar to pedestrians

Safety

• Reduced conflict points over other interchange 

forms

• Reduction from 10 to 2 crossing conflicts 

compared to standard diamond

• May have potential for wrong-way maneuvers at 

crossovers 

• Unusual sight distance considerations at crossovers 

and ramp movements

Operations

• Two-phase signals reduce lost time at interchange 

and increase capacity

• Left-turns onto freeway may be free-flowing 

• Ability to coordinate through traffic or left turns 

from freeway 

• Potential for significant delay and travel time 

savings over standard diamond

• Significantly reduced queue spillback potential, 

especially between ramp terminals

• Potential lower right-turn capacity from freeway 

where RTOR not allowed

• Challenging to coordinate through traffic in both 

directions

• Increased operational challenges at adjacent 

intersections due to crossover, two-phase operation at 

DDI

• Potential driver unfamiliarity with crossover design 

and merges from left

Access Management

• Provide full access control through interchange • Does not allow exit ramp to entrance ramp 

movement

• May require access control beyond interchange to 

prevent weaving maneuvers

• May require relocating or removal of adjacent 

streets/driveways to accommodate crossover and 

reverse curves

Traffic Calming

• Reduced speed through the interchange from 

crossover geometry and reverse curves

• Turns onto the freeway may have high speeds due to 

lack of signal control

Space

• Generally fits within existing interchange right-ofway and bridge structure

• Lower footprint compared to parclo interchanges 

• Some additional ROW width may be needed to 

accommodate crossovers at tight diamonds due to 

reverse curves

Construction and Maintenance

• Generally fits on or under existing bridge structure

• Low-cost design compared to other interchange 

forms

• Less queuing on the arterial may reduce pavement 

rutting and wear

• May require additional lighting due to unique 

geometry

• Additional signage and pavement markings are 

needed beyond the levels of a conventional diamond 

interchange

• More complex signal design requires early 

consideration in design
Diverging Diamond Interchange Informational Guide

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CHAPTER 3— MULTIMODAL CONSIDERATIONS

This chapter presents guidance for accommodating pedestrians, bicyclists, transit, and heavy 

vehicles at a DDI. While many existing interchanges do not have accommodations for 

pedestrians, bicyclists, or transit, almost all of the DDIs constructed to date included some 

combination of such facilities. The overall objective is to develop a design, regardless of the type 

of intersection, compatible with a Complete Street. A Complete Street is a facility that serves 

many types of users including freight, transit, and non-motorized users. This chapter discusses 

both the challenges and benefits of DDIs as an alternative intersection choice that is safe and 

efficient for multimodal users.

Multimodal benefits of DDIs include:

• Reduced overall right-of-way footprint compared to a conventional diamond interchange 

• Two-phase traffic signal control with reduced pedestrian wait time

• Minimized crossing distances 

• Simplification of conflicts to one-directional vehicular traffic

• Opportunities for bicycle lanes and multiuse paths through the interchange 

Some of the challenges of multimodal provisions at DDIs include: 

• Altered travel paths with travel in the center of the interchange between vehicular lanes

• Traffic approaching from unexpected directions 

• Unfamiliar signal phases

• Uncontrolled crossing of turn lanes 

The reduced number of signal phases can make it easier to serve non-motorized movements 

compared to a multi-phase signal. At multi-phase signals, the need to provide adequate 

pedestrian clearance may result in the pedestrian movement controlling the phase lengths, 

leading to longer cycle lengths and greater pedestrian delay. In contrast, vehicle movements 

typically control phase length at two-phase DDI signals, which most often provide sufficient 

time per phase to also serve pedestrians. Although pedestrian crossings at the crossovers are 

signalized, pedestrian crossings of the turn lanes to and from the freeway may not be signalized. 

These potentially uncontrolled crossing locations require special attention and consideration to 

assure pedestrian safety. 

A DDI avoids the need for left-turn pockets within the interchange, which frees up right-of-way 

compared to a conventional diamond. This right-of-way can be used for multimodal facilities in 

the form of sidewalks, bicycle lanes, or even transit facilities. 
Diverging Diamond Interchange Informational Guide

30 

At the crossover pedestrian signals, pedestrian clearance phases are generally short as crossing 

distances are shortened to only cross one direction of traffic at a time. At the same time, pretimed DDI signals can provide for extended pedestrian walk phase times, which can reduce 

pedestrian delay, and can provide added time for pedestrians with disabilities. The reduced 

crossing distances can also benefit bicyclists, who have a reduced exposure time within the 

crossover intersection, thereby minimizing the chance for vehicular conflicts. 

Finally, through the separation and channelization of the two directions of vehicular traffic, 

pedestrians interact with one direction of traffic at a time. This simplifies the pedestrian gap 

acceptance process and reduces the risk for pedestrian–vehicle conflicts, provided pedestrians 

understand from which direction traffic is coming from at a given crossing. 

DESIGN PRINCIPLES AND APPROACH

The fundamental design features of the DDI are the ramp terminal intersection directional 

crossovers serving movements to and from the freeway facility. The geometric design necessary 

to allow for these crossover movements can result in reverse curvature in advance of the 

crossover. These “bulb-outs” can result in a locally wider right-of-way footprint and large 

channelization islands that provide opportunities for providing pedestrian and bicycle facilities. 

Similarly, the two directions of through traffic within the interchange are often separated by a 

median (again a function of the bulb-outs needed for the crossover), that could be used for a 

share-use pathway between the streets. These features result in several options for locating

pedestrian and bicycle facilities, as illustrated in the schematic in Exhibit 3-1. 

Exhibit 3-1. Schematic of DDI right-of-way availability for multimodal facilities. 
Diverging Diamond Interchange Informational Guide

31 

Exhibit 3-1 illustrates possible locations for multimodal facilities in the center of the interchange

or outside of the traffic lanes. Multimodal facilities on the outside are common for most 

transportation facilities and interchanges, but the DDI also provides room in the center between 

the two through movements. Large channelization islands between the crossovers, bulb-outs, and 

channelized turn lanes provide additional room to locate facilities. The right-of-way can also be 

used to add bicycle lanes adjacent to the travel lanes. 

Anticipating Multimodal Needs, Behavior, and Patterns 

A fundamental challenge in developing any new intersection or interchange form is deciding 

how to best provide for pedestrian and bicycle movements, and anticipating the desire lines

between different origins and destinations for these modes (e.g., how they travel through the 

intersection or interchange). Forecast volumes for non-motorized users are rarely available, and 

if they are, they typically do not capture travel patterns within the intersection or interchange. 

However, the majority of DDIs constructed to date feature pedestrian and bicycle facilities. For 

many retrofit sites, the existing pedestrian and bicycle facilities were improved with construction 

of the DDI. For example, DDIs at MO 13 in Springfield, MO; Dorsett Road in Maryland 

Heights, MO; and Harrodsburg Road in Lexington, KY all added shared-use paths through the 

interchange (see Exhibits 3-2, 3-3, and 3-4).

At all three of these sites, the construction of multimodal facilities was a priority for agencies, 

garnering positive feedback from local residents and users of the facility. At interchanges, land 

use development can sometimes lag interchange construction, resulting in pedestrian and bicycle 

needs to not be apparent from opening day. But early consideration and provision for pedestrian 

and bicycle movements should be a priority consideration for any DDI, and should be accounted 

for in even early design concepts. 

Exhibit 3-2. Center walkway at MO 13 (Springfield, MO).(14)
Diverging Diamond Interchange Informational Guide

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Exhibit 3-3. Outside walkway at Dorsett Road (Maryland Heights, MO).(14)

Exhibit 3-4. Shared-use path on outside at Harrodsburg Road (Lexington, KY).(14)

This chapter describes the unique characteristics of the four primary non-auto modes 

(pedestrians, bicyclists, transit, and heavy vehicles) that should be considered when analyzing 

and designing DDIs. The transportation professional needs to work to identify and understand 

the needs of these various users in order to produce a balanced design that serves them all. 

PEDESTRIANS

The inclusion of pedestrian facilities should receive attention and consideration throughout the 

design process. This section describes DDI design features, considerations, and trade-offs 

relating to the pedestrian mode. 

Pedestrian facilities should be planned, designed, and constructed to emphasize pedestrian 

convenience and safety, which is achieved through appropriately sized sidewalks, vertical and 

horizontal separation from adjacent travel lanes, minimized pedestrian crossing distances, clearly 

defined pedestrian paths, adequate time to cross, and low vehicular speeds. Landscaping and 

other aesthetic treatments can contribute to a positive pedestrian experience.
Diverging Diamond Interchange Informational Guide

33 

Pedestrians Outside Versus Center

Pedestrian facilities can be provided as walkways outside the vehicular through travel way 

(standard for most diamond interchanges with pedestrian facilities) or as a walkway in the 

median, between the vehicular directions of travel. For an overpass DDI, pedestrian facilities in 

the center of the interchange (within the median) may be recommended to minimize conflicts 

with left-turning traffic to and from the freeway and to allow crossing the interchange in all 

directions i.e., travel along the arterial and crossing the arterial from one side to the other). For 

underpass DDIs, pedestrian walkways may need to be located on the outside to avoid conflicts 

with bridge columns placed between the two directions of vehicular traffic. 

A concern for outside pedestrian facilities is the lack of marked crosswalks to allow pedestrians 

to cross the arterial street at the DDI. While this has been the practice at many DDIs with outside 

facilities to date, it is not desirable as it requires pedestrians to travel to the next adjacent signal 

to cross the arterial. As many DDIs are associated with access management treatments and relocated adjacent intersections, the additional travel distance for pedestrians to the next crossing 

opportunity may be large, and may result in frequent non-compliance and jaywalking by 

pedestrians. It should be expected that pedestrians will need and want to cross the arterial, and 

the presence of transit facilities or certain types of development will only make this more likely. 

It is strongly recommended that all pedestrian movements be provided for at any intersection, 

including a DDI interchange. 

Exhibit 3-5 gives an example of a pedestrian facility located in the center of the DDI (MO 13 in 

Springfield, MO). Example 3-6 gives an example of an outside pedestrian walkway (Dorsett 

Road in Maryland Heights, MO). The center facility is at an arterial overpass, while the outside 

facility is at an arterial underpass. In general, both configurations are feasible for both arterial 

overpasses and underpasses, although with some challenges as discussed below. The exhibits

further highlight which crossings have pedestrian signals, and which are unsignalized crossings. 

For pedestrian crossings without signals, vehicle movements should be controlled to assure 

pedestrian safety and comfort by controlling vehicle speeds, and supplementing the crossing with 

pedestrian safety enhancements including a raised crosswalk, high-visibility markings, or 

rectangular rapid-flashing beacons (RRFB). In some cases, a pedestrian signal may be needed to 

assure driver compliance with the pedestrian crossing. Higher-speed and free-flowing vehicle 

turning movements should only be used where pedestrian facilities are not present. For multilane turns with pedestrian crossings, pedestrian signals are required to make the crossing 

accessible to pedestrians with disabilities. Additional information related to special consideration 

given to pedestrians with disabilities including accommodating pedestrians with vision or 

mobility impairments can be found in the policies related to ADA and Public Rights-of-Way 

Accessibility Guidelines (PROWAG).(23) 
Diverging Diamond Interchange Informational Guide

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Exhibit 3-5. Pedestrian facilities in center of DDI (Springfield, MO).

(26)

In the case of the overpass with center walkways in Exhibit 3-5, traffic signals are needed to 

control vehicle crossover movements. Pedestrian facilities to cross into the center can be colocated with these vehicle signals, and a pedestrian crossing phase provided with the concurrent 

vehicle phase. The right turns are shown as unsignalized pedestrian crossings and should be 

configured in a way to promote low vehicle speeds, good sight distance to the crosswalk, and 

high driver yielding behavior. Additional treatments are available to further promote yielding. In 

general, the center walkway needs to be separated by a barrier wall for pedestrian comfort and 

safety. The center walkway should not be extended to adjacent intersections outside the 

interchange. 

In the case of the underpass with outside walkways in Exhibit 3-6, pedestrians in each direction 

cross four vehicle turn lanes with eight total pedestrian crossings. These can all be unsignalized, 

but may be signalized for pedestrian safety or enhanced with other treatments that may improve 

yielding. In this example the right-turn off the freeway is signalized, and the right-turn to the 

freeway is unsignalized. Notice that in this case, no pedestrian crossing of the arterial street is 

provided, which is undesirable. An example of pedestrian-focused design of a DDI sidewalk 

along the perimeter that also allows for an arterial crossing is discussed in a later section. 
Diverging Diamond Interchange Informational Guide

35 

Exhibit 3-6. Pedestrian facilities on outside of DDI (Maryland Heights, MO).(26)

The advantages and challenges of center and outside pedestrian facilities are summarized in 

Exhibits 3-7 and 3-8, respectively.
Diverging Diamond Interchange Informational Guide

36 

Exhibit 3-7. Center walkway pedestrian safety and comfort. 

Advantages Challenges

Street

Crossings

Crossing of the arterial street provided at 

DDI for full pedestrian access

Crossing of free-flow right-turn 

movements to/from freeway

Crossing one direction of traffic at a time Pedestrians may not know to look to the 

right when crossing to center 

Short crossing distances Wait at center island dictated by length of 

signal phase for through traffic 

No exposure to free-flowing left turns to 

freeway

Location of pedestrian signals can 

conflict with vehicular signals at 

crossovers

Protected signalized crossing to walkway

Pedestrian clearance time generally provided 

in crossover signal phasing 

Pedestrian delay to center minimized by 

short cycles at two-phase signals 

Walkway 

Facility

Side walls provide a positive barrier between 

vehicular movements and pedestrians 

Center walkway placement counter to 

typical hierarchy of street design

Walls low enough to avoid “tunnel” effect 

that could impact pedestrian comfort 

Potential discomfort from moving 

vehicles on both sides of walkway

Recessed lighting can provide good 

illumination of walkway Sign and signal control clutter
Diverging Diamond Interchange Informational Guide

37 

Exhibit 3-8. Outside path/sidewalk pedestrian safety and comfort.

Advantages Challenges

Street

Crossings

Crossing one direction of traffic at a time Crossing of free-flow right-turn 

movements to/from freeway

Ramp crossing distances are often shorter 

than through traffic crossing distance due to 

fewer travel lanes

Conflict with free-flow left turns to 

freeway, where fast vehicle speeds are 

likely (acceleration to freeway)

 

Crossing of the arterial street sometimes 

not provided at DDI 

Potential sight obstruction of pedestrian 

crossing left turns from behind barrier wall

Pedestrians may not know which direction 

to look in, when crossing turn lanes 

Unnatural to look behind to check for 

vehicles before crossing when traveling out 

of the DDI (depends on angle of approach 

and direction of travel) 

Signalized crossings require more 

complicated timing

Walkway 

Facility

Extension of existing pedestrian network 

(natural placement on outside of travel 

lanes)

Need for widened structure on outside for 

overpass

Pedestrian typically has view of path ahead 

(depends on sight lines and obstructions)

Potential for additional right-of-way for 

underpass or construction of retaining wall 

under bridge

Walkway doesn't conflict with center 

bridge piers (at underpass) Need for additional lighting for underpass 

Opportunity to use right-of-way outside of 

bridge piers (at underpass) 

Pedestrian-Focused DDI Design

In general, pedestrian safety and comfort can be enhanced by reducing vehicle speeds, improving 

sight distances between drivers and pedestrians, and appropriately locating the crosswalks. At 

DDIs, the required channelized right and left-turn lanes to and from the freeway presents an 

opportunity for pedestrian-focused designs through the use of reduced curve radii and other 

geometric changes. Exhibits 3-9 and 3-10 illustrate this concept for a DDI with center walkway 

and a DDI with outside facilities, respectively. 
Diverging Diamond Interchange Informational Guide

38 

Exhibit 3-9. Pedestrian-focused DDI – center walkway. 
Diverging Diamond Interchange Informational Guide

39 

Exhibit 3-10. Pedestrian-focused DDI – outside walkway. 

The key concepts underlying pedestrian-focused design of DDIs include four primary principles: 

• Tighten vehicle curve radii to reduce speeds at the crosswalk. Lower vehicle speeds have 

been linked in research to increased driver yielding rates, as well as lower risk of serious 

injury or death for the pedestrian in the case of a crash. 

• Provide adequate sight distance for vehicle approaches to crosswalks by locating the 

crosswalk in the tangent portion of the approach, or in the beginning portions of the 

curve. A crosswalk located in the middle of a large swooping turn is difficult for drivers 

to see and react to. Improved vehicle sight distance also provides enhanced pedestrian 

sight distance to make adequate gap crossing decisions at unsignalized crossings. 

• Provide one vehicle length of storage downstream of the crosswalks for yield-controlled 

vehicle movements. Similar to the crosswalk placement at roundabouts, this separates the 

driver decision points of yielding to pedestrians at the crosswalk and screening for gaps at 

the yield sign. It also prevents drivers waiting at the yield line from blocking the 

crosswalk with their vehicle. 

• Locate crosswalks behind the stop bar for signalized vehicle turns, consistent with driver 

and pedestrian expectations at signalized intersections. 
Diverging Diamond Interchange Informational Guide

40 

These configurations apply equally to DDIs with center walkway or outside pedestrian facilities. 

One key difference between the two is that the turn radius for the left-turns to and from freeways 

can be selected independent of pedestrian considerations for the center walkway, because 

pedestrians do not cross these turn movements with a center walkway DDI configuration. 

Conflict Points

Anywhere a pedestrian walkway crosses vehicular travel lanes, a pedestrian–vehicle conflict 

point exists. The number and type of conflict points for a DDI with center and outside walkways

are illustrated in Exhibits 3-11 and 3-12, respectively. Note that a center walkway inherently 

enables pedestrians to cross from one side of the arterial to the other. For outside walkways an 

opportunity to cross the arterial street should be provided, although some existing DDIs did not 

provide crossings of the arterial. In those cases crossings of the arterial would have to take place 

at an upstream or downstream intersection, which is undesirable. The exhibits distinguish 

between conflict points where vehicles are stopped at signals or decelerating, and those where 

vehicles are more likely to be free-flowing or accelerating. Free-flow vehicles may also be 

present at the first type of conflict points (for example when the vehicle signal is green), but 

there are at least some intervals in the signal cycle where traffic is stopped at those locations. For 

the free-flow or accelerating conflict points, traffic rarely has to stop or slow down, other than 

when drivers yield to pedestrians. 

Exhibit 3-11. Pedestrian-vehicle conflict points at DDI with center walkway. 
Diverging Diamond Interchange Informational Guide

41 

Exhibit 3-12. Pedestrian-vehicle conflict points at DDI with outside walkways.

Exhibit 3-11 shows a total of eight pedestrian-vehicle conflict points for the center walkway. 

Two of these conflict points are free-flowing or accelerating (right turns to the freeway), while 

the other six conflict points are stopped or decelerating, with traffic having to stop at regular 

intervals. For the outside walkway, a total of twelve conflict points exist if pedestrian facilities 

are also provided to cross the arterial street. Four of the conflict points are accelerating and the 

remaining eight are decelerating. The two additional accelerating conflict points, compared to a 

center walkway, are for the free-flowing left-turn movements onto the freeway. These 

movements require designers to use extra care and attention to assure a safe pedestrian crossing, 

as discussed below. 

Note that the diagrams depict a single lane in each direction of travel, while DDIs are typically 

constructed at interchanges were the arterial has multiple through lanes in each direction. In 

practice, a mainline crossing will likely have more conflict points than a channelized turn lane 

crossing. 

For comparison, Exhibit 3-13 shows pedestrian-vehicle conflict points for a typical diamond 

interchange with channelized right turns. Exhibit 3-14 shows the pedestrian-vehicle conflict 

points for a typical diamond interchange with pedestrian crossing of the arterial. The diamond 

interchange has a total of eight conflict points for pedestrians with four decelerating and four 

accelerating points. When a crossing of the arterial is provided, an additional four decelerating 

conflict points are created. 
Diverging Diamond Interchange Informational Guide

42 

Exhibit 3-13. Pedestrian-vehicle conflict points at conventional diamond. 

Exhibit 3-14. Pedestrian-vehicle conflict points at conventional diamond with pedestrian 

crossing of arterial.

In summary, a DDI with center walkways has six stopped or decelerating and two free-flow or 

accelerating conflict points, while the DDI with outside walkways has eight stopped and four 

free-flow conflicts. Overall, a DDI with center walkway minimizes the overall number of 

conflict points, including accelerating conflicts, while providing full access of pedestrians to also 

cross the arterial street. 
Diverging Diamond Interchange Informational Guide

43 

Exhibit 3-15. Pedestrian-vehicle conflict point comparison.

Type of conflict

DDI with center 

walkway

DDI with 

outside 

walkways

Conventional 

diamond

Conventional 

diamond with 

pedestrian crossing

Free-flow 2 4 4 4

Stopped 6 8 4 8

Total 8 12 8 12

Free-Flow Left Turn Conflict

If the decision is made to place pedestrian facilities on the outside, the designer should pay close 

attention to placement, visibility, and vehicular speeds for the pedestrian crossing at the freeflowing left turn. Exhibit 3-16 shows one example of such a left-turn crossing. The image shows 

an exclusive left-turn lane approaching the crosswalk, with cars accelerating toward the freeway 

entrance ramp. The exhibit further shows potential visibility limitations for pedestrians crossing 

from the middle of the DDI. The waiting area is obstructed by a shadow in the photo, but even at 

other times of day the line of sight between the waiting position and the approaching truck in the 

left-turn lane is partially obstructed by the barrier wall on the bridge structure. Free-flowing 

vehicle movements, elevated speeds, acceleration, and insufficient sight distance can contribute 

to low yielding and an increased chance of conflicts at these crossing locations. 

Exhibit 3-16. Example of pedestrian crossing at free-flow left onto freeway.

(6)

To overcome the visibility and sight distance challenges, several potential treatments could be 

considered, including: 

• Revising the left-turn geometry towards a pedestrian-focused DDI design with reduced 

turn radii, reduced vehicle speeds, and improved sight distances as described above 
Diverging Diamond Interchange Informational Guide

44 

• Relocating the crosswalk further upstream in the turn-lane for improved sight distances 

• Adding raised crosswalks or other geometric modifications to control vehicular speeds in 

the vicinity of the crosswalk 

• Installing rectangular rapid-flashing beacons (RRFBs) or other pedestrian-activated 

devices to alert drivers of the presence and crossing intent of a pedestrian

• Providing a pedestrian-activated signal to supply a crossing opportunity with a steady red 

phase for vehicular traffic (an example of this treatment is shown in Chapter 8) 

While this discussion has focused on the channelized left-turn to the freeway, similar 

considerations should be applied to channelized right-turn lanes to and from the freeway, and the 

channelized left turn from the freeway (if yield-controlled). 

Another key consideration for the left-turn crossing is that pedestrians may not be aware of the 

direction from which conflicting traffic is coming. While this is also true for the crossings of the 

mainline at the center walkway, it is a particular consideration for the left- turn crossings, as they 

are typically unsignalized. Some agencies have provided special pedestrian signage or pavement 

marking to guide the pedestrian, as shown in Exhibit 3-17. APS devices can provide this 

guidance with a speech message.

Exhibit 3-17. Pedestrian markings to indicate directionality of traffic (Maryland Heights, 

MO).

(14)

This type of information should further be provided in auditory fashion to provide equal access 

to pedestrians with vision disabilities. A speech message recording saying for example “traffic 

coming from your left” needs to be added to an accessible pedestrian signal at signalized 

crossing, or to an audible push-button device at unsignalized crossings. Accessibility 

considerations of DDIs are discussed further below and in Chapter 8. 
Diverging Diamond Interchange Informational Guide

45 

Signalized and Unsignalized Pedestrian Crossings

Pedestrian crossings at a DDI can be signalized or unsignalized. For center walkways, the 

crossings from the channelization island to the center walkway are signalized, while the 

crossings from the island across the right-turn lanes are often unsignalized. Special attention to 

pedestrian visibility, sight distances, and vehicular speed control should be given to these 

channelized right-turn lanes. These considerations are not unique to a DDI and are similar in 

principle to channelized turn lanes at signalized intersections. For outside walkways, both the 

right-turn and left-turn channelized movements are crossed, and both may be unsignalized. 

Special considerations for the channelized left turns were given previously. 

The visibility of the pedestrian signals should be explored in the early stages of the design phase 

of the DDI, especially in the case of the narrow “noses” at the ends of the center walkway. Sight 

lines to and from the center walkway can further be obstructed by vehicles using the crossovers, 

as well as by poles and signage located in the median. The actual pedestrian signal may not be 

visible when approaching the crossing in the center walkway, and it may be unclear what the 

intended crossing direction is. Signal pole placement and orientation should be screened and 

selected carefully to assure that pedestrians can intuitively find and understand the intention of 

these traffic control devices. An accessible pedestrian signal with locator tone is required to help 

pedestrians with vision impairments locate the push-button. 

Pedestrian Channelization and Wayfinding

Because the DDI crossover is unusual for drivers, human factors considerations are emphasized 

throughout the design process. Human factors considerations also apply to the pedestrian 

environment, which is different from what pedestrians are used to at conventional interchanges. 

With center walkways, crossing to the center of the street is unusual compared to the typical 

hierarchy of street design, in which pedestrian facilities are typically placed outside of vehicular 

traffic. This hierarchy is satisfied for outside walkways, but pedestrian discomfort can arise as 

pedestrians have to cross a freeway bridge or walk through an underpass adjacent to vehicular 

traffic. Depending on the design vehicle and speed of the DDI, the channelization islands 

separating the right- and left-turning movements can be quite large, and pedestrians need clear 

guidance and information on where they should and should not walk, and where they should and 

should not cross. 

Cut-through island designs can be used to provide positive guidance to pedestrians as to where 

walkway and crossing locations are provided. A cut-through walkway can guide the pedestrian 

directly to the intended crossing point and can be angled to support pedestrians in viewing 

oncoming vehicular traffic and potential conflicts. The channelization islands at DDIs provide 

the opportunity for wide walkways. The cut-through walkway should be at least eight feet wide 

to comfortably accommodate pedestrians, including those with wheelchairs and other mobility 

devices. The actual curb ramp landing should be aligned perpendicular to the street centerline, 

which minimizes crossing distance and orients pedestrians to access ramps. 

As an alternative to the cut-through design, landscaping (grass or gravel) can be used to define 

the boundaries of the pedestrian walkway and give pedestrians a sense of place in what can be 
Diverging Diamond Interchange Informational Guide

46 

very large channelization islands. Examples of cut-through and landscaping designs are provided 

in Exhibits 3-18 and 3-19, respectively. 

Exhibit 3-18. Channelization toward center crosswalk.

(26)

Exhibit 3-19. Channelization toward outside crosswalk.(26)

ADA and PROWAG Accessibility Considerations

Accessibility was previous described in Chapter 2 in the broader contexts of considering a 

project’s contextual environment and the ability for various users to approach a desired 

destination or potential opportunity for activity using highways and streets (including the 

sidewalks and/or bicycle lanes provided within those rights-of-way). In this section, accessibility 

is explicitly focused on the policies related to ADA and Public Rights-of-Way Accessibility 

Guidelines (PROWAG).(23) Special consideration should be given to pedestrians with disabilities 

including accommodating pedestrians with vision or mobility impairments. Being relatively new, 
Diverging Diamond Interchange Informational Guide

47 

specific guidance for “Accessible DDIs” is not yet available. However, general accessibility 

principles can be borrowed from other forms of intersections and applied here. The United States 

Access Board provides many additional resources on accessibility and specific requirements for 

Accessible Public Rights of Way, which the transportation professional should refer to and be 

familiar with.(23)

The basic principles for accessible design can be divided into the pedestrian walkway and the 

pedestrian crossing location. For the pedestrian walkways, the following considerations apply: 

• Delineate the walkway through landscaping, curbing, or fencing to assist with wayfinding 

for blind pedestrians. Example of such wayfinding provisions are shown in Exhibits 3-20 

and 3-21. Note the use of fencing under the bridge structure where landscaping is more 

difficult to maintain. 

• Provide sufficient space (length and width) and recommended slope rates for wheelchair 

users and other non-motorized users such as people pushing strollers, walking bicycles, 

and others 

• Construct an appropriate landing with flat slope and sufficient size at crossing points. 

Exhibit 3-20. Example of pedestrian wayfinding provision at DDI via curbing.

(26)
Diverging Diamond Interchange Informational Guide

Exhibit 3-21. Example of pedestrian wayfinding provision at DDI using an urban fence.

(26)

48 

For pedestrian crossing locations, these additional considerations apply: 

• Provide curb ramps and detectable warning surfaces at the end of curb ramp and 

transition to the street

• Provide audible speech message to communicate directionality of traffic (from left or 

from right) at all crossing points. Audible speech messages should be used where spacing 

between APS devices is less than 10 feet or where additional narrative for the expected 

direction of traffic is needed (from left or from right; see discussion in Chapter 8)

• Provide accessible pedestrian signals with pushbutton locator tone at signalized crossings 

• Locate push-buttons to be accessible by wheelchairs and adjacent to the crossing at a 

minimum separation of 10 feet 

• Align the curb ramp landing to the intended crossing direction 

• Crosswalk width through the intersection should be wide enough to permit pedestrian and 

wheelchairs to pass without delay from opposing directions, and the medians should 

provide sufficient storage for all non-motorized users to safely wait when 2 stage 

crossing are required.

• Pedestrians with vision, mobility, or cognitive impairments may benefit from targeted 

outreach and additional informational material created with these specific users in mind. 

These outreach materials include information on crosswalk placement and intended 

behavior, as well as answers to frequently asked questions. For blind pedestrians, 

materials need to be presented in an accessible format with sufficient description of all 

features of the DDI crossing. 
Diverging Diamond Interchange Informational Guide

49 

BICYCLISTS 

Bicyclists at DDIs may be commuters or recreational users. While the latter category depends to 

some extent on adjacent land uses and the presence of bicycle facilities, bicycle commuters are 

commonplace in many locations and should be explicitly considered in the design of most DDIs. 

Bicycle-friendly design should focus on minimizing bicycle–vehicle conflicts, providing 

adequate lateral space between vehicles and bicycles, minimizing the speed differential between 

bicycles and vehicles, and managing any bicycle–pedestrian conflicts. 

Options for Bicycle Accommodation at DDIs 

Three basic options exist for accommodating bicyclists at a DDI. These options include 

providing: 

1. A marked bicycle lane through the DDI. For this option, pavement markings can 

reinforce to drivers that the bicycle lane is not a shoulder (Exhibit 3-22) 

2. A separated bicycle path or shared-use path (Exhibit 3-23) 

3. Shared-lane or on-street bicycle accommodations, which would mean that bicyclists

would use the vehicular travel lane. For this option, sharrow markings can reinforce to 

driers that bicyclists are legal road users(Exhibit 3-24) 

Each of these options may be viable in certain locations; the choice depends to a large extent on 

the expected use of the facility by bicyclists, the expected behavior of these bicyclists (commuter 

versus recreational), and the available right-of-way at the interchange. Without a bicycle lane, 

experienced bicyclists are likely to use the vehicular travel lanes (see Exhibit 3-24) while 

recreational bicyclists are more likely to use the sidewalk. But this may also lead to the 

interchange becoming or being perceived as an obstacle that would keep cyclists from using the 

facility. 
Diverging Diamond Interchange Informational Guide

Exhibit 3-22. Bicycle lane maintained through DDI between crossovers.(20)

50 

Exhibit 3-23. Bicycle provisions on shared-use path.

(14)
Diverging Diamond Interchange Informational Guide

51 

Exhibit 3-24. Bicyclist in travel lane at DDI crossover.

(14)

Conflict Points

The conflict points for a bicycle using the street generally include two crossing conflicts at the 

crossover, four merging conflicts with traffic coming from the freeway, and four diverging 

conflicts for traffic moving to the freeway. Bicyclists using the sidewalk system or a shared-use 

path experience the same conflict points as pedestrian movements, which were discussed in a 

previous section. Sight distances should be checked for these conflict areas; walls, railings, tall 

landscaping, or other obstructions may limit sight distance.

Bicycle Lane Guidance

Bicycle lanes are recommended at a DDI to provide bicyclists with safe and convenient facilities 

to travel across the interchange. The bicycle lanes should be located to the right of the travel 

lanes for motorized traffic, which is generally where bicyclists expect to travel. Protected bicycle

lanes are recommended where actual vehicle speeds exceed 35 miles per hour (mph), but add 

comfort for bicyclists for all facilities. Bicycle lanes wider than five feet may be appropriate on 

an arterial where the lanes would be placed adjacent to a concrete barrier.

(27) For example, a 

seven-foot bicycle lane provides additional room between the bicyclist and the concrete barrier 

without encroaching on the travel lanes. Buffered bicycle lanes should be used when a bicycle

lane width in excess of seven feet is desired, as greater widths may encourage motor vehicle use. 

Green-colored pavement and/or dashed bicycle lane lines can be used to connect the solid 

bicycle lane lines at intersections. The bicycle lane should only be interrupted by stop bars at the 

signalized crossovers. The bicycle lane should continue through the exit ramps from freeway, 

where motorized traffic would generally be required to yield to arterial traffic, including cyclists. 

Where bicycle lanes cross exit ramps, the use of colored pavement may increase cyclist visibility 
Diverging Diamond Interchange Informational Guide

52 

to motorists. Exhibits 3-25 and 3-26 show visualizations of a DDI bicycle lane in Reno, NV that 

highlight some of these features. 

Exhibit 3-25. Visualization of bicycle lane, approach of first crossover into DDI.

(20)

Exhibit 3-26. Visualization of bicycle lane, approach of second crossover out of DDI.(20)
Diverging Diamond Interchange Informational Guide

53 

Left versus Right Side Bicycle Lane 

When bicycle lanes are provided, it is generally preferred to locate them to the right of motorized 

vehicle traffic, consistent with generally expected bicyclist behavior. At a DDI with a center 

barrier wall adjacent to a bicycle lane, the lane should provide adequate width to avoid cyclists 

from feeling “trapped.” It is recommended that the implementing agency works with the local 

bicycling community to discuss this and other issues. The right side bicycle lane configuration is 

shown in Exhibit 3-27. 

Exhibit 3-27. Schematic for bicycle lane placement on right side of vehicular traffic. 

As an alternative, the bicycle lane could be marked on the left of the approach street, resulting in 

bicyclists being on the outside between the crossovers and away from the center buffer wall. This 

design is not preferred in most cases as it creates the need for bicyclists to shift lanes prior to the 

DDI, increases the risk of a diverge conflict with left-turn traffic to the freeway, and requires 

another lane change back to the right-hand side after the DDI. No DDIs to date have left-side 

bicycle lanes. 
Diverging Diamond Interchange Informational Guide

54 

Bicycle-Pedestrian Conflicts

Potential pedestrian-bicycle conflicts exist in those cases where bicycles are expected to use the 

pedestrian walkway and sidewalk system. Just as there is a speed differential between motorized 

traffic and bicycles, there is also a speed differential between bicycles and pedestrians, unless 

bicyclists choose to dismount on the sidewalk. The minimum width of a shared -use path is ten 

feet except in very rare circumstances.(28) Wider paths, or fully separated bicycle paths, may be 

needed in cases where high pedestrian and bicycle volumes are expected at a DDI, such as when 

a path or trail system goes through the interchange. 

TRANSIT

Buses on Arterial

Buses operating on the arterial will benefit from the reduced number of signal phases and 

potentially lower delay when traveling through a DDI. In general, bus stops are not placed within 

interchanges unless there is transit service within the freeway corridor that stops at the 

interchange.

Transit on Freeway

If rail transit or a busway is located within the freeway right-of-way and a station is provided at 

the interchange, a wider median on the arterial would allow buses on the arterial to stop directly 

above or below the station. This would facilitate passenger transfers, with passengers boarding 

and alighting directly from a center walkway with an elevator and stairs connecting the walkway 

to the freeway station platform. The added median width would provide room for both the 

elevator and stairs, and for bus pullouts on the far side of the station access points to serve the 

bus stops. 

Another transit possibility is that express bus service is provided on the freeway and transit 

service is also provided on the arterial, with a transfer between the two services. At a 

conventional diamond interchange, the typical approach would be to have the express bus service 

exit using the exit ramp, cross the arterial at the ramp terminal intersection, and serve a far-side 

stop before re-entering the freeway via the entrance ramp. Because a DDI does not support 

through movements between off- and entrance ramps, an alternative configuration would be 

needed. One option would be a “freeway flyer” stop where a bus-only collector-distributor road 

is provided adjacent to the freeway between the ramps, providing a safe location for buses to 

stop and serve passengers. Transfers between the freeway express and the arterial transit service 

would be facilitated via the center walkway in much the same manner as described previously. 

Light Rail on Arterial 

Median-running light rail could remain in the median and go “straight” at the crossover 

intersections, Light rail remaining in the median would be served by a dedicated signal phase at 

crossover intersections. Extra clearance between the light rail tracks and the adjacent travel lane 

may be required to accommodate the trains’ dynamic envelope (and some form of positive 

separation may be desirable to discourage vehicles from encroaching onto the tracks when going 

through the curves at the crossover points. 
Diverging Diamond Interchange Informational Guide

55 

One such example of a median-running light-rail line through a DDI is shown in Exhibit 3-29. 

This light rail line remains in the median through the DDI.

Exhibit 3-28. Aerial view of DDI with center-running light rail at I-494 and 34th Avenue in 

Bloomington, MN.

(29) 

Geometric design considerations for a DDI with a center-running light rail include: 

• Crossover requires longer tangent length to clear rail line on straight trajectory 

• The spacing of the crossovers needs to be long enough to store the entire length of the 

train, unless signal phasing allows the light rail to cross the entire DDI without stopping 

• Pedestrian facilities need to be located on the outside, or two center walkways are needed 

(with the light rail tracks in the middle). 

• Pedestrian crossings of the arterial street are possible, but placement of a median refuge 

area may be challenging due to the presence of the light rail tracks. 

• Drivers are more likely to feel they are on the "wrong" side of the road by seeing train 

movements to their right when between crossovers intersections. 

Traffic signal considerations for a DDI with center-running light rail include: 

• Added signal phase to serve light rail line through the crossover. This third phase can be 

served concurrent with both right and left turns from the freeway. 
Diverging Diamond Interchange Informational Guide

56 

• Signal stop bars moved additional distance away from crossover to prevent drivers from 

stopping too close to the light rail line

• Potential advance pavement markings and signing alerting drivers of the rail crossing

• Potential use of supplemental blank-out sign displaying a train graphic and message such 

as "Light Rail Crossing" when a train is approaching 

HEAVY VEHICLE CONSIDERATIONS

The geometry of crossover curves and channelized turn lanes should generally accommodate 

heavy vehicles without off-tracking over curbs or into adjacent lanes. Recommended lane widths 

and design details for channelized turn lanes are documented in Chapter 9 of the AASHTO 

Green Book.(8) At the time of writing, little guidance is available for the design of the successive 

curves associated with the crossover points, as this feature is unique to a DDI. In general, lane 

widths should be increased relative to the arterial’s typical section through these curves, and 

vehicle turning template software should be used to check the swept paths of trucks. Chapter 7 

provides more detailed guidance on design techniques to accommodate heavy vehicles. 
Diverging Diamond Interchange Informational Guide

57 

CHAPTER 4— SAFETY

DDIs, like many alternative intersection and interchange designs, offer several potential safety 

advantages compared to conventional designs. At this time, little is known about the long-term 

safety performance of DDIs through rigorous studies as there have been insufficient numbers of 

interchanges open for an extended period of time. The best glimpse of safety comes from the 

first DDIs opened to traffic in Missouri and other states. This chapter provides the best overall 

assessment of safety at DDIs at this time based on studies completed by state agencies and recent 

research efforts. 

SAFETY PRINCIPLES

The number of conflict points present at an intersection and volume of conflicting traffic present 

may serve as a surrogate measure of intersection safety. Conflict points are defined by the 

location where the paths of various traffic modes cross, including motor vehicle, bicycle, or 

pedestrian movements. Vehicle-to-vehicle conflicts are most often described as merging, 

diverging, or crossing, where crossing conflicts represent the greatest risk for higher severity 

crashes as they denote the location where severe angle collisions take place. Although traffic 

control devices can reduce the propensity for crashes by eliminating conflict points or controlling 

them through signing, signals, or pavement markings, traffic control and intersection geometry 

cannot completely eliminate the human error factor that contributes to collisions.

Exhibits 4-1 and 4-2 present vehicle-to-vehicle conflicts for a conventional diamond interchange 

and a DDI, while Exhibit 4-3 provides a direct comparison of the conflicts present at both 

interchanges. This section focuses on vehicle-to-vehicle conflicts; bicycle and pedestrian 

conflicts are described in Chapter 3.

Exhibit 4-1. Conflict point diagram for DDI. 
Diverging Diamond Interchange Informational Guide

58 

Exhibit 4-2. Conflict point diagram for conventional diamond. 

Exhibit 4-3. Conflict point comparison. 

Crossing Merging Diverging Total 

Conventional diamond 10 8 8 26 

Diverging diamond 2 6 6 14 

Conventional diamond interchanges have 26 conflict points, and DDIs have 14. The reduction in 

conflict points is related to the unique crossover movements. The crossovers remove all exit 

ramp to entrance ramp through movements and eliminate several left-turning conflicts between 

the ramp and cross street. Crossing conflicts from left-turn movements are reduced from 10 to 

two. Theoretically speaking, the DDI offers a safety benefit due to reduced conflicts—especially 

crossing conflicts—and speed-controlling curvature of the crossover movements. 

Despite the theoretical safety benefits, the unusual design of the DDI creates concerns related to 

user expectation and perception errors. This chapter summarizes the most up-to-date safety data 

available, providing the profession an overall sense of the safety performance of the DDI 

compared to the conventional diamond interchange. The discussion highlights general areas of 

concern and describes safety performance measures in terms of crashes, conflicts, erratic 

maneuvers, and wrong-way maneuvers. Other safety considerations related to the DDI are also 

summarized.

Human Factors Principles and Considerations

Focus groups and surveys conducted near several DDIs captured the general public’s options. (14)

Respondents generally reported that driving through a DDI interchange was no more complex 

than a conventional diamond interchange. However, respondents also reported it was common to 

observe confusion by other drivers at DDI interchanges. This finding suggests experienced 

commuters responding to the survey were not likely to experience driver confusion, but novice 

drivers with less exposure may initially have issues.
Diverging Diamond Interchange Informational Guide

59 

The majority of participants in the focus groups didn’t use the pedestrian and bicycle facilities 

but the comments from the surveys expressed some concern with them. If the pedestrian 

walkway is designed to use the center of the road, designers should recognize that pedestrians 

will immediately want to go back to the left or right side of the road once they are past the 

crossover. Care should be taken to make sure that these facilities have good sight distance for 

pedestrians and drivers, and if insufficient gaps are not available, signalization or some other 

treatment may be necessary to make sure pedestrians or bicyclists who use the sidewalk are 

safely accommodated at crosswalks that are otherwise unsignalized. Last, if bicycle lanes are on 

the arterial, they should be maintained through the interchange and adequate operating space 

must be provided adjacent to any barrier on the right hand side of the roadway between the two 

crossover intersections. 

GENERAL SAFETY CONCERNS

As DDIs have opened across the U.S. in the past five years, the most common safety concerns 

perceived by the transportation profession are associated with exit ramp movements, heavy 

vehicles, bicyclists and pedestrians, and emergency vehicles. A brief summary of each concern is 

provided below: 

Right Turn at Exit Ramp 

Traffic between the two crossovers travels on the left side of the road, which is counter to the 

expectation of right-turning drivers from the freeway exit ramp. RTOR operation is often desired 

at this movement for capacity reasons, similar to the conventional diamond interchange. 

However, if RTOR operations are allowed, right-turning drivers may look on the wrong side of 

the road when checking for conflicting vehicles because traffic is crossed over. Intersection sight 

distance may also be limited by barriers or other obstacles between the crossovers. Design 

techniques to address these issues are provided in Chapter 7. Exhibit 4-4 shows the oncoming 

traffic for the right turn at the exit ramp. 
Diverging Diamond Interchange Informational Guide

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Exhibit 4-4. Oncoming traffic for the right turn at the exit ramp. 

Left Turn at Exit Ramp 

At this time, approximately two-thirds of states allow left turn on red (LTOR) from a one-way 

street to another one-way street. The unique design of the DDI legally allows LTOR operations 

in these states. However, there are safety concerns related to intersection sight distance and 

drivers looking down the wrong approach. Design techniques to address these issues are 

provided in Chapter 7. Exhibit 4-5 shows the oncoming traffic for the left turn at the exit ramp. 

Exhibit 4-5. Oncoming traffic for the left turn at the exit ramp. 
Diverging Diamond Interchange Informational Guide

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Heavy Vehicles 

Truck considerations at the ramp and crossover movements require curve radii and lane widths 

sufficient to accommodate trucks without off-tracking into adjacent lanes. Radii, crossover angle, 

and lane width values that generally achieve this are presented in Chapter 7.

Wrong-way Maneuvers 

At most interchanges, including conventional diamonds, wrong-way maneuver concerns are 

primarily related to drivers entering the freeway in the wrong direction. There is nothing inherent 

about the design of the DDI that would increase the likelihood of this, and the channelization of 

movements may decrease the likelihood of wrong-way maneuvers at freeway exit ramps. 

However, the crossovers on the cross street create the potential for wrong-way movements on the 

cross street. The results of a field study are presented later in this chapter, and design techniques 

to discourage wrong-way maneuvers are presented in Chapter 7. In general, wrong-way 

maneuvers appear to occur more frequently at crossover intersections with flat crossover angles 

and during low-volume periods when conflicting vehicles are not present. 

Pedestrians 

Channelization of all turns onto and off of the ramps is required to discourage wrong-way 

maneuvers and to move ramp terminal intersections away from the crossover intersection. 

Channelization, especially for unsignalized turns, creates pedestrian safety concerns due to the 

potential for high speeds and sight distance limitations. Minimizing the radius and speed of 

channelized turns with crosswalks is recommended to improve pedestrian safety. Signalizing 

turns may also improve pedestrian safety. 

Many DDI designs have been associated with upgrades to pedestrian facilities and/or multi-use 

paths, and best practices for the designs of these facilities are still developing. Considerations 

include locating the pedestrian facilities (outside versus median), unusual crossing points, radius 

and speed of turning movements, and whether to signalize turns at exit ramps – especially left 

turns onto entrance ramps with walkways along the outside of the DDI where speeds are higher 

and there may be limited sight distance issues. Design guidance and other information can be 

found in Chapters 3 and 8. 

Bicyclists 

The right-of-way gained by eliminating left turn bays for entrance ramp movements can often be 

considered for bicyclists. Designers have several options for bicyclists including using vehicle 

lanes, use of a shoulder, or use a dedicated bicycle lane. For the latter two options, designers 

must carefully consider the amount of space necessary for safe travel next to the median barrier.

Design guidance and other information can be found in Chapters 3 and 8. 

OBSERVED SAFETY PERFORMANCE

The primary measure of safety performance is long-term annual average crash frequency; 

however, there is little of this available for DDIs. Crash modification factors (CMFs) based on 

significant samples of data from treatment and reference sites that remove bias found in most 
Diverging Diamond Interchange Informational Guide

62 

safety studies, especially regression-to-the-mean, have not yet been developed. Therefore, this 

section provides initial safety effects from early DDI implementations and surrogate measures 

collected at several DDIs in a recent FHWA study. Other safety considerations, including wrongway maneuvers, are discussed and data is provided where possible. 

Crash Observations 

Generally speaking, empirical safety studies are rarely conducted at interchanges. However, the 

unique design of the DDI has raised concerns from the general public, decision-makers, and 

within the transportation profession about its safety performance. Transportation agencies are 

eager to learn about the interchange safety performance as apprehensions about its design are 

often raised in public meetings and by the news media. 

Twenty-four DDIs have opened to traffic in the U.S. since the first DDI opened at MO 13 and I44 in Springfield, MO, and several state and local agencies have begun reporting initial crash 

results. Agencies should use caution when using these results as they likely contain biases found 

in before-after safety studies. Agencies should look for CMFs to be developed from U.S. data 

and published in the future. The initial safety findings are presented below. 

MO 13 at I-44 – Springfield, Missouri

A comparison of five years of before data and one year of after data from the first DDI in the 

U.S. indicates:(16)

• A decrease in total crashes

• A decrease in left-turn crashes 

• “After” crash patterns similar to a conventional interchange

• No increase of any crash type

The analysis was a simple comparison of before/after data without adjustments for potential 

biases such as changes in volume or regression to mean. 

Harrodsburg Road (US 68) at KY 4 – Lexington, KY

A comparison of four years of before data and two years of after data from the Lexington, KY

DDI indicates:

(11)

• On the two road segments between the crossover intersections and adjacent signalized 

intersections:

o Total crashes increased on one segment and decreased on another 

o Injury crashes remained about the same on both segments 

• On the arterial within the interchange: 
Diverging Diamond Interchange Informational Guide

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o Total crashes increased and injury crashes decreased

o Sideswipe crashes increased and rear-end crashes decreased

The analysis was a simple comparison of before/after data without adjustments for potential 

biases such as changes in volume or regression to mean.

Winton Road at I-590 – Rochester, NY 

A comparison of three years of before data and eight months of after data from the Rochester, 

NY DDI indicates:(30)

• Crashes decreased at the two crossover intersections, primarily due to the reduction of 

rear-end crashes

• Crashes increased to a lesser extent at a conventional signalized intersection adjacent to 

the DDI, primarily due to an increase in overtaking crashes 

The analysis was a simple comparison of before/after data without adjustments for potential 

biases such as changes in volume or regression to mean. NYSDOT noted rear-end collisions at 

the adjacent intersection were still problematic but adjustments to signal offsets between the DDI 

and adjacent signal appear to be helping with progression. No incidents related to wrong-way 

movements were reported. 

Traffic Conflicts and Erratic Maneuvers

Traffic conflicts are defined as events involving two or more moving vehicles approaching each 

other in such a way that a traffic collision would ensue unless at least one of the vehicles 

performs an emergency maneuver.

(31) Erratic maneuvers are similar to conflicts but include 

single vehicle events that have the potential to cause a collision. In an ongoing FHWA field 

study of seven DDIs, traffic conflicts and erratic maneuvers were used as surrogate safety 

measures since sufficient collision data requires several years to collect.(14) This study assumed 

the same factors resulting in crashes result in conflicts. In most cases, surrogate studies are 

typically accepted in practice while waiting for additional crash data; however, human 

observation is a complicated process that some argue provides too much subjectivity. As such, 

the findings provided in this guide related to conflicts and erratic maneuvers should be judged 

carefully and in light of initial safety studies conducted to date.

Each ramp terminal intersection was broken into eight potential conflict zones, shown in Exhibit 

4-6. Of these potential conflict zones, conflict areas 1 through 4 were presumed to be the most 

critical for evaluating the safety performance and studied in the field. Conflicts were divided into 

a variety of categories (e.g. rear-end maneuver, lane-changing, and so on) and by severity 

maneuver (conflict or near-collision). 
Diverging Diamond Interchange Informational Guide

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Exhibit 4-6. Conflict zones at ramp terminal intersection.

Conflict areas 1 and 2 correspond to the two approaches to the DDI crossover points. These areas 

were monitored for various conflicts and erratic maneuvers including rear-end, wrong-way 

maneuvers, illegal U-turns, red-light-running, and lane-changing conflicts. Conflict area 3 

corresponds to the left-turn maneuver exiting the freeway. Similarly, conflict area 4 corresponds 

to right-turn maneuver exiting the freeway. The divergent conflicts in conflict areas 5 through 8 

were not monitored since they are largely consistent with what would be expected at a 

conventional diamond interchange. 

A description of notable events from the seven DDIs studied is summarized below:(14)

• Signal compliance and lane changing were the primary causes of traffic conflicts.

• Some sites experienced an unusual amount of lane changing in the crossover intersection, 

while one site had considerable lane change events at the split of the exit ramp where a 

right and left were separated at the island. The lane change event was most often noted as 

a single vehicle event (erratic maneuver).

• Allowing exit ramp RTOR may be problematic when the arterial is an underpass because 

the bridge structure limits sight distance.

• Where RTOR was allowed, several merge events were noted for the right turn exit ramp. 

RTOR was allowed at two sites where the median barrier limited sight distance to 
Diverging Diamond Interchange Informational Guide

65 

oncoming vehicles. A modification in alignment of this movement to discourage drivers 

looking down the wrong approach was implemented after data collection for the project 

was complete. 

• U-turns were rare with the exception of one DDI where an adjacent intersection with a 

local street was converted to right-in-right-out control and residents of the street used the 

crossover intersection to make U-turns. 

Wrong-Way Maneuver Observations

Wrong-way maneuvers are often identified by the public as a safety concern prior to the opening 

of a DDI. Plan drawings and simulations of DDIs in public meetings often give the impression 

that wrong-way maneuvers will be easy for drivers to unintentionally execute. This section 

summarizes the findings from a six-month monitoring effort of wrong-way maneuvers at five 

DDI locations and provides some basic findings related to factors that may lead to wrong-way 

maneuvers. 

Many engineers have hypothesized the crossover angle may affect the frequency of wrong-way 

maneuvers, and it is recommended the crossover angle be as close to 45 degrees as possible.(13)

Many DDIs have been built at sites with relatively low traffic volume along the cross street and 

higher left-turn volume onto the freeway, thus providing less conflicting traffic in the opposing 

lane to discourage a wrong-way maneuver. 

Exhibit 4-7 provides the number of wrong-ways identified at each site along with the crossover 

angles and crossroad annual average daily traffic (AADT) at each intersection. Intentional 

wrong-way events made by construction vehicles or emergency vehicles were excluded from the 

analysis. Wrong-way maneuver rates at the interchange crossovers were low at most sites, 

ranging from 0.3 to 24.5 wrong-way events per 1,000,000 vehicles. Crossover angles were 

measured using aerial photography and the centerline pavement markings in lieu of plan 

drawings. AADTs were found using state traffic volume maps. 
Diverging Diamond Interchange Informational Guide

66 

Exhibit 4-7. Wrong-way maneuvers at five DDI sites along with crossover angle and 

crossroad AADT.

Site

Total Wrong 

Ways (% of 

total)*

Crossover 

Angles

CrossStreet 

AADT

Wrong-Ways per 

1,000,000 vehicles

Bessemer Street and US 129, 

Alcoa, TN 5 (3%) 47 ̊ / 52 ̊ 10,850 2.5 

Dorsett Road and I-270, Maryland 

Heights, MO 16 (10%) 34 ̊ / 36 ̊ 46,000 1.9 

Front Street and I-435, Kansas 

City, MO 97 (62%) 32 ̊ / 28 ̊ 21,733 24.5 

Harrodsburg Road (US 68) and 

KY 4, Lexington ,KY 2 (1%) 38 ̊ / 37 ̊ 38,463 0.3 

Winton Road and I-590, 

Rochester, NY 36 (23%) 47 ̊ / 40 ̊ 24,984 7.9 

* Total events after removing intentional wrong-way maneuvers

The frequency and rate of wrong-way maneuvers is considerably higher at Front Street than 

other sites. Some basic summaries for each site are provided based on the factors described 

earlier and other qualitative factors.

Front Street: The low crossover angles at Front Street along with the low AADT and 

predominant left-turn-on-freeway movement may contribute to wrong-way maneuvers. 

Winton Road: Wrong-ways at Winton Road appear to be related to the low traffic volumes and 

predominant left turn onto the freeway. Another possible factor that may have led to some 

wrong-way maneuvers at the southern intersection crossover was the removal of a third through 

lane. The outermost lane was closed off with pavement markings as the additional capacity was 

not needed (Exhibit 4-8). This lack of guidance from a curb along the outside edge of the lane 

line seems to provide a greater opportunity for wrong-ways at this crossover. It may be more 

desirable to mark off the inside lane instead or provide improved channelization with curb rather 

than pavement markings. 

Exhibit 4-8. Southbound outer-most lane closed and marked with pavement markings.(26)
Diverging Diamond Interchange Informational Guide

67 

Dorsett Road: The crossover angle at both intersection crossovers was smaller than all other 

sites excluding Front Street. However, high traffic volumes at this site resulted in opposing 

vehicles generally being present at the crossover. Similar to outbound movements at Front Street, 

pavement markings were used instead of raised curb to guide inbound drivers through the 

crossover (Exhibit 4-9).

Exhibit 4-9. Pavement marking used to channel vehicles through crossovers.(26)

Bessemer Street: Crossover angles exceed the recommended values; however, the single-lane 

crossovers and curbing on approaches channelize vehicles to a greater extent than approaches 

with more lanes at other sites.

Harrodsburg Road: Crossover angles are lower than recommended; however, high traffic 

volumes at this site resulted in opposing vehicles generally being present at the crossover. 

The research team attempted to identify supplemental trends that might lead to wrong-way 

maneuvers. Several variables were considered such as inbound versus outbound movements, 

time-of-day, vehicle type, weather, and peak versus off-peak travel times. Graphs of trends for 

each factor are presented in the Appendix. Summaries of these trends seem to indicate the 

following: 

• Wrong-way maneuvers were more likely to happen for inbound movements. 

• Wrong-ways were much more likely to take place during night time and off-peak times of 

day. These time periods would provide more opportunity to make a wrong-way with 

conflicting traffic on the opposing approach. 

• The majority of events took place during dry conditions; however, 11 (8%) wrong-ways 

did take place during rain or snow events. No corresponding data was recorded on the 

frequency of rain or snow events. 

• The predominant vehicle type was a passenger car or truck, with only three heavy 

vehicles making a wrong way at Dorsett Road (1) and Front Street (2). 
Diverging Diamond Interchange Informational Guide

68 

INCIDENT RESPONSE CONSIDERATIONS

The response times of emergency responders are a universal concern. The design of a DDI 

provides a much better experience for emergency vehicle responders. Several concerns are 

summarized below: 

• The barrier between travel directions and crossovers prevents passenger vehicles or 

emergency responders from driving on the wrong side of the road to pass or respond to 

broken down vehicles. This is similar to a conventional interchange with a divided 

arterial. In most cases, decreased delay and queuing at a DDI compared to a convention 

diamond interchange mitigates the need for travel on the wrong side of the road. Longterm video monitoring efforts at seven DDIs for a six-month period of time found no 

unusual problems clearing crashes.(14)

• In most cases, decreased delay and queuing at a DDI compared to a conventional

diamond interchange is beneficial to emergency vehicles. The right turn on the freeway 

exit ramp often does not allow RTOR, causing queues to be longer than at a conventional 

diamond. This may impede emergency vehicles making right turns off of the freeway.

• Ramp-to-ramp movements are sometimes made by emergency vehicles when congestion 

on the freeway does not allow passage of an emergency vehicle. The unique crossover 

design of a DDI does not allow through movements at the ramps, and strategies for 

mitigating this issue are limited.

SAFETY EVALUATION CONSIDERATIONS

Although no CMFs exist for DDIs at this time, they will be available in the future. CMFs will be 

provided in a future edition of the Highway Safety Manual and on FHWA’s CMF 

Clearinghouse.(32)

 The CMF will likely apply to the entire interchange facility and not individual 

crossovers. 
Diverging Diamond Interchange Informational Guide

69 

CHAPTER 5— OPERATIONAL CHARACTERISTICS

This chapter discusses the operational characteristics of DDIs, describes the different movements 

and operational considerations, introduces signal timing concepts for DDIs, and summarizes past 

comparative studies on DDI field performance. It is intended to help transportation professionals 

understand the unique operational characteristics of DDIs and prepare them for conducting 

operational analysis as described in Chapter 6. 

INTRODUCTION

DDIs improve traffic flow for motorists turning left onto the freeway. By moving through traffic 

to the left side of the street between the crossovers, left-turn movements onto the freeway do not 

conflict with opposing traffic. As opposed to conventional diamond interchanges where left 

turning vehicles are stored between the two ramp terminals, no such turn storage is required at 

the DDI. This reduces the required width of the cross street which can result in significant cost 

savings compared to conventional diamond interchanges. 

Traffic signals at DDIs operate with two phase intervals compared to three at conventional 

diamond interchanges. This reduction in phase intervals improves overall signal efficiency and 

benefits through traffic on the cross street and left-turn traffic from the freeway. Tight turn 

movements are accommodated similar to a conventional diamond. The reduction of signal 

phases is one of the fundamental principles underlying alternative intersection and interchange 

design. 

The key operational characteristics of a DDI are shown below in Exhibit 5-1. 
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70 

Exhibit 5-1. Key operational characteristics of a DDI. 

Naming Convention for Movements 

A naming convention is introduced here to clearly define each of the critical origin-to-destination 

movements at the DDI. Each directional movement was named based on the direction of the 

origin and destination as shown in Exhibit 5-2. 

In Exhibit 5-2, through movements on the crossroad are shown as E to W (east to west) or W to 

E movements. This orientation, rather than north of south, is arbitrary for the sake of discussion 

in this chapter. 
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Exhibit 5-2. Naming convention of movements at a DDI (east-west cross street).

Operational Zones

DDIs have five unique operating zones: (1) Approach Zone; (2) Crossover Zone; (3) Exit ramp

Zone; (4) Entrance ramp Zone; and (5) Departure Zone. These five zones are shown in Exhibit 5-

3, followed by a discussion of key operational considerations. 
Diverging Diamond Interchange Informational Guide

Exhibit 5-3. Operational consideration zones for DDIs. 

72 

The Approach Zone (A) refers to the segment of street between an upstream intersection and the 

first DDI crossover. Operational considerations in this zone include queue spillback, demand 

starvation (no traffic present when the signal phase is green, generally because it is held up at an 

upstream signal), and signal progression. 

The Crossover Zone (B) refers to the crossover intersection itself and the entries and exits of this 

intersection. Operational considerations in this zone include signal progression between 

crossovers, lane utilization of approach traffic, saturation flow rate at the crossover, and speed 

profiles through the crossover. 

The Exit ramp Zone (C) refers to the area between the freeway diverge area and the cross street.

Operational considerations in this zone include vehicle speed profiles, performance of right-turn 

movements, and performance of left-turn movements. 

The Entrance ramp Zone (D) refers to the area between the second crossover and the freeway 

merge area. Operational considerations include speed profiles through the turns, the merge area 

capacity, and potential ramp metering effects. 

The Departure Zone (E) refers to the area between the second crossover and the next adjacent 

signal on the cross street. Operational considerations in this zone include queue spillback from 

the downstream signal into the DDI, signal progression, and weaving maneuvers from the 

freeway exit ramp to a left turn at the next downstream intersection. 

For all five zones, emergency vehicle accommodations are also a key operational consideration. 

Exhibit 5-4 summarizes operational considerations and their applicable zones, followed by 

additional discussion in the next section. 
Diverging Diamond Interchange Informational Guide

73 

Exhibit 5-4. Operational considerations at DDIs. 

# Consideration DDI Zones Description

1 Queue spillback A, E Spillback from DDI to adjacent signal or from that 

signal to the DDI

2 Demand starvation A Green time at DDI external signal that does not serve 

any demand due to adjacent signal timing

3 Signal progression A, B, C, E Ability to coordinate movements through the DDI 

(through movement or turns)

4 Lane utilization B Equal versus unbalanced utilization of all approach 

lanes at the crossover

5 Saturation flow rates B The theoretical per-lane capacity through the DDI 

6 Speed profiles B, C, D Vehicle free-flow speeds through the various geometric 

elements of the DDI

7 Left turn at exit ramp C Movement can be yield-controlled or signalized

8 Right turn at exit ramp C Movement can be free-flowing, yield-controlled, or 

signalized

9 Ramp area merge 

capacity

D The right turn from cross street typically yields to freeflow left turn 

10 Ramp metering impacts D A ramp meter to control freeway entry volumes may 

impact DDI operations

11 Weaving maneuvers E Right turns from freeway may have to weave across 

arterial lanes for downstream left turn

12 Emergency vehicles A, B, C, D, E Emergency vehicles will be present throughout the 

interchange

13 Corridor improvement 

strategies

A, E Strategies for improving the corridor traffic flow with 

adjacent signalized intersections. 

A summary of the key operational considerations unique to DDIs is provided in the next section, 

along with DDI-specific design strategies where applicable. 

OPERATIONAL CONSIDERATIONS

Queue Spillback

DDIs generally have fewer signal phases than adjacent intersections and thus provide greater 

capacity and throughput. As a result, queue spillback is likely to occur in the departure zone 

where a downstream intersection cannot readily process all the demand processed by the more 

efficient upstream DDI. An example of this effect is shown in Exhibit 5-5. 
Diverging Diamond Interchange Informational Guide

74 

Exhibit 5-5. Queue spillback into DDI from downstream adjacent signal.(14)

Queues occur less frequently within the DDI because of more efficient signal operations (fewer 

phases). Thus, queuing tends to occur external to the DDI. 

Demand Starvation 

Demand starvation occurs when a signal phase is green and no traffic is present because it is held 

up at an upstream signalized intersection. Demand starvation is most likely to occur in the 

approach zone link where the capacity at the downstream end of the link (the approach to the 

first DDI crossover) is greater than the capacity of the upstream end of the link (the receiving 

approach from the adjacent intersection). When demand starvation occurs, the actual throughput 

of the DDI movement will be less than its potential capacity. 

Signal Progression 

Through movements at a DDI cannot run concurrently due to the cross over and thus are required 

to operate with split phasing. As such, this limits the ability to progress movements through the 

interchange. Signal timing and progression is discussed in more detail further into this chapter. 

Lane Utilization

Interchanges generally have unbalanced lane utilization because drivers making turning 

movements tend to preposition in advance of the interchange. Field observations show that prepositioning is more likely to occur at a DDI compared to a conventional diamond interchange, 

resulting in lane imbalance.

(14) Some of the lane imbalance is likely attributable to the lane 

configuration at the second crossover. In Exhibit 5-6, the left-hand image represents a three-lane 

road with a left and through shared lane at the second crossover, whereas the right-side image 

represents a three-lane road with an exclusive left-turn lane. The shared lane configuration is 

expected to result in a mix of left-turn and through traffic in the leftmost lane at the first crossover, while the exclusive lane is expected to limit the left lane to left-turn traffic only. 
Diverging Diamond Interchange Informational Guide

75 

Exhibit 5-6. Different lane configurations at three-lane DDIs.

Lane utilization at the first crossover may be imbalanced in both cases as a function of the 

amount of left-turning traffic in the traffic stream. Field observations indicate that left-lane 

utilization can be predicted by the left-turn demand ratio as seen in Exhibit 5-7. 

Exhibit 5-7. Left lane utilization prediction for two-lane DDI crossovers.

(14)
Diverging Diamond Interchange Informational Guide

76 

Saturation Flow Rate 

The unique geometric configuration of the crossover has been linked to reductions in saturation 

flow rate. FHWA’s Saxton Lab is currently developing a saturation flow prediction equation for 

DDI movements. 

Speed Profiles

Free-flow speeds are limited by the geometrics of the DDI. Field studies at DDIs across the U.S. 

have shown that free-flow speeds through and between the crossovers are lower than the posted 

speed limit even without interaction effects of other traffic. Free-flow speeds for the left-turn and 

right-turn movements are also limited by geometry. 

Free-flow speeds impact the capacity of traffic movements. Speed-limiting geometry may be 

unexpected to drivers, and a transition zone in advance of a crossover (e.g. through the 

introduction of a reverse curve upstream of the crossover) may be beneficial. This is discussed 

further in Chapter 7. 

Free-flow speeds also directly impact the safety and comfort of pedestrian and bicycle 

movements at DDIs. For bicyclists in a shared lane or striped bicycle lane, faster speeds have 

been linked to a reduced quality of service for cyclists and further cause safety concerns. Faster 

speeds have also been correlated with a decreased propensity of drivers to yield to pedestrians at 

unsignalized crossings, as well as a greater chance of serious of injury or death in the event of a 

pedestrian-vehicle collision. As such, slow speeds through the interchange can greatly benefit 

non-motorized users of the facility. 

Field free-flow speeds at seven DDIs are summarized in Exhibit 5-8. 

Exhibit 5-8. Field-measured speed parameters for DDI sites.

(14)

Turn Speed for Speed Between 

Speed Crossover Lefts from Crossovers 

Interchange

Limit 

(mph)

Speed (mph) Freeway (mph) (mph)

Avg. StdDev. Avg. StdDev. Avg. StdDev.

MO 13, Springfield, MO 40 24.0 3.5 15.0 2.5 25.0 3.5

National Ave, Springfield, MO 40 25.0 3.0 21.0 2.9 29.0 4.0

Bessemer St, Alcoa, TN 35 26.0 2.5 15.5 3.0 32.0 4.0

Dorsett Rd, Maryland H., MO 30 26.0 3.0 23.5 3.0 31.0 4.0

Harrodsburg Rd, Lexington, KY 45 26.2 3.2 22.8 2.8 29.7 3.5

Front St, Kanas City, MO 35 24.1 3.1 20.0 3.2 26.8 3.1

Winton Rd, Rochester, NY 45 28.9 3.7 18.6 2.3 31.1 3.5

Exhibit 5-8 suggests the crossover speeds are a fairly consistently in the 24 to 26 mph range 

independent of the speed limit. Similarly, the speed on or beneath a bridge between the 

crossovers appears to range from about 25 to 31 mph. Consequently, the geometric design of the 

DDI appears to control free-flow vehicle speeds more than the posted speed limit. This may 

reduce lane capacity but also offers traffic calming benefits.
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77 

Left Turn at Exit Ramp

Left-turn movements from the freeway exit ramp at DDIs operate with either signal or yield 

control. Yield control is applied under low to moderate volume levels when left turns can 

generally get processed when the conflicting through traffic is stopped at the nearby crossover. 

Chapter 3 provided a discussion and examples of pedestrian-focused DDI design, where a 

reduced radius and modified alignment of the left-turn movement for vehicles can lead to (a) 

reduced speeds at the crosswalk, and (b) improved sight distances between drivers and 

pedestrians. 

Signalization is the recommended control strategy for exit ramp left turns where pedestrian 

facilities are provided on the outside; this provides a safe crossing opportunity. For DDIs with 

center walkways and thus no pedestrian crossings at left-turn exit ramp, refer to the MUTCD for 

general guidance on choosing traffic control devices. 

DDIs allows for left-turn-on-red (LTOR) operations, provided state laws allow this maneuver 

and adequate sight distance is achieved. 

Right Turn at Exit Ramp

In most cases RTOR movements are not allowed at DDI exit ramps because conflicting traffic 

along the cross street travels from right-to-left instead of left-to-right which is counter intuitive 

for most drivers. As such, capacity for right-turn movements at exit ramps is generally lower at 

DDIs compared to conventional diamond interchanges where RTOR movements are allowed. 

This results in higher delay and longer vehicle queues for right-turn movements at DDIs 

compared to conventional interchange configurations. Exhibit 5-9 below shows a heavy rightturn movement at a DDI exit ramp. 

Exhibit 5-9. Queue spillback caused by heavy traffic volumes and a no RTOR condition.(14)

Entrance Ramp Merge Capacity

Right-turn movements from the cross-street to the freeway typically operate with yield control in 

the presence of free-flow left-turning movements to the freeway. As such, the capacity of the 

right turn is a function of the conflicting left-turn traffic demand and the upstream crossover 
Diverging Diamond Interchange Informational Guide

78 

signal timing which affect arrival patterns for left-turning vehicles. For DDIs with high rightturning demands, additional lanes or alternate merge configurations may be appropriate. 

Ramp Metering Impacts

Ramp meters may be applied at entrance ramps to control the rate of flow from the entrance 

ramp to the freeway. Queue spillback from a ramp meter can adversely impact the operations of 

the DDI. This issue can be mitigated by providing additional storage prior to the ramp meter 

(Exhibit 5-10). An extended acceleration lane on the freeway could allow additional queue 

storage as the ramp meter can be moved closer to the gore. An acceleration lane helps reduce 

turbulence at the ramp influence area by providing drivers with additional time to reach freeway 

speed. 

Exhibit 5-10. Ramp meter with added lane for queue storage at a DDI.(6)

Additional considerations for ramp meters include how to merge multiple entrance ramp lanes 

downstream of the meter, signal timing settings of the ramp meter, freeway mainline detection to 

set ramp meter timing, and queue spillback detection to potentially override the ramp meter. 

These topics are beyond the scope of this guide, but additional guidance on ramp metering is 

available in the FHWA Freeway Management and Operations Handbook and the FHWA Ramp 

Management and Control Handbook.(33, 34) 

Weaving Maneuvers 

Weaving maneuvers result when adjacent intersections are in close proximity to the DDI 

crossover signal, as shown in Exhibit 5-11. Many DDIs prohibit RTOR maneuvers from the 

freeway exit ramp. This has the benefit of reducing the number of conflicts associated with 

weaving maneuvers. Besides RTOR restrictions, weaving conflicts can be minimized by 

lengthening the distance to the adjacent intersection.
Diverging Diamond Interchange Informational Guide

79 

Exhibit 5-11. Weaving maneuver and conflict points for DDI right turn from freeway. 

Emergency Vehicles

Widened shoulders or separation of turn lanes with a vein island can allow emergency vehicles 

to bypass stopped vehicular traffic as seen in Exhibit 5-12. Lane separation is typically provided 

to prevent off-tracking by heavy vehicles in turns, and the additional pavement creates the 

potential for emergency vehicles to pass other vehicles. 

Exhibit 5-12. Lane separation for heavy and emergency vehicle accommodation.(26)
Diverging Diamond Interchange Informational Guide

80 

Corridor Improvement Strategies

Several strategies exist for improving the traffic flow of a DDI in a corridor context. Field 

observations at various DDIs in the U.S. have shown that often the observed operational 

concerns and queuing impacts at the DDI are in large part due to queue spillback effects and 

capacity constraints at downstream signals. This section discusses several strategies to help with 

corridor operations of a DDI. 

It is noted that each of the strategies below is geared at maximizing traffic flow and vehicle 

throughout through the DDI and adjacent signals. Any of the strategies below should be 

considered in light of trade-offs of this increased auto efficiency for other modes of 

transportation or other impacts. 

Optimize Timing and/or Meter Traffic at Upstream Signalized Intersections

The adjacent signalized intersection downstream of a DDI should be optimized so that 

throughput can be maximized with minimal queue spillback into the DDI. If spillback still 

occurs, the throughput of the upstream adjacent signal can be artificially reduced to match the 

capacity of the downstream adjacent signal. This will reduce the chance for queue spillback 

through the outbound DDI movement. 

If a downstream adjacent signal does not pose problems with queue spillback into the DDI, the 

upstream adjacent signal should be looked at carefully to make sure that demand starvation is not 

occurring at the inbound DDI signal. The DDI signal is expected to provide for more throughput 

than the upstream adjacent signal. This creates an opportunity to use additional green time at the 

DDI to progress right and/or left turns from the side street at the upstream intersection. 

Eliminate Signal Phases at Adjacent Intersection

With the DDI providing efficient two-phase operations, an agency may consider opportunities 

for also reducing phases at adjacent intersections. This will tend to increase the overall corridor 

throughput. For upstream and downstream intersections, phases could be eliminated by changing 

protected movements to permitted or permitted/protected movements, or by providing split sidestreet phasing where a low volume side street may be skipped. 

The above strategies are retrofit applications for existing intersections. For a new construction 

DDI or for a major corridor improvement projects, other alternative intersections like the MUT 

and RCUT may provide additional benefits for a DDI corridor. 

Alternate Side-Street Phases

The phasing scheme at an adjacent intersection to the DDI could be adjusted to provide 

additional capacity to a mainline approach while only allowing one or more side-street 

movements every other cycle. This unusual phasing scheme could be used in a time-of-day plan 

when through traffic demand is high and when there is a great need for additional capacity to 

prevent queue spillback into the DDI. On the next cycle, the mainline is again serviced followed 

by the alternate minor approach. 
Diverging Diamond Interchange Informational Guide

81 

Half Cycle

Using half cycles at some intersections may provide for improved progression of the off-peak 

traffic at the DDI by opening the green band more often. This would also discourage red light 

running at DDI intersections, which only run two phases as compared to other signals within the 

same system. Since the adjacent six- or eight-phase signals are likely to require longer cycle 

lengths, a half-cycle DDI can help reduce driver frustration due to long wait times at the 

crossover. Half-cycling may also be useful in progressing both the through movement from the 

adjacent upstream signal and a potentially heavy left turn from the side street at that intersection. 

Half-cycling was employed recently at the first DDI in Atlanta, GA with some success.

Lead/Lag Phasing

Lead/lag phasing allows signal designers flexibility to choose when to start the left-turn phase in 

a specific ring so that maximum bandwidth can be achieved along the coordinated movement for 

the purpose of progressing traffic. In a DDI corridor, lead/lag phasing at adjacent signals can 

help with progression of vehicle traffic to or from the DDI. 

Queue Detection and Clearance

At freeway exit ramps with high demands from left- or right-turn movements, preemption could 

be used to help flush queues that are on the verge of spilling back onto the freeway. The 

movement that is usually more problematic is the right turn since RTOR is often not allowed at a 

DDI. In a queue detection and clearance implementation, a loop detector is placed just 

downstream of the exit ramp gore. When the queue approaches the loop, the signal is preempted 

to call a dedicated phase for left and right turns from the ramp as shown in Exhibit 5-13. This 

ramp preemption method is used regularly by several states; the first known DDI location to use 

this method was in St. George, Utah. 

Exhibit 5-13. Phasing schematic with exit ramp queue clearance phase.

Preemption provides a mechanism for alleviating excessive queuing (i.e. “flushing”); however, it 

hould be used sparingly when other signal timing strategies and design strategies (i.e. longer 

amps or more lanes) are infeasible. Preemption of a signal on a recurring basis could cause 

xcessive delays and queues to form on the arterial. 

s

r

e
Diverging Diamond Interchange Informational Guide

82 

SIGNAL TIMING AND COORDINATION

The timing and coordination of signals at DDIs is unique compared to conventional diamond 

interchanges because DDI signals operate with “split phasing” to allow both crossover 

movements to proceed independently. This presents unique considerations for traffic operational 

analyses and signal design plans. This section describes the basic principles and considerations 

for timing signals along a DDI. 

Pre-Timed versus Actuated Control

In a coordinated signal system, actuation is used to give additional time to heavy (mainline) 

movements if that time is not needed for oftentimes lower-volume side-street or turning 

movements. Through the use of detectors, the traffic signal measures the traffic demand at all 

movements continuously and allows minor movements to gap out if there is no traffic demand. 

This enhances the capacity and efficiency of the mainline movement. 

At the DDI, actuated control may not provide the same level of benefit as at a conventional 

intersection, as there is no “side-street” movement at the signal. The major street intersects with 

itself, and ramp turning movements run concurrent with the oftentimes higher-volume through 

movements. As such, there is less of an opportunity for phases to gap out, and a pre-timed 

system may perform equally efficient as an actuated controller. 

Another argument for pre-timed operations at a DDI is the desire to achieve some level of traffic 

progression for both directions of traffic to the extent possible. A pre-timed signal assures that 

the green bands and progression opportunities are maintained across multiple cycles. 

While a pre-timed signal may be recommended at a DDI, actuated control offers some benefits. 

Example applications for actuated control at a DDI include an unusually heavy movement (that 

needs more time whenever possible) or an unusually low-demand movement (that can gap out 

without much impact on progression). When a very heavy movement is combined with an 

unusually low movement in the opposing direction, actuated control may provide opportunities 

to enhance flow for the predominant movement. 

Actuated control is also recommended for any pedestrian-only signals at the DDI (e.g. for 

pedestrians crossing the signalized left turn onto the freeway). The pedestrian movement is likely 

to have low and/or intermittent demand, and actuated signals limit impacts to vehicular traffic to 

the times when pedestrians are actually present. 

Phase Sequence and Timing

DDI signals can be controlled by one or two controllers. Each crossover signal alternates 

between the two directions of the arterial, which are crossing each other. A simplified timing 

sequence is shown in Exhibit 5-14 for illustrative purposes. 
Diverging Diamond Interchange Informational Guide

83 

Exhibit 5-14. Simplified DDI timing sequence.

For a single-controller configuration, typically one ring in the NEMA timing sequence is 

dedicated to each crossover. The resulting two-ring design for both crossovers typically does not 

employ a barrier. For example, in Exhibit 5-14, Ring 1 alternates between two phases for the 

East crossover, while ring 2 alternates between two phases for the West crossover. The 

transitions occur at different points in the cycle to create progression between the crossovers. 

Assignment of phases to these movements varies based on agency preference and capabilities of 

the specific controllers in use.

Signalized right-turn and left-turn movements from the freeway run concurrent with the nonconflicting through traffic in Exhibit 5-14. However, additional efficiency can be achieved by 

introducing additional phase intervals (such as dummy phases and overlap phases) into the DDI 

timing sequence to minimize loss time during all-red intervals. An example of this advanced 

DDI timing sequence is shown in Exhibit 5-15 and is employed at most DDIs across the country. 
Diverging Diamond Interchange Informational Guide

84 

Exhibit 5-15. Example use of overlap phases in DDI timing sequence. 

Additional clearance time is needed at DDIs to allow the “outbound” crossover movement to 

sufficiently clear the right-turn movement at the exit ramp. Without an overlap phase, the 

“inbound” crossover movement must wait for the outbound movement to traverse the entire 

distance from the near side of the crossover intersection to the far side of the exit ramp, which 

can take up to seven seconds. With an overlap phase, the “inbound” crossover movement can 

begin after the “outbound” crossover movement clears the intersection (typically three to four 

seconds) and thus gain up to three seconds of green time per cycle. While this may seem subtle, 
Diverging Diamond Interchange Informational Guide

85 

it can result in significant gains in capacity and eliminate driver frustration associated with 

waiting at “empty” intersections. 

Favoring Cross Street versus Exit Ramps

With the two-phase signal operation at the DDI, a signal designer has the option of favoring the 

left turns off the freeway. While most DDIs tend to favor the through traffic on the cross street, 

the option of favoring the exit ramps is available for sites or time periods of heavy freeway to 

arterial demand; this is illustrated in Exhibit 5-16. 

Exhibit 5-16. DDI timing favoring cross street versus turns. 

Phasing favoring the cross street allows through traffic on the arterial to pass through both 

crossover signals in one movement without stopping between crossovers. Phasing favoring the 

exit ramps allow the left turns off the freeway to proceed through the DDI in one movement. 
Diverging Diamond Interchange Informational Guide

86 

Notice that the left turns to freeways are equally accommodated with these two designs as that 

movement generally never stops at the second crossover unless a pedestrian signal is provided at 

the freeway entry ramp or there is queue spillback from the second crossover. 

Coordination 

Progression bandwidth at DDIs is generally wider than at adjacent signals. It is also “staggered” 

in both directions as the two directional crossovers cannot be green at the same time. Exhibit 5-

17 shows an illustration of the time-space diagram for a hypothetical four-intersection corridor 

with standard signalized intersections. The exhibit shows the through movements in both 

directions receiving a green indication at the same time. At the equal intersection spacing and the 

correct progression speed (slope of the dashed lines in the figure), both directions of travel 

experience approximately equal bandwidth. 

Exhibit 5-17. Time-space diagram for corridor of conventional intersections.

Exhibit 5-18 shows the same hypothetical corridor, but with the two DDI crossovers at 

intersections 2 and 3. The exhibit illustrates that since the DDI crossover cannot display green 

for both crossover movements simultaneously, the southbound bandwidth is interrupted. The 

progression in the example favors the northbound direction. 
Diverging Diamond Interchange Informational Guide

87 

Exhibit 5-18. Time-space diagram for corridor with DDI.

The example illustrates that coordination is possible at the DDI, but is likely to favor one 

direction of travel. Signal offsets can be shifted in other periods to favor the opposing direction, 

or to provide partial progression for both directions. 

If separate controllers are used for each ramp terminal intersection, communication should be 

provided between them to prevent clock drift from disrupting coordination. 

Signal Spacing Considerations

The spacing between the signalized ramp terminal intersections has a direct effect on the 

capacity of the DDI. Shorter spacing between the two signals will tend to provide higher capacity 

as signals can be better coordinated, and less time is need to travel between crossovers. However, 

the shorter spacing also results in less space for vehicle queues and potential for spillback from 

the downstream crossover into the upstream crossover. 

Larger spacing increases the chance for demand starvation at the downstream crossover signal 

for through traffic. Exhibit 5-19 provides a theoretical example of the percent of unused time by 

through vehicles at the second crossover due to the travel time between crossovers. The graph is 

based on an assumed vehicle speed of 30 mph and shows three different cycle lengths (80, 120, 

and 160 seconds). As spacing is increased, the travel time between crossovers takes up an 

increasing portion of the cycle, assuming both signals turn green at the same time. In practice, 

parts of this unused time may be captured by setting signal offsets to match the travel distance, 

but the opportunity for unused time still increases with larger spacing. 
Diverging Diamond Interchange Informational Guide

88 

Exhibit 5-19. Example of percent demand starvation. 

COMPARATIVE PERFORMANCE STUDIES

Early DDI Studies

In early DDI evaluation studies, MoDOT conducted simulation analyses of the DDI and a 

conventional diamond.(35) Some of the operational advantages of the DDI were as follows: 

• The DDI was likely to double the throughput of the left-turn lanes

• The DDI design had more storage capacity between the ramp terminals (550 feet for the 

DDI compared to 350 feet for the compressed diamond) 

• The DDI allowed for simpler signal timing and geometry 

• The smaller ramp intersections in a DDI enabled shorter clearance times and delays

Chlewicki analyzed the DDI and compared results to the conventional diamond interchange.(36)

The operational analysis was performed using Synchro® to compare phasing and geometrics and 

SimTrafficTM for the simulation. Key findings were as follows: 

• Total delay for the DDI was three times less compared to a conventional diamond 

interchange

• Stop delay was four times less compared to a conventional diamond interchange 

• The total number of stops was approximately half as those of a conventional diamond 

interchange
Diverging Diamond Interchange Informational Guide

89 

Bared, Edara, and Jagannathan conducted a study of two different designs of DDIs.(37) Key 

operational findings of that study include: 

• For higher traffic volumes, the DDI offered lower delays, fewer stops, lower stop times, 

and shorter queue lengths as compared to the performance of the conventional design. 

For lower volumes, the performance of the DDI and conventional diamond intersections 

were similar. 

• Service volumes for all signalized movements were higher for the DDI as compared to 

the conventional diamond. The service volume of left-turn movements was twice that of 

the corresponding left-turn service volumes of the conventional diamond. This analysis 

indicated the DDI design was operationally superior to the conventional diamond because 

exclusive left-turn lanes were not necessary for the DDI. 

• If a conventional diamond had six lanes on the bridge section (two through lanes and one 

left-turn lane in each direction) and higher service volumes were needed, converting to a 

six-lane DDI instead of pursuing the more costly option of widening bridges and 

approaches to provide dual left lanes in each direction is viable. 

Before and After Field Results

In a before and after comparison of two DDIs in Kansas City, MO and Rochester, NY, an 

FHWA-sponsored study found significant reductions in delay by converting a conventional 

diamond interchange to a DDI. Exhibit 5-20 shows the before-after interchange delay 

comparison results for both sites.(14) 
Diverging Diamond Interchange Informational Guide

a) Kansas City, MO – AM Peak b) Kansas City, MO – PM Peak

c) Rochester, NY – AM Peak d) Rochester, NY – PM Peak

Exhibit 5-20. Before-after interchange delay comparison at two DDIs.(14)

90 

The Kansas City DDI showed the largest improvements through conversion with reductions of 

one to four LOS letter grades for directional movements between the before and after time 

periods. In the after period delay analysis, the interchange operated at LOS C or better for all 

movements, whereas several movements showed LOS F in the before period. 

At the Rochester DDI, during the before and after periods, all movements at the interchange 

operated at LOS D or better. Some movements experienced a better LOS in the before period, 

and some had a better LOS in the after period. Left turns from the arterial generally showed

reductions in delay between the before and after periods, although the delay for some other 

movements increased.
Diverging Diamond Interchange Informational Guide

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CHAPTER 6— OPERATIONAL ANALYSIS

The previous chapter presented operational characteristics unique to DDIs. To support decisions 

regarding the choice and design of a DDI, there needs to be an appropriate level of traffic 

operations analysis corresponding to the stage of the project development process. The level of 

analysis needs to be consistent with the available data, and that data needs to support the applied 

analysis tools. Vehicular traffic operations coincide with multimodal considerations. Final 

intersection configurations and associated signal timing should be in balance with multimodal 

needs for each unique project context. 

A DDI has two signalized intersections. As such, an operational analysis needs to consider the 

effects of and relationship between the multiple signals. 

Available data could include the following elements:

• Average daily traffic (ADT)

• Speed (posted, design, or 85th percentile)

• Weekday and weekend peak hour turning movement counts 

• Weekday and weekend off-peak turning movement counts 

• Pedestrian volume at the intersection

• Bicycle volume at the intersection

• Proportion of the traffic stream composed of heavy vehicles 

Measures of effectiveness are used to evaluate the operational efficiency of a particular design 

like the DDI. The FHWA Traffic Analysis Toolbox has identified the following seven basic 

measures of effectiveness:(38)

• Travel time: average time spent by vehicles traversing a facility, including control delay, 

in seconds or minutes per vehicle 

• Speed: rate of motion (expressed in distance per unit of time)

• Delay: additional travel time experienced by travelers at speeds less than the free-flow 

(posted) speed (expressed in seconds or minutes) 

• Queues: length of queued vehicles waiting to be served by the system (expressed in 

distance or number of vehicles) 

• Stops: number of stops experienced by the section and/or corridor (based on a minimum 

travel speed threshold) 
Diverging Diamond Interchange Informational Guide

92 

• Density: number of vehicles on a street segment averaged over space (usually expressed 

in vehicles per mile or vehicles per mile per lane)

• Travel time variance: a quantification of the unexpected non-recurring delay associated 

with excess travel demand (can be expressed in several ways)

The final two measures, density and travel time variance, are less applicable to an interchange

treatment than an uninterruped flow facility, but may still be considered during operational 

analysis. The most difficult performance measure to incorporate into DDI analysis is queuing. 

This is because the short spacing created between the intersections within the DDI may cause 

queue spillback when they are not coordinated. Individual performance measures such as queues, 

stops, and delay across multiple intersections of a typical vehicle progressed through the 

intersection provides more meaningful comparisons versus simply adding or averaging the 

performance measures from each intersection.

OPERATIONAL ANALYSIS TOOL OVERVIEW

According to FHWA’s Traffic Analysis Tools, several tools are available to analyze traffic 

operations at intersections, including the following:(38)

• Planning Level Analysis (such as critical lane volume and Cap-X)

• Highway Capacity Manual (HCM) Analysis

• Microsimulation Analysis

One major factor distinguishing these three types of analysis is the level of time required to 

evaluate each scenario. HCM analysis may take several times as long as planning analysis, and 

microsimulation is typically an order of magnitude greater than HCM analysis. Planning-level 

tools are useful in the initial feasibility analysis and to conduct a high-level comparison of the 

approximate number of lanes for a DDI. An operational analysis using a deterministic method, 

such as the HCM, is useful to perform a more detailed peak-hour performance analysis and to 

estimate performance measures like delay, travel time, and queue lengths.(9) The HCM analysis 

may provide insight on additional geometric design and signal timing details. Microsimulation is 

useful for analysis of interactions between intersections 

PLANNING-LEVEL ANALYSIS

Planning-level analysis methods are appropriate for conducting feasibility assessment, 

determining lane geometry, and comparing the operational performance of several interchange 

forms at a planning level. Planning-level analyses generally provide an assessment of the 

sufficiency of individual traffic signals at a DDI expressed in terms of capacity or volume-tocapacity ratio at the intersection level. Planning-level analysis methods generally do not estimate 

performance by individual lanes or approaches and they generally do not provide estimates of 

delay or queues. Planning-level analysis can be performed by hand or software.

Two principal tools and methods are available for planning-level evaluation of DDIs: (1) a 

critical-movement analysis implemented in FHWA’s Cap-X tools, and (2) an interchange type 
Diverging Diamond Interchange Informational Guide

93 

selection methodology in the HCM. Both are introduced below, and details can be found in the 

referenced documents. 

Critical Movement Analysis

The Capacity Analysis for Planning of Junctions (CAP-X) developed by FHWA is a tool that can 

be used to evaluate selected types of innovative junction designs (eight intersections, five

interchanges, and three roundabouts) using given peak flow volumes.(39) The intersections and 

interchanges are evaluated using the method of critical lane volume summation to provide 

planning capacity assessment at each crossing. The tool includes the ability to evaluate a DDI. 

Implemented in a spreadsheet workbook, CAP-X is a simple and cost-effective sketch-planning 

tool that will help users focus on more effective intersection/interchange designs prior to 

conducting more demanding traffic simulation. 

The inputs for the methodology include turning movement counts at both crossovers, heavy 

vehicle percentages, and the number of lanes at each point where two movements cross (the two 

crossovers, the two right-turn entry points from freeway, and the two left-turn entry points from 

the freeway). The method further allows for an estimation of impacts from a future-year growth 

rate. The outputs are the approximate v/c ratio at each of the six crossing points. Exhibit 6-1 is a 

screen capture from the spreadsheet that is downloadable from the Transportation Systems 

Institute website, A Federal Highway Administration Project in partnership with the 

Transportation Systems Institute at the University of Central Florida (note that Cap-X refers to 

the DDI as Double-Crossover Diamond, or DCD).(39) 
Diverging Diamond Interchange Informational Guide

94 

Exhibit 6-1. Cap-X Planning Level Tool Screen Capture.(39) 

Double Crossover Diamond Interchange (E-W)

Design and Results

Project Name: Cap-X Sample Evaluation Critical Lane Volume Sum

Project Number: 12345 < 1200 1200 - 1399 1400 - 1599

Location City, State

Date January 1, 2014

221 275 206 255 270 274

0.14 V / C 0.17 V / C 0.13 V / C 0.16 V / C 0.17 V / C 0.17 V / C

Note: This diagram does not reflect the actual lane configuration of the Intersection

≥ 1600

VOLUME / CAPACITY 

RATIO: 0.17

HCM Interchange Type Selection Procedure

An alternative planning-level method is provided in the Interchange Ramp Terminal chapter of 

the 2010 HCM. The Interchange Type Selection Procedure provides a method to quickly 

compare different interchange forms including a conventional diamond, tight diamond, singlepoint diamond, roundabout interchange, and several partial cloverleaf interchange options.(9) The 

DDI was added to that procedure through research by FHWA and is expected to appear in future 

releases of the HCM.

The HCM methodology is similarly based on the estimation of the sums of critical flow ratios 

through the interchange and their use to estimate interchange delay. A combination of simulation 

and field data was used to develop critical relationships for the methodology. Input variables for 

the methodology include interchange origin-destination (O/D) movements for the peak hour, the 

number of lanes for each movement, and the spacing between the two ramp terminals. Additional 

information is available in the 2010 HCM or the source report for the research.

(9)
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Advantages and Disadvantages 

The key advantage of planning-level analysis approaches are that the methods can generally be 

applied quickly and with minimum resources. The methods make use of simplifications and 

assumptions which may or may not be appropriate for any given condition. The resulting highlevel assessment of DDI performance is useful to determine the initial feasibility of a DDI, or to 

explore the necessary number of lanes based on a given set of design volumes. 

The key disadvantages of these approaches are an oversimplification of operational performance 

and a lack of consideration of signal timing and design details that may impact operations. 

Planning-level approaches do not consider time-of-day timing plans, coordination settings, or 

any of the unique signal timing attributes as discussed in Chapter 5. 

As a general rule, the planning-level results provide estimates of expected performance and are 

useful in informing the initial DDI feasibility and high-level design features. For example, if a 

planning-level analysis shows a v/c ratio of 1.3 for a DDI, it is unlikely that a more detailed 

operational analysis would show such a design to work satisfactorily. Similarly, if the planninglevel analysis shows a peak-hour v/c ratio of 0.3, it is likely that the number of lanes could be 

reduced in the design. For any planning-level results that are close to a given v/c threshold, a 

more detailed operational analysis should be performed. 

Special Considerations

An important consideration in the application of planning-level tools for DDIs is the assumed 

capacity of the DDI intersections. Field observations have shown that the capacity at the 

crossover approaches is slightly lower than at a conventional intersection approach for similar 

green-to-cycle ratios.(14) Similarly, the imbalanced lane utilization observed at many DDIs 

(external signal) reduces the potential throughput through the crossover. However, while these 

two DDI characteristics may suggest a reduction in the critical lane volume, the saturation flow 

and lane utilization effects are likely offset by reduced lost time in the two-phase signal

operation. 

HIGHWAY CAPACITY MANUAL (HCM) ANALYSIS

Highway Capacity Manual analysis tools are deterministic (similar to planning-level analysis 

tools) but provide more detailed performance measures such as delay, travel time, and queue 

lengths, and assess performance at a lane-group level as opposed to an overall intersection level. 

HCM analysis is performed using software but individual calculations can be checked by hand. 

An HCM procedure specifically for DDI intersections is under development by FHWA. The 

procedure will be included in an update of the 2010 HCM scheduled for completion in 2015. The 

DDI operational analysis methodology is planned to be incorporated in the HCM as part of 

Chapter 22: Interchange Ramp Terminals. That chapter further contains analytical methods for 

evaluating diamond interchanges, partial cloverleaf interchanges, roundabout ramp terminal 

intersections, and single point diamond interchanges. As such, the chapter and methodology will 

be well-suited for a high-level comparison of the DDI with other interchange forms. 
Diverging Diamond Interchange Informational Guide

96 

This section provides a high-level overview of the interchange LOS definition, computational 

steps, and input data requirements. For details, the reader is referred to the source chapter in the 

HCM.

(9)

Level of Service Definition

The LOS of an interchange in the HCM is based on the sum of the control delay for each O/D

movement. At a diamond interchange, a movement experiences delay at more than one location, 

and the individual delays are summed before determining the LOS. For example, a DDI through 

movement or left turn from freeway could experience delay at two signals, while a left turn to the 

freeway only experiences delay at the first crossover (provided no ramp signals or queue 

spillback at the second crossover for left turns are present). The thresholds for LOS A through F 

are based on the signalized intersection methodology in the HCM, but multiplied by a factor of 

1.5 (as opposed to 2.0) to reflect the fact that drivers tend to evaluate the two interchange signals 

as being part of the same system. The corresponding control delay thresholds are shown in 

Exhibit 6-2. That exhibit also shows that any movement receives an automatic LOS F if either 

the v/c ratio exceeds 1.0 or the queue-storage ratio, RQ, exceeds 1.0 (i.e. queue storage 

overflow). 

Exhibit 6-2. LOS criteria for DDIs (based on HCM Exhibit 22-11).(9)

O/D LOS

Control Delay 

(s/veh)

v/c < 1 and RQ < 1

for Every Lane Group

v/c > 1

for Any Lane Group

RQ > 1

for Any Lane Group

≤15 A F F

>15–30 B F F

>30–55 C F F

>55–85 D F F

>85–120 E F F

>120 F F F

Computational Steps

The basic methodology for operational analysis of a signalized interchange is shown in Exhibit 

6-3. The method contains up to ten steps, though not all may apply to every interchange. The 

steps are: 

1. Gather input data 

2. Estimate O/D flow rates from turning movement counts 

3. Adjust volumes for heavy vehicles and other factors 

4. Estimate lane utilization

5. Estimate saturation flow rate
Diverging Diamond Interchange Informational Guide

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6. Check queue length for internal links 

7. Estimate effective green adjustment due to interchange operations

8. Estimate effective green adjustment for adjacent intersections if applicable

9. Calculate capacity and v/c ratio

10. Estimate performance measures

The interchange methodology mirrors much of the HCM signalized intersection method, but 

with additional consideration for imbalanced lane utilizations, additional saturation flow rate 

effects, additional lost times due to downstream queues, additional lost time due to demand 

starvation, and additional lost times due to interactions with closely spaced intersections. The last 

three lost time effects translate into the effective green adjustments in steps 7 and 8 of the 

method. 
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Exhibit 6-3. HCM methodology for DDI evaluation (adapted from HCM Exhibit 22-14).(9)

Required Input Data

A variety of input data are required to apply the HCM methodology for the evaluation of a DDI. 

These generally fall into the categories of geometric conditions, traffic conditions, and 

signalization conditions. Exhibit 6-4 shows all input data needed for the evaluation of a DDI 

using the HCM methodology. 
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99 

Exhibit 6-4. Input data for HCM DDI evaluation (based on HCM Exhibit 22-15).(9)

Type of 

Condition Parameter

Geometric 

conditions

Area type

Number of lanes (N) 

Average lane width (W, ft) 

Grade (G, %)

Existence of exclusive left- or right-turn lanes 

Length of storage for each lane group (La, ft)

Distance corresponding to the internal storage between the two intersections in the 

interchange (D, ft)

Distances corresponding to the internal storage between interchange intersections and 

adjacent closely spaced intersections (ft)

Turning radii for all turning movements (ft)

Traffic 

conditions

Demand volume by O/D or turning movement (V, veh/h)

RTOR flow rates

Base saturation flow rate (s0, pc/hg/ln) 

Peak hour factor (PHF) 

Percent heavy vehicles (HV, %) 

Approach pedestrian flow rates (vped, ped/h) 

Approach bicycle flow rates (vb, bicycles/h)

Local bus stopping rate (NB, buses/h) 

Parking activity (Nm, maneuvers/h)

Arrival type (AT) 

Upstream filtering adjustment factor

Approach speed (SA, mi/h)

Signalization 

conditions

Type of signal control

Phase sequence

Cycle length (if appropriate) (C, s)

Green times (if appropriate) (G, s)

Yellow-plus-all-red change-and-clearance interval (intergreen) (Y, s)

Offset (if appropriate)

Maximum, minimum green, passage times, phase recall (for actuated control)

Pedestrian push button 

Minimum pedestrian green (Gp, s) 

Phase plan

Advantages and Disadvantages 

One of the advantages of the operational-level analysis approach in the HCM is that it balances 

operational detail with reasonable data input needs and analysis resource requirements. The 

HCM method provides more detailed output in the form of delays, travel time, and queue 

estimates than the planning-level method, while allowing for more customization and 

consideration of geometric variability and signal timing details. At the same time, its methods are 

typically applied more quickly than a more resource-intensive simulation analysis. 

Another key advantage of the HCM over simulation analysis is that the deterministic analysis 

framework offers consistency in performance estimation across analysts and interchange options. 
Diverging Diamond Interchange Informational Guide

100 

The HCM is generally regarded as the benchmark for operational performance estimation, and its 

equations and LOS stratification form the basis of comparison with other tools. While the HCM 

has limitations as discussed above, it does provide the consistency that agencies need for 

evaluating alternatives. The HCM is an international reference manual that is overseen by an 

independent committee of experts in the field, and thus is often the basis for policy decisions and 

LOS thresholds for interchange selection. 

Disadvantages of the HCM include a limited scope of applicable geometry and a lack of focus on 

network and system effects, including the interaction of the DDI with the freeway facility it 

serves. Other operational characteristics at the DDI not handled by existing HCM methodologies 

that may require simulation-based analysis include: 

• Queuing on the links between the two crossover signals 

• Demand starvation at signalized approaches leaving the DDI 

• Queue blockage of left-turn entrance ramp movements on shared lanes 

• Impact of reverse curves on speed patterns and progression 

• Estimation of pedestrian or bicycle level of service

• The effect of pedestrian and bicycle activity on vehicles 

• The interaction between freeway and arterial traffic if queues from one facility impact 

operations on the other 

Special Considerations

The DDI procedure in the 2015 HCM update will implement many of the operational 

considerations were discussed in Chapter 5. For example, the method will estimate saturation 

flow rates specific to DDIs, predict DDI lane utilization, and consider unique attributes of DDI 

signal timing in the performance estimation such as demand starvation and other adjustments to 

the effective green time. 

One unique attribute of the HCM DDI method is that the signal timing input will be simplified to 

assume the more simple two-phase design introduced in Chapter 5, which runs the right and left 

turns from the freeway concurrent with the respective through movement at the crossover. The 

method will allow the ability to evaluate the advanced use of overlap and dummy phases by 

adding a greater lost time to the respective turn phase. In essence, rather than extending the DDI 

through phase with the dummy phase, the HCM method will code the full overlap phase for the 

through movement, and then subtract parts of that phase duration for the right/left turn through 

an additional lost time. The resulting green times to cycle length (g/C) ratio and movement 

capacity will be unchanged. 
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Optimization

One of the key considerations in DDI evaluation in an HCM context, as well as in a 

microsimulation environment, is the question of traffic signal optimization. The HCM does not 

include a methodology for optimizing DDI traffic signals, requiring the use of other tools for 

optimizing DDI signal timing plans prior to implementation in the HCM or simulation 

environment. 

To overcome this challenge, analysts to date have relied on off-the-shelf signal optimization 

tools like deterministic software available from private vendors to arrive at signal timing 

parameters for DDIs. Many of these approaches generally relied on “tweaking” existing software 

tools to adapt to the unique characteristics of DDIs, as opposed to being specifically geared at 

DDI optimization. A few sources in the literature have focused specifically on DDI optimization

and more research is expected as DDIs become more common.(40)

MICROSIMULATION ANALYSIS 

Microsimulation tools employ a series of algorithms for car following, lane changing, and other 

parameters to model the movements and interaction of individual vehicles on a sub-second 

interval basis. Most of the simulation algorithms are stochastic in nature, meaning that they 

include one or more random variables and distribution of variables rather than a fixed 

deterministic input (e.g. for vehicle speed). For the evaluation of DDIs, simulation tools have 

been the status quo, as many were able to directly account for the unique geometry and signal 

timing of this interchange form. While other planning and operational tools have become 

available and can be highly useful in alternative selection and design refinement, simulation 

remains inherently suited for DDI analysis. Microsimulation analysis is performed exclusively 

using software. 

A variety of simulation tools are available to model and evaluate DDIs. All microsimulation 

tools vary in user interface and the list of available features; a full discussion is beyond the scope 

of this guide. Among the more critical features that are required to accurately model the DDI is 

the ability to replicate the crossover geometry and accurately code the DDI signal timing 

sequence. The analyst should further review the list of calibration factors and validation 

parameters described below to assure that the selected tool can adequately provide these. 

Advantages and Disadvantages 

Many analysts to date have turned to microsimulation to estimate the operations of new and 

planned DDIs. Simulation tools have the clear advantage as, in many cases, they allow for 

flexible customization and configuration of geometry, signal timing, and other operational 

parameters. This allows for the direct estimation of DDI performance, rather than approximating 

certain effects through equations that may have derived based on only a few sites. Another key 

advantage of simulation is that the DDI can readily be evaluated as part of a broader network of 

intersections, as well as with consideration of interaction effects between the surface street and 

the freeway. Many simulation tools allow the modeling and evaluation of different modes of 

transportation, and thus incorporate evaluation of pedestrian, bicycle, and transit modes, as well 

as their interaction with vehicular traffic. 
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102 

In addition to an improved operational evaluation and extended features list, a simulation 

analysis provides visualization of traffic patterns and street geometry, which can be an invaluable 

asset for communicating operations and geometry of the still relatively new DDI configuration to 

a non-technical stakeholder audience. Especially in areas where DDIs have not been used 

previously, animation files generated by simulation are key tools to communicate the proposed 

design and expected operations to the public. 

The greatest disadvantage of simulation is the increased resource requirements, as every DDI 

model needs to be built and configured. Another limitation of simulation is the need to calibrate 

and validate the effort, as well as the potential implications of failing to do so. The analyst needs 

to understand the many unique operational attributes of the DDI including saturation flow rate, 

speed profiles, lost time, etc., as well as how to replicate those in simulation. 

Related to the need for calibration and validation is an expected variability in the results of DDI 

evaluations performed by different analysts. This lack of consistency in output can be an 

important limitation of a simulation analysis, especially for DDIs with estimated performance 

close to some threshold. In this case, a benchmarked analysis method like the one provided in the 

HCM may be considered to further inform the decision-making process. 

Calibration Factors 

The key calibration factors that serve as inputs into the simulation are: 

1. Origin-destination volumes at the DDI and adjacent signals 

2. Look-back distances from route decision points to control lane positioning 

3. Field-measured free-flow speeds through the DDI, as well as geometrically-constrained 

free-flow speeds at the crossover and for turning movements 

4. Field-implemented signal timing schemes as obtained from field controller settings at the 

DDI and adjacent signals

Interchange O/D route percentages are no different at a DDI than at a conventional interchange. 

O/D routes should be drawn through the entire interchange and adjacent signals. 

Look-back distance is the distance upstream from a diverge point at which simulated vehicles are 

affected by the diverge and initiate any necessary lane changes. In field observations of DDIs, 

drivers were observed to pre-position themselves well in advance of the DDI for downstream 

turning movements.(14) This phenomenon was especially pronounced for left-turning movements 

from arterial to freeway and impacted lane utilization at upstream signals. Consequently, the 

look-back distance for these movements should be specified in a way that it extends through the 

upstream signals. Exhibit 6-5 illustrates the look-back distance concept. 
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103 

Exhibit 6-5. Concept of look-back distance in simulation.

For calibration, speeds at DDI crossovers were observed to be below the free-flow speeds on 

tangent sections of the arterial, as discussed in Chapter 5. It is therefore recommended to use 

speed reduction zones to control free-flow speed at crossovers and turns. The speed distributions 

should be modeled as normal distributions with mean and standard deviation estimated from 

field data or adapted from Chapter 5. 

Exhibit 6-6 shows the use of reduced speed areas to model slow speeds at turns and crossovers, 

and desired speed decision to switch between the approach speed limit and the generally lower 

observed speeds throughout the DDI. 
Diverging Diamond Interchange Informational Guide

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Exhibit 6-6. Simulation speed settings.

To accurately model signalized control of the DDI interchange, the analyst needs to explore 

whether the interchange is modeled with one versus two controllers. The selected tool should 

employ signal control logic that is flexible enough to allow modeling of two-controller twophase signal control, as well as four rings on a single controller. Details on DDI signalization

schemes are given in Chapters 5 and 8. 

Validation Parameters 

Three key validation parameters are recommended for accurately modeling existing DDIs in 

simulation: 

1. Interchange travel times, as defined by travel time segments through the two DDI signals 

(through routes), as well as left-turning routes through the DDI (to and from the freeway) 

2. Route travel times, as defined by travel time segments through the DDI and adjacent 

signals for through movements, left turns from freeway to arterial, and left turns from 

arterial to freeway

3. Comparison of average and 95th percentile queue lengths, estimated from maximum 

queue lengths on a per-cycle basis
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The interchange travel time includes the two DDI signals and any queues immediately upstream 

of the DDI. The route travel time segments include at a minimum the adjacent signals upstream 

and downstream of the DDI. For left-turn routes, the travel time segments start or end at the top 

of the freeway exit ramp or entrance ramp, respectively. 

The travel time data can be collected from travel time runs during field visits. A floating car 

technique should be employed to assure the travel time vehicle is representative of the travel 

stream. For the queue measurements, cycle-by-cycle queues can be observed through manual 

observation in the field on a per-lane basis. Detailed operational study protocols are documented 

in the FHWA DDI study.(14)
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CHAPTER 7— GEOMETRIC DESIGN

This chapter describes the typical DDI design approach and provides guidance for geometric 

features. It requires input from the multimodal considerations (Chapter 3), safety assessment 

(Chapter 4), and the traffic operational analysis (Chapters 5 and 6). The guidance in this chapter 

is intended to supplement national resources on intersections that apply basic design principles.

DESIGN APPROACH

The geometric design of a DDI requires balancing competing objectives. Street geometry is 

intended to result in consistent speeds through interchange crossovers and ramp movements. The 

majority of the geometric parameters are governed by the design vehicle requirements. 

Therefore, designing a DDI requires carefully considering safety, operations, and geometric 

performance while accommodating the design vehicle and non-motorized users.

The design objectives for any interchange can be different and depend upon the surrounding 

environment and project context. At urban locations, right-of-way (ROW) footprint, access 

management in the vicinity of the interchange, and pedestrian and bicycle considerations will 

likely influence many of the project design decisions. However, the cross street approach speed 

in the interchange influence area is likely established by the street. For rural locations, ROW is 

likely less constrained by adjacent land uses and there may or may not be pedestrian or bicycle 

facilities. The design vehicle and approach speeds may be relatively higher, compared to urban 

locations. Although location does govern some design decisions, the majority of decisions are 

independent of location. This chapter provides design guidance based on existing DDIs, relevant 

resource manuals, and the state-of-the-practice from experienced street design professionals. The 

primary focus of this chapter will be on the intersection crossover as this is the most defining 

characteristic of the DDI. A key secondary focus will be the cross street ramp terminal 

intersections.

Exhibit 7-1 DDI characteristics. 
Diverging Diamond Interchange Informational Guide

108 

The contents of this chapter are intended to serve as guidance and should not be interpreted as a 

standard or rule. As with all street designs, designing a DDI is an iterative process where a 

variety of design objectives must be considered and balanced within site specific constraints and 

project context. Maximizing the operational performance and safety for a DDI, while 

understanding the potential ROW and access management constraints, results in solutions 

adapted to each project’s unique location. Throughout this chapter, ranges of typical values will 

be provided for many geometric elements. However, deviating from these values does not 

automatically create a fatal flaw or unsafe condition provided that the design principles and 

optimized operational performance for each user can be achieved.

GEOMETRIC DESIGN PARAMETERS

This section describes several design constraints common to a DDI. A multitude of topics 

surface when DDIs are considered in the planning and design stages; however, this section 

focuses on three unique areas that can readily be addressed in the early design stages. First, the 

decision to build over or under the limited access facility can affect the safety and operation of 

various transportation modes. Second, ROW constraints may limit a designer’s ability to provide 

desired vehicle alignments through the crossover. Last, and not unique to the DDI, is the 

proximity of adjacent signalized intersections to the interchange crossover movements. Nearby 

adjacent intersections have been found to hinder the ability to provide traffic to or receive traffic

from the DDI as efficiently as it was intended.

Overpass versus Underpass

The interchange design will be directly affected by whether or not the arterial passes over or 

under the limited access facility. In most cases, DDIs with a cross road designed as an overpass 

offer the most design flexibility in serving pedestrians. With a cross road passing under the 

highway, bridge abutments and columns can impede sight distances. Horizontal alignments and 

curb radii may also be constrained by bridge and retaining wall elements. For new bridge 

projects, this factor could be considered during the preliminary design stage when analyzing 

existing grades of the limited access facility and cross road. However, the majority of DDIs

evaluated have reconstructed the existing diamond interchanges, and the decision to go over or 

under the limited access facility had already been determined. 

Overpass Designs

Exhibit 7-2 provides several cross-sections observed at cross road overpasses. Overpass designs 

can use a single, dual, or even triple bridge structure. Most observed overpass DDIs use an 

existing single bridge design, as seen in examples A–D. A single overpass is frequently the least 

cost overpass design option as it usually uses an existing bridge. The center crosswalk is more 

feasible with an overpass as there are rarely obstructions that prevent its design. Center 

crosswalk designs for underpasses would require clear span limited access bridges (which has no 

center columns). Where mast arms are used for signalizing exit ramp movements, the mast can 

be accommodated in the center median if sufficient space is available in the crosswalk (example 

B). If sufficient space is not available, an option could be to install the mast on the outside of the 

structure (example C). 
Diverging Diamond Interchange Informational Guide

109 

A) Center Walkway, Single Structure B) Center Walkway, Single Structure

MO 13 - Springfield, MO Botts Rd. – Branson, MO

C) Center Walkway, Single Structure D) Center Walkway, Single Structure

Ashford Dunwoody Road – Dunwoody, GA MO 248 – Branson, MO

E) Outside Walkways, Dual Structure

S 500 E St. – American Fork, UT

Exhibit 7-2. Examples of various cross sections utilized at overpasses in the U.S.(26)

Another advantage of an overpass design is the ability to add lanes to existing facilities by 

adding a parallel structure. This was the case in several designs in Utah (example E). An 

additional structure also provides distance for a tangent section between reverse curves. This 

becomes easier for the designer as the distance between the two through movements is increased. 

Crossover alignments are discussed in more detail later in this chapter. To date, DDI facilities 

with multiple bridges have provided pedestrian facilities along the outside of the DDI; however, 

center walkway designs are also possible in those cases.
Diverging Diamond Interchange Informational Guide

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Underpass Designs 

Underpass facilities tend to have less flexibility in their design. At an underpass facility, the ROW

between the two crossover intersections is directly affected by any components of the bridge 

substructure. Since most U.S. DDIs are reconstructed from conventional diamond interchanges, 

the substructure components cannot be relocated. For instance, available ROW in the center of the 

street may be impacted if columns with interior bent caps are designed (or already exist) in the 

median or outside shoulders of the cross road. For pedestrians, this poses limitations on locations 

for the pedestrian facilities (if at all) and, in the case where columns are in the median, limit the 

placement of lanes if unbalanced designs are under consideration. 

Several illustrations of cross road underpass designs are provided in Exhibit 7-3. These show 

varying examples of column designs directly affecting the available ROW for vehicle lanes and 

pedestrian and bicycle facility considerations. Examples A and B provide limited ROW for 

vehicular traffic lanes. Pedestrian facilities were limited or non-existent. Examples C and D 

provide sufficient ROW for vehicular traffic and pedestrian facilities. Pedestrians were routed 

through the center as no columns were present in the median. Example D also provides some 

accommodation for bicycle lanes within the ROW. Examples E, F, and G each show examples of 

pedestrian facilities along the outside. Note the varying use of barrier protection between 

vehicles and pedestrians ranging from no protection or landscaping strip in example E to fencing 

and concrete barriers used in examples F and G, respectively. 
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A) No Walkway, Columns Outside B) Single Walkway Outside, Columns in Median

SR-92 – American Fork, UT Thurbers Ave. – Providence, RI

C) Center Walkway, Columns Outside D) Center Walkway, Bicycle Lane, Columns 

Front St. – Kansas City, MO Outside

Moana Ln. – Reno, NV

E) Outside Walkways w/ No Barrier, No Columns F) Outside Walkways w/ Fencing, Columns in 

Winton Rd. – Rochester, NY Median

Dorsett Rd. – Maryland Heights, MO

G) Outside Walkways w/ Barrier, Columns in 

Median and Outside

Harrodsburg Rd. – Lexington, KY

Exhibit 7-3. Examples of various cross sections utilized at underpasses in the U.S.(26)
Diverging Diamond Interchange Informational Guide

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Right of Way 

DDI configurations are similar to conventional diamond forms, but may have slightly different 

area or footprint requirements. In some cases, the DDI requires additional area and in others, 

less. Exhibit 7-4 provides a sample overlay of a DDI design over a conventional diamond 

interchange. The figure highlights footprint differences between a conventional diamond and 

DDI. Note that street footprint differences do not correspond to right-of-way differences if the 

area is between ramps and thus within the overall interchange footprint area. 

Exhibit 7-4. Conventional diamond interchange overlaid with a DDI design.(13)

DDIs at reconfigured conventional diamond forms rarely require additional ROW. However, 

they may create conflicts with existing utilities, sidewalks, or drainage. The three areas differing 

between DDIs and conventional diamond forms are described below: 

1. Left-turn storage between intersections: The DDI eliminates storage needs for exclusive 

left turn lanes on the cross road that are necessary at a conventional diamond interchange. 

This area is often used for supplementary lanes or updated facilities dedicated to 

pedestrians or bicycles when an existing conventional diamond is converted to a DDI and 

the existing bridge is reused. 

2. Left-turn curve radii onto the limited access facility: Because vehicles travel on the left 

side of the road between the ramp terminal intersections, the design vehicle radius must 

be accommodated for the left turn onto the ramps. This requires additional pavement and 

potentially additional bridge structure in this area. Because this area is within the 

interchange, it does not require acquisition of additional ROW. 

3. Right-turn curve radii onto the cross road: Right-turning vehicles off the ramps may 

require a sharper angle of approach compared to the conventional diamond because 

drivers should be aligned perpendicular to the outbound crossover lanes if the right-turn 
Diverging Diamond Interchange Informational Guide

113 

movement is yield-controlled or if it is signalized and RTOR is allowed. A sharp 

approach angle provides a sight line helping to deter drivers from looking down the 

wrong approach for conflicting vehicles. 

Other ROW differences exist when comparing a DDI to a tight diamond (TDI) or skewed 

diamond interchange. A TDI generally has signalized ramp terminal intersections spaced 250 to 

600 feet apart, and they are often relatively close to the interchange bridge.(41) Skewed 

interchanges with ramp terminal intersection spacing greater than the TDI have a decreased 

effective spacing because the distance between the intersections is reduced to account for vehicle 

swept paths and resulting stop bar locations. In either case, a DDI retrofit design may require 

locating crossover intersections further away from the bridge structure. This may be needed to 

accommodate the minimum curve radii and tangents through the crossovers to transition traffic 

to the opposing side of the road. In rural or suburban areas where ROW is sometimes more 

available, relocating the ramps may be possible. In confined urban areas or other constrained 

sites, it may be more difficult to implement a DDI. 

Access Management Considerations

Access management is addressed in detail in Chapter 2, and some specific issues relative to the 

DDI are also discussed in this section. 

Adjacent intersections considerations are common when constructing new or reconstructing 

existing interchange configurations and should be assessed by those considering, implementing, 

and operating DDIs. In particular, the transportation professional should consider how the DDI 

will impact operations and safety. 

From an operational perspective, the DDI’s two-phase signals often provide higher throughput 

than can be provided from or served by adjacent signals. These adjacent signals infrequently 

have two-phase signal control and are, relatively, less efficient than the DDI signalized ramp 

terminal intersections. This, combined with what can sometimes be the relatively close proximity 

of the adjacent intersection, provides inadequate queue storage at the adjacent intersection, which 

can result in queue spillback into the DDI. Similarly, adjacent signals with more than two phases 

may, in some conditions, not have the efficiency to serve the available capacity of the two-phase 

signals at the DDI.

To the motoring public, the DDI design may appear to be ineffective when, in fact, the DDI is 

operating as intended. The distance between the DDI ramp terminal intersections can also 

exacerbate weaving conditions that arise from motorists turning right from an exit ramp and 

weaving across traffic to make a left at the adjacent intersection. Similar weaving can occur from 

through traffic on the cross street weaving across right-turning traffic from the DDI exit ramp. 

This can potentially be countered by not permitting RTOR operations at the exit ramps. 

These operational conditions are similar to those of a conventional diamond interchange form. In 

new construction conditions, adjacent access spacing may trigger supplemental traffic operations 

analysis beyond the ramp terminal intersection analyses. These analyses results may help inform 

access management decisions and intersection spacing.
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114 

Transportation agencies considering the DDI at locations with nearby signalized intersections 

and congested cross roads have made geometric and signal design modifications to nearby 

intersections. Some potential geometric treatments that may improve operations (and potentially 

also safety performance) include:

• Closing the closest signalized intersection or converting it to unsignalized right-in/rightout control. These treatments were used at Dorsett Road in Maryland Heights, MO. 

• Using grade separation to eliminate one or more movements at the adjacent intersection. 

This treatment was used at National Avenue in Springfield, MO where a left turn into a 

hospital was modified to take a right, followed by another immediate right turn, leading 

to an undercrossing passing under the cross road and accessing the hospital. 

• Alternative intersection designs could potentially be used to reduce the number of 

necessary signal phases at adjacent intersections along the corridor. This treatment has 

yet to be used in practice; however, an entire corridor of alternative intersection designs 

is being designed along Poplar Tent Road in Charlotte, NC. 

GEOMETRIC DESIGN PRINCIPLES

Several overarching principles guide users in conceptualizing and designing DDIs. The 

following principles may support DDI concept development, considering the context of the 

interchange and nearby adjacent intersections:

• Accommodate design vehicles at the crossover ramp terminal junctions 

• Promote reduced and consistent design speeds (25-35 mph) through the interchange 

• Channelize inbound and outbound movements in the crossover design at each 

intersection to encourage drivers to use the intended lanes. Create a vehicle path 

alignment that directs vehicles into appropriate receiving lanes 

DDI concept design involves balancing and optimizing trade-offs associated with user 

performance, capacity, costs, maintenance, and construction staging, among other items. For 

instance, considering heavy vehicle design at the crossover may lead designers to contemplate 

larger design radii or wider lanes; however, this could promote higher speeds through the 

crossover for other vehicle types. Instead, to provide adequate facilities for the design vehicle 

while maintaining safe speeds for other motorists, designers may want to consider designs 

resulting in offsetting one or more of the approaches to the DDI. This method may increase street

alignment radii resulting in comparatively narrower lanes to serve design vehicles over tracking.

Design Vehicle

DDI configurations are affected by the need to accommodate the largest vehicle likely to use the 

interchange. Turning path requirements for this vehicle, referred to as the design vehicle, will 

dictate many of the dimensions of the DDI. Selecting the design vehicle and determining the 

corresponding swept paths using turning templates or a CAD-based vehicle turning path program 

will establish lane widths and channelization configurations. 
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Three key areas of the DDI are directly affected by the design vehicle: 1) through movements at 

the crossover, 2) left turns at the exit ramp, and 3) right turns at the exit ramp. The first two areas 

are unique to the DDI with respect to interchange design; however, they build on concepts used 

for designing other street facilities such as roundabout and one-way street designs. The third 

area, right turning vehicle paths, is not unique to DDIs. 

The choice of design vehicle will vary depending on the cross road facility type and surrounding 

environment. Most often, state or local agencies and project stakeholders help determine the 

appropriate design vehicle for each site. The most common design vehicle used at DDI facilities 

is the WB-67. In more urbanized areas, it may be more appropriate to use smaller design vehicles 

such as the WB-62 or WB-40. At a minimum, fire trucks, transit vehicles, and single-unit 

delivery trucks should be considered in urban areas. In rural areas, farming or other construction 

vehicle types may govern design vehicle needs. 

In some cases, it may be appropriate to consider different design vehicles for different 

approaches. For example, there may be oversized and overweight vehicles travelling certain 

routes through the interchange. These larger vehicles would need to be accounted for in the 

design of certain movements, with the balance of the movements designed to serve a smaller 

design vehicle. AASHTO Green Book provides dimensions and turning path requirements for a 

variety of highway vehicles.(8)

Oversized vehicles may need to be considered at a DDI just like at any other interchange form. 

These vehicles often require a special permit to travel on the street; however, if they are expected 

to use the DDI, special consideration should be given to geometrics; signal height, placement, 

and installation; and, most importantly, to the structural soundness of the facility. Dimensions for 

special vehicles can be established by working with local haulers or the industry or agricultural 

enterprises served by those vehicles. CAD-based turning path programs allow the designer to 

customize the electronic template for these vehicles. An example of an oversized load making a 

left turn onto I-44 from MO 13 in Springfield, MO is provided in Exhibit 7-5. 

Exhibit 7-5. An oversized load making a left turn onto I-44 at MO 13 (Springfield, MO).(16)
Diverging Diamond Interchange Informational Guide

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Design Speed

Design speed at a DDI affects the reverse curve radii and configuration through the two 

intersection crossovers. The crossovers’ chief purpose is to create the contraflow operations 

between the ramp terminal intersections that reduce signal phases for conflicting approaches. 

Target crossover angles of no less than 45 degrees facilitate efficient passage through the 

crossover. The angle and resulting design is a product of considering ROW constraints outside 

the DDI and available cross-section over or under the bridge. Reverse curves provide the 

transition from parallel to conflicting through movements. These curves provide the necessary 

approach angle through the crossover, but also act as a traffic-calming device to control speeds. 

The cross street and exit ramp left- and right-turn speeds should also be considered; however, 

their design is generally understood given the similarities to other interchanges. Factors directly 

influencing the design speed selection of a DDI are traffic volumes, percentage of trucks, ROW,

and other existing safety performance and site context conditions. The assumed speeds for each 

of these movements will determine the minimum turning radius for each location. For the 

crossover movements, traffic operations and safety performance will benefit from designing for 

speeds ranging from 25 to 35 mph. For exit ramp turning movements, considerations are similar 

to other service interchange forms and include pedestrian crossing conditions and intersection 

and stopping sight distance. 

Additional information related to design speed can be found in the Design Guidance section of 

this chapter. 

Crossover Design

State DOTs recommend crossover angles of no less than 45 degrees between opposing 

approaches. Research findings indicate a higher correlation between lower crossover angles and 

the likelihood for increased wrong-way maneuvers into opposing lanes. This is especially 

apparent at sites where the predominant movements are left turns on and off the limited access 

facility. Exhibit 7-6 shows vehicle paths through a crossover. 

Several factors influencing crossing angles:

• Wrong-way maneuvers: Minimizing the likelihood of a wrong-way maneuver into 

opposing traffic is a key consideration in DDI design. The greater the crossing angle, the 

less the intersection will appear different than a conventional location. Minimizing skew 

angle is a common objective at any intersection type.

• Right-of-way constraints: The surrounding environment will influence a DDI 

configuration. For instance, a reconstruction design may be constrained by bridge 

abutments and built-out developments on either side of the crossover. These constraints 

can make it difficult for designers to attain reverse curve crossover angles of 45 degrees 

or greater. 

• Driver discomfort: Greater crossing angles require corresponding reverse curves. Smaller 

curve radii increase traffic calming effects and promote reduce speeds. Overall speed 
Diverging Diamond Interchange Informational Guide

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profiles approaching, navigating crossovers, and departing the interchange ideally result 

in speed reductions between successive movements of less than 15 to 20 mph.

• Exposure: As with any skewed intersection, larger crossing angles decrease the amount 

of time that a vehicle is exposed to conflicting traffic and reduce the potential for angle 

collisions.

• Heavy vehicles: Greater crossing angles will increase the potential for overturning and 

centripetal forces acting on the driver. Minimizing speed reduction differences between 

successive geometric elements can mitigate this. Horizontal alignment upstream of, 

through, and departing DDIs that provides smooth and consistent speed transitions will 

better serve all motor vehicles. 

Path alignment at the crossovers should clearly direct vehicles to the receiving lanes. Reverse 

curves between crossovers should include sufficient tangent length between curves to provide a 

direct alignment. Curve radii in the middle of the crossover movements place the point of 

curvature or tangent in the intersection where drivers do not typically turn. A lack of tangents 

between reverse curves or indirect path alignments can lead to vehicle path overlap, or even 

worse, inadvertently guide motorists into opposing traffic. This is especially true for vehicles at 

rest behind the stop bar waiting for a green light. 

Exhibit 7-6. Vehicle paths through crossover. 

Additional information related to crossover design can be found in the Design Guidance section 

of this chapter. 

ALIGNMENT ALTERNATIVES

This section discusses various alignment alternatives for the DDI and the required intersection 

crossover separation for each alternative. The location of the two crossovers is highly dependent 
Diverging Diamond Interchange Informational Guide

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upon the spacing and location of the ramps. Queue storage needs must also be addressed. There 

is flexibility in placing crossovers when ROW is available; however, if more or less distance is 

needed between crossovers due to queue storage needs, reverse curve design, skew angle, or 

other constraints, the ramp entrance or exit locations will be affected. 

Minimizing the distance between crossovers can positively enhance traffic operations by 

increasing the distance to nearby signals. Reduced distances can also limit needed ROW. DDIs 

are typically designed using a symmetrical alignment. Several other options exist that can 

provide minimum cross-sections over or under a bridge or minimum distance between crossovers 

when ROW is at a premium. 

Minimize Cross Sections Over or Under a Bridge

There are three alignment alternatives resulting in a minimum cross-section along the cross road 

regardless of whether the facility is an over- or underpass. These are presented graphically in 

Exhibit 7-7. These alternatives are common to reconstructing an existing conventional diamond 

interchange with limited cross-sections between the bridge abutments. Reducing the median 

width on the cross street is a way of minimizing the cross-section of the over- or undercrossing. 

Two specific alignment methods can be used to create alignments to reduce cross road crosssection. These alignments use four reverse curves in each direction at each crossover, as shown 

in Exhibit 7-7: 

• Symmetrical Alignment (A): A symmetrical alignment where both streets are deviated to 

achieve the 45 degree crossover and then returned to the minimum cross-section. As 

noted earlier, this design has been used at constructed DDIs. Reducing the cross road 

cross-section requires curvature combinations to narrow the cross road between crossover 

intersections. 

• Shifted Alignments (B/C): This design approach shifts the alignment of one of the cross 

road travelled ways to asymmetrically to achieve the 45 degree angle at each crossover 

intersection. This can be accomplished by shifting one of the streets to either side of the 

cross road projected centerline. These alignment alternatives are used, as applicable, to 

adapt to site constraints or ROW restrictions. This approach may also facilitate traffic 

maintenance during construction by using the existing cross road while a parallel bridge 

is constructed. 
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Exhibit 7-7. Alignment alternatives that minimize cross-sections over or under a bridge.(14)

Minimize the Distance between Crossovers and Amount of Reverse Curvature

Reverse configurations that minimize cross-section between crossovers typically have at least 

four reverse curves in each direction at each crossover. The number of curves and providing 

needed tangents between reverse curves increases the overall spacing requirements between 

crossovers. Eliminating some of reverse curves reduces driver work load and allows shorter 

spacing. With a wider median, the number of reverse curves between the cross overs can be 

minimized. In new construction or reconstruction where sufficient width exists across the limited 
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120 

access facility, a wider median between crossover intersections can minimize the alignment 

deflection (amount reverse curvature) and the number of curves. These alternatives achieve 

minimum distances by reducing the number of reverse curves necessary between crossovers. 

There are five basic alternatives to minimize the distance between crossovers on the cross road. 

These alignment approaches result in as few as two reverse curves in each direction at each 

crossover. The alternative approaches are presented graphically in Exhibit 7-8. Using two reverse 

curves per crossover provides a spacing of approximately 400 to 500 feet between the 

crossovers. This assumes the angle between the limited access facility and cross road is close to 

90 degrees. Removing the number of reverse curves reduces the opportunity for vehicle over 

tracking, path alignment issues, and driver workload. 

The alternatives to minimize the distance between crossovers and amount of reverse curvature, 

as shown in Exhibit 7-8, are described as:

• Symmetrical Alignment (A): The alignment is symmetrical to the cross road centerline

with both travelled ways of the cross road realigned evenly to achieve the 45-degree 

crossing. 

• Offset Alignments (B/C): One of the cross road travelled ways remains on the cross street 

centerline alignment. The other cross road travelled way is realigned to one side of the 

cross road centerline to achieve the 45-degree crossover. Either of these alignment 

alternatives may be used depending upon ROW availability. 

• Shifted Alignments (D/E): The alignment alternatives shift both cross road travelled way 

alignments to either side of the cross road centerline to achieve the 45-degree crossover. 

These may be used where there are ROW constraints on one side of the cross road or the 

other. This concept is also applicable when the ROW constraints are in opposite 

quadrants on either side of the limited access facility. 
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Exhibit 7-8. Alignment alternatives to minimize the distance between crossovers and 

amount of reverse curvature.(14)
Diverging Diamond Interchange Informational Guide

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These alignment alternatives would be most appropriate with new construction, unconstrained 

reconstruction projects, and where the crossover intersections are located as far as possible from 

adjacent intersections. These approaches are most commonly used in reconstructed projects with 

a cross road overpass configuration. In these conditions, an additional bridge is built parallel to 

the existing structure to provide the necessary space between the crossovers. 

Alignment A of Exhibit 7-8 displays the center lines of the cross road travelled ways with crosssectional dimensions and the distance between the crossover intersections. These dimensions 

apply to all alternative designs (alignments A-E) discussed in the exhibit. Based on the 

assumptions used, centerline to centerline dimensions require approximately 75 to 80 feet. These 

values may increase or decrease depending on treatments and dimensions outside the edge of 

pavement (EOP). 

To better understand the spacing required between the outer bridge abutments, the horizontal 

alignment and cross-sections are provided in Exhibit 7-9. The cross-sections shown include the 

cross road over or under the limited access facility. The actual dimensions will vary based on 

lane widths, shoulder width, and bridge parapet wall design. 
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Exhibit 7-9. Example of horizontal alignment and cross-sections for under and overpass 

designs that minimize the distance between crossovers.(14)

For this specific geometric design, the distance between the crossover intersections is 410 to 470 

feet depending upon the bridge length or the width of the limited access facility over or under the 

cross road. These ranges are provided to begin conceptual planning, and actual values will be 

determined via design refinements. With the cross road over the limited access facility, the cross 
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124 

road may be on two separate bridges. With the cross road under the freeway, there is ample 

space for a pier between the two travelled ways protected by either a guard rail or concrete 

barrier. Pedestrians can be accommodated on the outside or median; however, if an overpass 

design is utilized with multiple bridge structures the median treatment could be more 

problematic. Bicycle lanes can be added with widths ranging from 5 to 7 feet for those abutting 

the median barrier between the crossovers. More guidance on these facilities is provided in 

Chapter 3. 

Exhibit 7-10 provides an example of a DDI design at Dorsett Road in Maryland Heights, MO. 

This interchange uses a symmetrical design to minimize the distance between crossovers. ROW

was constrained in all quadrants of this DDI. The cross road and freeway intersect at an angle of 

approximately 80 degrees, providing a total approximate distance between the interchange 

crossovers of 500 feet. For comparative purposes, this would translate to a distance of 490 feet if 

perpendicular. 

Exhibit 7-10. Symmetrical alignment with no reverse curves between crossovers (Dorsett 

Rd., Maryland Heights, MO).

(26)

The cross-sectional distances for the example provided above are given in Exhibit 7-11. The 

cross-sectional distances between the edge of pavement and the outer barrier walls align well 

with the horizontal alignment example provided in Exhibit 7-9, which both provide three travel 

lanes in each direction.
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Exhibit 7-11. Cross-sectional distances for a symmetrical alignment with no reverse curves 

between crossovers (Dorsett Rd., Maryland Heights, MO).

(26)

Exhibit 7-12 provides an example of a DDI design at Pioneer Crossing in American Fork, UT. 

This cross road overpass example minimizes the distance between crossovers. Right-of-way was 

constrained in both quadrants north of the DDI; however, ROW was available to the south to 

accommodate the interchange crossovers. An alignment shift of approximately 35 to 40 feet to 

the south was used to take advantage of the available ROW. The cross road and freeway intersect 

at an angle of approximately 28 degrees, providing a total approximate distance between the 

interchange crossovers of 920 feet. For comparative purposes, a perpendicular crossing would 

have resulted in a crossing distance of approximately 430 feet. 

Exhibit 7-12. Shifted alignment south of centerline (red) with no reverse curves between 

crossovers (Pioneer Crossing, American Fork, UT).

(26)

The cross-sectional distances for the example provided above are given in Exhibit 7-13. The 

cross-sectional distances between the edge of pavement and the outer barrier walls align well 

with the horizontal alignment example provided in Exhibit 7-9, with the primary difference being 

the number of lanes; four lanes are present in both directions of travel at Pioneer Crossing 

instead of three. 
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Exhibit 7-13. Cross-sectional distances for a shifted alignment with no reverse curves 

between crossovers (Pioneer Crossing, American Fork, UT).(26)

DESIGN GUIDANCE

While the previous sections provided general geometric parameters and principles related to the 

DDI, this section provides more specific guidance to assist designers in configuring DDI 

components. The primary source for geometric design guidance is the AASHTO Green Book.

(8) 

This chapter augments AASHTO Green Book guidance to support decisions with respect to 

DDIs.

Design Speed

The relationship between horizontal curvature and travel speed is documented in the AASHTO 

Green Book. The predicted speed associated with minimum radii can be determined using the 

equations provided below. The equations apply a simplified relationship between speed and 

radius based on the most common superelevation rates of +0.2 and -0.2 and the corresponding 

side friction factors based on assumed speeds. 

V = 3.4415R0.3861, for e = + 0.02 

V = 3.4614R0.3673, for e = - 0.02 

where 

V = predicted speed, mph 

R = radius of curve, ft

e = superelevation, ft/ft
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127 

Exhibit 7-14 provides a quick reference of the speed-curvature relationship for both 

superelevation rates.

(42) For DDIs, it is reasonable to assume the superelevation rates and side 

friction factors used below will provide a reasonable estimate of speeds at crossover and ramp 

movements. 

Exhibit 7-14. Speed-radius relationship.(42)

40

35

30

h)

25

d (mp

20

epeS15

10

5

0

0 50 100 150 200 250 300 350 400

Radius (ft)

e=+0.02 e=-0.02

Three key areas of the DDI are directly affected by the design vehicle: 1) through movements at 

the crossover, 2) left turns at the exit ramp, and 3) right turns at the exit ramp. The first two areas 

are unique to the DDI with respect to interchange design; however, they build on concepts used 

for designing other street facilities such as roundabout and one-way street designs. The third 

area, right turning vehicle paths, is not new to DDI. 

For scenario 1, it is desirable for through movements to progress though the crossover of a DDI 

at 20 to 30 mph without encroachment of vehicles on adjacent lanes. This corresponds to 

minimum curve radii in the range of 100 to 300 feet, respectively. For designs minimizing the 

spacing between crossovers, a larger median width is required as there are no reverse curves (just 

the reverse curve of the crossover itself) on the closest side to the bridge abutments. This 

additional space is sometimes used to accommodate larger turning radii, which in turn requires 

smaller lane widths to accommodate design vehicle swept paths. The design speeds usually range 

from 25 to 35 mph, which correlates to minimum curve radii of approximately 175 to 400 feet, 

respectively. 

Field observations at five DDI sites documented average free-flow speeds through the crossovers 

for inbound and outbound movements ranging from of 22.3 to 31.1 mph. This corresponds to 

curves with radii between 180 to 350 feet.(14) Based on these findings, where space is available, 
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128 

design speeds upward of 35 mph can be used while providing narrower lane widths to serve the 

design vehicle swept path. 

For scenarios 2 and 3, turning movements to and from ramp terminal intersections typically 

accommodate slower speeds in the realm of 15 to 20 mph, correlating to curve radii ranging from 

approximately 50 to 100 feet. These design speeds are similar to other interchange forms. The 

DDI configuration has unique sight lines compared to other interchange forms; as such, the 

intersection design will reflect the need to provide sight angles specific to the upstream crossover 

movement. 

Crossover Design

Crossover angles of 45 degrees or more support operations and safety performance targets. 

However, MoDOT recommends crossover angles range from 40 to 50 degrees, while UDOT 

recommends angles be 30 degrees or greater. Generally speaking, it is desirable to provide the 

largest crossing angle while adapting to each site’s unique conditions. 

Based on documented ongoing research efforts, seven DDIs in various states used crossover 

angles ranging from 28 to 52 degrees. As noted in Chapter 4, lower crossover angles of 40 

degrees or less had the highest number of wrong-ways movements, especially for sites that 

progressed traffic on and off the limited access facility and not along the cross road. These initial 

findings seem to align well with recommendations made by MoDOT. However, there are DDI 

designs with crossover angles below 40 degrees that integrate different design criteria and 

features to discourage wrong-way movements. 

Commonly used treatments supplement the crossover angle as a means of discouraging wrong 

way movement at the crossovers include: signing at the gore, pavement markings, and signal 

heads with arrows. These treatments are summarized in more detail in Chapter 8. UDOT 

recommends installing a vertical barrier to block the line of sight of the opposing movement in 

the crossover area. This is intended as a means of discouraging right turns into the conflicting 

approach at the crossover.(43) Exhibit 7-15 illustrates of the barrier versus a raised channelizing 

traffic island at the inbound and outbound approaches of two different DDIs. If this raised barrier 

is used, sight distances should be accommodated.
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1. Barrier Installation, Right Side, Entry Approach

Pioneer Crossing, UT

2. Barrier Installation, Left Side, Exit Approach

Pioneer Crossing, UT

3. Traffic Island, Right Side, Entry Approach

MO 13 - Springfield, MO

4. Traffic Island, Left Side, Exit Approach

MO 13 - Springfield, MO

Exhibit 7-15. Barriers and traffic islands to discourage wrong-way movements.(26)

According to UDOT, these concrete walls emphasize the desired continuous movement through 

the crossover more effectively than raised traffic channelizing islands. While this treatment has 

not been evaluated to document its effectiveness in decreasing the likelihood of wrong-way 

maneuvers, it can be considered in addition to more traditional treatments. 

Tangent through Crossovers 

Tangent alignments through the crossover promote desired vehicle tracking and reduce driver 

workload by separating driving tasks. The tangent section in the reverse curves is consistent with 

fundamental highway design principles. The curve-tangent-curve sequence promotes a selfenforcing street. The curve of the crossover enforces the relatively slow desired target speed. The 

tangent creates a self-describing alignment and provides the means for drivers to see and prepare 

for the subsequent reverse curve of the crossover. These principles apply to any succession of 

reverse curves approaching, traveling through, and departing a DDI.

A tangent of approximately 100 feet is typically sufficient for most crossover designs.(16) The 

actual length may depend on the number of lanes being traversed and the crossover angle. The 

actual length should be adapted to the site-specific conditions, and as a minimum, result in 15 to 

25 feet (approximately a car length) of tangent leading to the stop bar and 10 to15 feet past the 

projection of the theoretical edge of the opposing traffic travel path. The latter promotes drivers 

navigating the desired path alignment through and exiting the crossover. Some sites have utilized 

no tangent section in reverse curve design and raise path alignment concerns, such as those 
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shown earlier in Exhibit 7-6. Exhibit 7-16 depicts tangent length approaching and departing the 

crossover. 

Exhibit 7-16. Tangent length approaching and departing the crossover.

While DDIs should include tangent sections, if none are provided, curves should be of sufficient 

radii to match intended operating speeds. Curve radii values corresponding to design speeds 

below intended operating speeds can lead to vehicles over tracking intended travel paths. If DDIs 

are being considered with no tangent section throughout the intersection for all lanes, other 

guidance such as signing, striping, and signal head placement should be considered to aid drivers 

to the proper lane and provide adequate visibility to the signal heads. 

Lane Widths

The lane widths of the crossover and ramp movement are determined based on the design vehicle 

and the likelihood of multiple heavy vehicles being side by side. Horizontal geometrics such as 

curve radius, crossover angle, and tangent segments at the crossover can influence lane width 

dimensions. Design vehicle swept paths using templates or software are useful for determining 

the necessary lane widths through different radius curves. 

Generally speaking and with other intersection design, smaller radius curves will require larger 

lane widths for the crossover, usually in the range of 15 feet. Where larger radii curves are used 
Diverging Diamond Interchange Informational Guide

131 

with large cross road medians, lane widths ranging from 12 to 14 feet have been sufficient. Lane 

width dimensions should be adaptive to each project’s unique context. 

Lane widths for left and right turn movements at entrance and exit ramp terminal intersections 

may be designed using similar methods for right turn ramp designs (entry and exit) employed at 

conventional interchanges. Left turn movements entering and exiting the cross road function in 

the same fashion as right turning movements at conventional diamond interchanges. Lane widths 

can be increased to accommodate design vehicles at entry or exit ramps, depending on the 

project context. 

Lane widths along the crossroad tangents typically range from 12 to 15 feet wide, depending on 

local design practice. Where necessary, lane widths should achieve their crossover lane width 

prior to the first curve approaching the crossover and at the end of the last curve departing the 

crossover. The intent would be to increase (on approach) and decrease (on departure) crossover 

lane widths through bridge abutments. The abutment may be a critical cross sectional constraint. 

Shoulders

Under most circumstances in street design, the outside shoulder is used by drivers needing to pull 

over. However for DDI’s, the inside shoulder is the recommended pull over location between 

crossovers. Reasons for using the inside shoulder include: 

• Existing outside cross road shoulders approaching and departing the DDI crossover are 

naturally transitioned to an inside shoulder after entering the interchange. 

• Inside shoulders between crossovers remove the conflict with potentially heavy left turn 

movements onto the limited access facility.

Shoulders are typically less effective than raised curbs for channelizing vehicles through the 

crossover. Therefore, to discourage wrong-way movements and appropriately direct vehicles, 

shoulders are not recommended within the vicinity of the crossover. 

Adding and Dropping Lanes

Auxiliary lanes may be beneficial at a DDI to serve weaving traffic. Auxiliary lanes serving turn 

lanes to and from entrance and exit ramp terminal intersections developed in advance of the 

crossover intersections could smooth traffic flow through the interchange. These turn lanes 

provide added capacity through the crossover intersections, particularly if they can be developed 

in advance of the interchange. Overhead signs can be used where these lanes develop with the 

appropriate messaging to communicate appropriate lane use. 

Lanes can be added or dropped on the left or the right side of through lanes onto the limited 

access facility. Several options exist for developing auxiliary lanes that become dedicated left or 

right lanes, shared through and left, and exclusive through lanes. Lanes added or dropped 

approaching or departing the first crossover signal will allow drivers to pre-segregate themselves

to the appropriate lane before the crossover and eliminate lane drop until after. This promotes an 

even lane distribution at the stop bar. Examples and descriptions are provided below. 
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132 

1. Dedicated left: Auxiliary left turn lanes added to the left side of existing lanes can take 

two forms. These are most often used where heavier left turns onto the entry ramp are 

present. The first form adds the dedicated left turn lane between the left turns from the 

exit and entry ramps as shown in Exhibit 7-17. This option does not provide additional 

capacity at the inbound crossover movement and is not preferred. It acts similar to a 

shared through and left turn lane with an early entry. Limiting left turn storage to the 

dimension between crossovers is similar to limiting the left turn storage between ramp 

terminal intersections at a compressed diamond. In either configuration, the inadequate 

left turn storage can result in queue spill back to the through lanes on the crossover. 

Exhibit 7-17. Auxiliary left turn lane between crossovers.

The second option provides an auxiliary left turn lane developed prior to the entry of the 

first crossover, as shown in Exhibit 7-18. This example shows the auxiliary lane develop 

approximately 200 feet prior to the crossover. Traffic operations evaluations will guide 

storage lengths for each interchange location. However, there are several examples 

developing this lane 1,000 feet prior to the crossover. The longer the length of this lane, 

the better the lane utilization is likely to be, improving capacity through the DDI. This 

configuration is similar to a tight diamond interchange configuration where left turns are 

developed in advance of the first ramp signalized ramp terminal intersection. In both 

cases, the left turn is advanced through the signal as a through movement. 

Exhibit 7-18. Auxiliary left turn lane developed prior to the first crossover.
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2. Shared through and left: Auxiliary lanes added to the left or right side of existing lanes 

often take the form of a shared through and left turn. This option is most often used 

where low to moderate left turns onto the entry ramp are present. An example is provided 

in Exhibit 7-19. 

Exhibit 7-19. Auxiliary lane, shared left and through, developed prior to the first crossover. 

The auxiliary lane is added along the right side of the road after crossing the intersection 

(left). This lane is continued through the first crossover where a shared through and left 

turn split at the left-turn entry ramp, continuing the through lane through the second 

crossover (right). Although the auxiliary lane was added on the right side of the road in 

this example, it would be more common to add the lane on the left side of existing lanes. 

3. Exclusive through: Auxiliary lanes added to the left or right side of existing lanes 

sometimes continue through both interchange crossovers. This is not a common 

movement as the DDI is not typically installed to service heavy through movements. An 

example is provided in Exhibit 7-20. 

Exhibit 7-20. Auxiliary through lane developed prior to the first crossover. 

The upstream through and right turn are developed prior to the conventional intersection 

(left). This auxiliary lane initially shares the through and right turn; however, the right 

turn separates into a long storage lane a few hundred feet downstream, allowing this lane 

to be a dedicated through lane when entering the first crossover. After crossing over to 

the left side of the road, this lane is continued through the DDI along the inside of the 

cross road and back through the second crossover (right). 
Diverging Diamond Interchange Informational Guide

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Ramp Design

This section provides guidance for traffic control at entry and exit ramp terminal intersections at 

DDIs. There are six basic types of traffic control options to accommodate left and right turn 

movements. These control options are not unique to the DDI and are the same for all interchange 

forms.

1. Stop control 

2. Yield control with no downstream acceleration lane 

3. Yield control with a downstream acceleration lane

4. Signal control with right or left-turn-on-red (RTOR/LTOR)

5. Signal control with no right or left-turn-on-red (RTOR/LTOR)

6. Free turn with downstream acceleration lane 

For exit ramp right and left turn movements at DDIs, sight lines through the crossover must be 

evaluated to provide designs with sufficient intersection sight distance for options 2 or 4. Traffic 

control assumptions discussed in this section assume these two safety considerations are 

addressed. Specific guidance on sight lines and intersection sight distance are discussed later in 

this chapter. 

Entrance Ramp Terminal Intersections

The defining concept of a DDI is the removal of left turn conflicts for vehicles turning left from 

the cross road onto a limited access facility. As such, left turns from the crossroad are freeflowing and unsignalized at the majority of DDIs constructed to date. However, at DDIs with 

outside pedestrian facilities, left turns from the cross-street conflict with pedestrians and 

signalization may be desirable. A separate pedestrian activated signal or beaconing device (i.e. 

pedestrian hybrid beacon, rectangular rapid flashing beacon, etc.) may be installed to bring 

attention to the crossing, making the movement safer. This topic is discussed in more detail in 

Chapter 3. 

Assuming the left turn entry ramp is unobstructed, the right turn entry ramp will take one of two 

basic designs: yield control with no acceleration lane and free turn with acceleration lane. These 

are described along with an illustration below in Exhibit 7-21 and Exhibit 7-22. The geometric 

considerations for pedestrians crossing the entrance ramp are discussed further in Chapter 3. 

• Yield control with no acceleration lane: This choice of traffic control is often dictated by 

conflicting traffic volumes, the downstream merge point onto the limited access facility, 

and/or available ROW. It is most often used where low to moderate turning movements 

are present for the left or right turn. The left and right turn lane can either be shared or 

exclusive. 
Diverging Diamond Interchange Informational Guide

Exhibit 7-21. Entry ramp right turn yield control - no acceleration lane. 

135 

• Free turn with acceleration lane: This form of traffic control is preferred if entry ramp 

volumes are significantly high for left or right turns. The downstream merge point onto 

the limited access facility should be designed no differently than other interchange 

designs. The left and right turn lane can either be shared or exclusive.
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136 

 

Exhibit 7-22. Entry ramp free right and left turns with acceleration lanes. 

A left turn movement at the entrance ramp may be very low in comparison to a right turn 

movement. Heavy right turn movements that are required to yield to low volume free-flow left 

turns have been shown to have considerable conflicts at one of seven sites studied in recent 

research efforts.

(14) In this case, it is permissible to have the left turn yield to the right turn as 

shown in Exhibit 7-23. Ideally, the left turn treatment would also be used at the opposing 

entrance ramp terminal for consistency. Designers should verify that queues will not back up into 

the arterial through lanes if the left turn is shared. 
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Exhibit 7-23. Entry ramp left turn yield control - no acceleration lane. 

Exit Ramp Terminal Intersections – Left-Turns

DDIs have separate lanes for left and right turning traffic on different sides of the crossover 

intersection to allow unopposed turns onto the cross street. Removing left turn conflicts from 

opposing through movements provides a similar movement to exit ramp right turn movements.

Although the traffic conflicts are similar, the traffic control options are limited due to several 

factors such as proximity to the two crossovers, limited cross section of the bridge, and state 

statutes regarding LTOR operations. The left-turn traffic control at the exit ramp terminal 

intersections will take on one of three basic forms as described below and illustrated in Exhibit 

7-24 and 7-25. 

• Yield control with no acceleration lane: This can be used where low to moderate traffic 

volumes exist for the left turn movement. A yield movement should have sufficient time 

to maneuver when the opposing through movement is stopped by a traffic signal at the 

crossover signal. 

• Signal control with no LTOR: This can be used where moderate to high traffic volumes 

exist for the left turn movement. It can be used for single or dual lane configurations. 

This form of signal control is more favorable than LTOR allowed because traffic is 

controlled by the signal.
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138 

• Signal control with LTOR: DDIs are typically not designed with LTOR operations. 

However, approximately two-thirds of states allow LTOR onto a one-way street. The 

unique design of the DDI allows an opportunity to use LTOR operations, provided state 

laws permit this maneuver and sight distance can be provided. Since LTOR creates a 

downstream weave through the next crossover, the distance to the downstream crossover 

will be a consideration. If weaving or queuing could create undesirable operations, LTOR 

might not be appropriate. LTOR operation is most favorable for single left turn signalized 

approaches.

Exhibit 7-24. Exit ramp left turn with yield control. 

Exhibit 7-25. Exit ramp left turn with signal control. 
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139 

Stop control is not recommended. Options using an acceleration lane downstream of the exit 

ramp left turn are not recommended since downstream signal may result in queue conflicts and 

or insufficient distance for safe weaving maneuvers on the cross road. 

Exit Ramp Terminal Intersections – Right-Turns

DDIs have separate lanes for left and right turning traffic to allow unopposed turns onto the cross 

street. The right turn exit ramp is the most complex ramp movement to design as it depends on 

conflicting traffic volumes and the type and location of access downstream along the cross road. 

These concepts are not unique to the DDI. An analysis procedure from the Institute of 

Transportation Engineers (ITE) “Freeway and Interchange Geometric Design Handbook” can be 

used to determine the distances required between intersections.

(41) Similar to other interchange or 

arterial traffic operations analysis, traffic simulation software can be a helpful tool to evaluate 

design options. 

For the purposes of this section, the type and location of downstream access is similar to other 

interchange right turn exit ramp terminal intersection considerations. The downstream access 

could be one or more of the following: 

A. Right-in/right-out (RIRO) 

B. Two-way stop control (TWSC) 

C. Signal control 

D. A combination of RIRO and TWSC or RIRO and signal control. 

The right-turn traffic control at exit ramp terminal intersections will consist of one of six basic 

configurations (noted at the start of this ramp design section) depending on the type and location 

of the four downstream access points mentioned above. As right turns at DDI’s are often heavy 

yet capacity constrained, considering the traffic control treatment is an important step in the 

design of the interchange. 

Angle of Approach 

Ideally, intersecting roads should meet at right angles. The crossover through movements are a 

special condition discussed earlier in this chapter; however, exit ramp right and left turn 

alignment at a DDI creates a condition where some drivers may look down the wrong approach. 

In other words, depending on the cross road alignment, the drivers may be looking at the 

departing traffic that has been transposed by the crossover. At the ramp terminal intersection, 

separate approach lanes will be provided for left and right turn movements. The approach angles 

for each movement should be considered separately as each has a unique conflict with differing 

through movements from the crossover. This is shown in Exhibit 7-26. 
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Exhibit 7-26. Alignment of exit ramp right and left turn movements. 

140 

Although a right angle alignment is usually desired, angles above 60 degrees are often permitted 

by agencies and result in a relatively small reduction in visibility compared to 90 degree 

intersections. For the DDI, more acute angles help facilitate yield control movements or where 

RTOR is allowed at signal controlled movements. The acute angle reduces the likelihood of 

looking down the wrong approach for available gaps in traffic. Acute angles can also result in 

undesirable viewing angles depending on where a driver stops for an available gap in conflicting 

traffic. The angle of approach can affect ROW needs, especially if trucks are taken into account 

as acute angles tend to limit visibility for those drivers more so than passenger car drivers. 

For sites with limited available ROW and capacity limitations for the left or right exit ramp 

terminal intersection movements, glare screens have been used to assist drivers looking down the 

correct approach. 

Sight Distance

Drivers approaching or departing an intersection should have an unobstructed view of traffic 

control devices and sufficient length along the cross road to safely navigate the intersection. 

Insufficient sight distance is a significant factor in street crashes and near collisions. As with any 

other intersection, DDI intersections should provide stopping sight distance (SSD) and 

intersection sight distance (ISD). Specific guidance is provided in the following sections along 

with strategies for increasing sight distance where appropriate. 

Stopping Sight Distance

SSD is the distance required by a driver to perceive and react to an object in the road and come 

to a complete stop before colliding with the object. SSD should be provided at every point of the 

DDI and on each approach where yield control is used. 
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141 

The AASHTO Green Book provides an equation for calculating SSD and is provided below.(8) 

SSD = (1.47)(V)(t) + 1.075

V2

a

where 

SSD = stopping sight distance, ft; 

t = perception-brake reaction time, assumed to be 2.5 s;

V = initial speed, mph; and 

a = driver deceleration, assumed to be 11.2 ft/s2. 

According to the AASHTO Green Book, SSD values are based on an assumed driver’s eye 

height of 3.5 feet and an object height of 2 feet.(8)

 At DDIs, SSD constraints will be most 

prevalent at the following locations where yield control or pedestrian crossings are provided: 

• Exit ramp left and right turns: Drivers must be able to stop at conflicts with approaching 

vehicles or crossing pedestrians at the exit ramp terminal intersection.

• Entry ramp left and right turns: Drivers must be able to stop at pedestrian conflicts at 

free-flow left turns and yield controlled right turns onto the entrance ramp. This topic is 

discussed in Chapter 3. 

For signalized intersections, SSD does not need to be considered. Approaching vehicles are 

regulated by the traffic control device and not the presence or absence of vehicles on intersecting 

approaches. Where traffic signals are not visible to oncoming drivers, advance signals can 

provide the necessary information to drivers regarding who has the right-of-way. Exhibit 7-27 

and 7-28 provides an example where the advance signal is provided to the left of the cross road. 
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142 

Exhibit 7-27. Supplemental advanced signal at crossover.

(26)

Exhibit 7-28. Plan view of supplemental signal. 
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143 

Intersection Sight Distance

ISD is the distance along a clear line of sight allowing a stopped driver from a minor approach to 

accept an appropriate gap in traffic when entering or crossing the major road. For a DDI, the 

minor approach is the exit ramp terminal. As drivers cannot cross from the exit ramp to the 

entrance ramp (i.e. crossing the major road), only traffic entering the major road as a left or right 

turn is considered. 

At DDIs, ISD constraints will be most prevalent at exit ramp left and right turns where yield 

control movements are provided. Yield control movements in this case refer to standard yield 

signs, RTOR at signals, flashing operations, or even power outages. 

The AASHTO Green Book equation for calculating ISD as provided below.(8) 

ISD = (1.47)�Vmajor��tg�

where 

ISD = intersection sight distance, ft; 

Vmajor = design speed of major road (mph); and

tg = time gap for minor road vehicle to enter the major road (s). 

The exit ramp maneuvers represent a Case C (Yield control on the minor road), maneuver 2 (left 

and right turns) as defined in the AASHTO Green Book.

(8) The left and right turns from the 

minor road are considered using different gap time requirements for each movement type; 

however, the exit ramp left turn is essentially the same as the right turn maneuver. Therefore, the 

requirements are the same. The time gap requirements based on the design vehicle are provided 

below in Exhibit 7-29. 

Exhibit 7-29. Time gap - Case C2, left and right turn from minor approach. 

Design Vehicle Time gap, tg (s)

Passenger Car (PC) 8

Single-unit truck (SU) 10

Combination (Comb) truck 12

Exhibit 7-30 presents ISD distance derived from the ISD equation using the time gap values for 

the three design vehicles. A figure of ISD for each design vehicle as well as SSD is provided in 

Exhibit 7-31. 
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144 

Exhibit 7-30. SSD and ISD calculations (Case C2). 

Design 

Speed 

(mph)

Stopping Sight Distance, 

SSD (ft)

Intersection Sight Distance, ISD (ft)

Passenger 

Cars (PC)

Single Unit (SU) 

Truck

Combination (Comb) 

Truck

10 46 118 147 29

15 77 176 221 265

20 112 235 294 353

25 152 294 368 441

30 197 353 441 529

35 246 412 515 617

40 301 470 588 706

45 360 529 662 794

Exhibit 7-31. Stopping and intersection sight distance for yield controlled left and right 

turns.

50

45

40

)h

35

mp (

30

deep25 S ngi 20 seD15

10

5

0

0 100 200 300 400 500 600 700 800 900

Length of Sight Triangle Leg (ft)

Stopping Sight Distance

ISD - Passenger Car

ISD - Single Unit Truck

ISD - Combination Truck

Appropriate ISD tied to the designed angle of approach is needed to support RTOR signal 

operations for exit ramp right turns. Assuming the angle of approach is designed such that 

drivers do not look down the wrong approach, the RTOR should only be allowed if ISD values 

are attained. If used, a median barrier between the crossovers can block driver sight lines. The 

end of the median barrier wall could be truncated or the wall height reduced to provide sufficient 

sight distance. Exhibit 7-32 demonstrates the lack of sight distance caused by median walls along 

cross roads under and over a limited access facility. 
Diverging Diamond Interchange Informational Guide

145 

Exhibit 7-32. Median barriers blocking ISD for exit ramp right turns.

Transit

Transit accommodations at a DDI have similar considerations as other intersection locations. 

This includes locating transit stops and the complementary pedestrian and bicyclist facilities.

Chapter 3 addressed multimodal considerations in general and those general considerations 

would need to be adapted if transit stops are provided at a DDI. Transit stops at a DDI would 

follow the basic categories as any other intersection: near side and far side stops in relation to the 

ramp terminal intersections. For instance, a transit stop just downstream of a DDI with a stop 

located in the outside lane will indirectly affect capacity while stops are taking place. Nearside 

stops may conflict with right turning traffic destined to the entrance ramp. Or transit stops could 

be located at crossovers near the stop bar. The location of near or far side transit stop locations 

will influence the type of pedestrian facilities provided at a DDI. Transit stops and pedestrian 

crossing locations could influence the type of pedestrian facilities provided (inside or outside 

pedestrian facilities). In theory, the best means of serving transit stops could dictate the 

configuration of a DDI. As with any transit stop location, the corresponding pedestrian facilities 

could influence pedestrian/transit user behavior. DDI layouts and associated transit stops that 

discourage jaywalking or other unintended safety consequences will be most desirable. 
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CHAPTER 8— SIGNAL, SIGNING, MARKING, AND LIGHTING

This chapter provides guidance on the use and installation of traffic control devices at a DDI to 

inform, guide, and control various traffic modes such as motorists, pedestrians, and bicycles. 

Details are provided on various elements of signals, signing, pavement marking, and lighting. 

Where appropriate, several key issues that arise during the design of traffic control plans are 

discussed, as well as the trade-offs and any potential solutions that could be considered to 

mitigate challenges that arise.

The Manual on Uniform Traffic Control Devices (MUTCD) remains the primary source for 

guidance and standards related to traffic control devices.

(10) This chapter is intended to augment 

and interpret existing MUTCD language for application to DDIs. The MUTCD (or its state-level 

equivalent) should be the overruling and primary source for traffic control device guidance, with 

this chapter providing supplemental information related to the DDI. 

DESIGN PRINCIPLES AND APPROACH

The key principle behind signalization, signing, and marking is to provide guidance to drivers, 

pedestrians, and bicyclists on how to act and how to travel through an intersection or 

interchange. The use of consistent and clear traffic control devices enhances driver 

understanding and expectancy of the interchange they are about to traverse. With the unique 

geometric configuration of the DDI, traffic control devices need to be applied and implemented 

carefully to assure that drivers understand what behavior is expected from them. 

In applying traffic control devices to a DDI, there are four primary objectives and messages that 

are conveyed to motorists and other travel modes: 

1. Priority describes which movement has the right-of-way at a crossing or conflict point. 

Priority is controlled through signalization or regulatory signage (yield sign), as well as

supplemented with pavement markings where appropriate (stop bar). If necessary, 

advanced warning signs may be geared at informing drivers of an upcoming conflict 

point and priority control. 

2. Directional guidance informs drivers about upcoming turning movements, as well as 

preventing movements against the intended direction of traffic (at the crossover). 

Directional guidance is provided through signing and pavement markings. 

3. Lane choice communicates to drivers which lane to choose and pre-position them to 

complete a specific movement such as the left turn onto the freeway. The lane choice at a 

DDI generally follows driver expectations to where drivers choose the right lane to turn 

right, and the left lane to turn left (as opposed to, for example, a cloverleaf interchange 

where a driver turns right to go left). Lane choice is communicated through signing and 

especially pavement marking. 

4. Information about other aspects of the interchange and surrounding area are provided 

mostly through signage. Informational signs are not necessary at a DDI and can 

contribute to the visual clutter, which distracts drivers from other necessary information. 
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148 

SIGNALS

There are several key decisions to be made when designing the traffic signals at a DDI. Each of 

these decisions has a significant impact on the signal timing, as well as the programming and 

wiring for signal technicians. Involving operations and maintenance personnel in the early stages 

decision-making process helps to ensure the design will meet their needs. Operations and 

maintenance personnel will also be familiar with the design parameters well in advance of the 

DDI opening. 

To maximize safety and comfort of pedestrians and bicycles, all turning movements from the 

freeway should initially be assumed as signalized. While in some cases, signalization of these 

movements may not be necessary for vehicular capacity, signalized turns are strongly desired to 

provide safe crossing opportunities for pedestrians. 

Signalization Principles

Chapter 5 introduced basic signalization principles at a DDI, and these concepts are expanded in 

the discussion below with some additional detail. Exhibit 8-1 shows a common signal timing 

configuration that has been employed at DDIs in the U.S. to date. The design is able to run both 

crossovers on a single controller by assigning one ring to each of the crossovers. 
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149 

Exhibit 8-1. DDI standard phasing scheme. 

At the heart of the DDI signal design are the signals at the crossover intersection that operate 

with main-line split phasing. As was discussed in Chapter 5, the two through phases at each 

crossover simply alternate between inbound and outbound traffic. All other phases for left and 

right turns generally run concurrent with either the inbound or outbound movement. 

The DDI signal plan generally uses four overlap phases for the four through phases (2, 4, 6, or 8, 

as applicable) and also includes a short dummy phase in each overlap. This dummy phase is 

timed to provide sufficient clearance time for the opposing through traffic to clear the concurrent 

turning movements. 

Chapter 5 provided additional options for potential configuration of the DDI signals, including a 

single-ring design and a four-ring design with signalized pedestrian crossings on the outside. 

Overlap Phasing 

If signals are installed for exit-ramp movements (left or right), an overlap should be used to 

provide necessary clearance time of the through movements prior to starting the left- and rightturn exit ramp movements. Providing this overlap allows the opposing through to start several 
Diverging Diamond Interchange Informational Guide

150 

seconds earlier than it otherwise would because the clearance interval at the crossover can be 

shorter. This use of overlap phasing is independent of the choice to use a single or multiple 

controllers at the interchange. 

The use of overlaps is recommended for enhanced signal operational efficiency and reduced 

driver frustration. Without the overlaps, through traffic at the crossover may have to wait an 

additional 5 to 10 seconds after all opposing vehicles have cleared the crossing point. This 

additional wait time should be avoided for the simple two-phase crossover junction. 

Overlaps change the wiring plan for the signal controllers and their use at DDIs may be 

unfamiliar to signal technicians. Signal system operators implementing the signal plans in the 

field or making future updates or fine tuning adjustments based on changes in demand or other 

needs also need to understand the intended use of the overlaps. 

Controllers

Due to the relatively low number of phases at DDI traffic signals, it is possible to run both 

crossovers/ramp terminal intersections on a single controller. This is the overwhelming choice by 

DDI designers to date. However, several states are weighing the trade-offs and considering 

multiple controllers. The trade-offs of each are listed in Exhibit 8-2. 

Exhibit 8-2. Considerations for one versus two controllers at a DDI. 

Single Signal Controller Two Signal Controllers

+ Reduced hardware cost + Ability to better control offsets

+ Used at most existing DDIs + More flexibility if all turns are signalized

+ Avoids need to set up communication between 

controllers

+ More transparency in signal design and cabinet 

set up

+ Improved flow during “free running” signal 

operation (late night) - Need for controllers to communicate

- Increased need for wiring across DDI - Additional hardware and installation cost

- More complicated signal design and cabinet setup- May result in undesirable gap out situations 

during low volume periods

Note: pros are shown with a (+) and cons with a (-).

With a single controller, several of the movements are controlled via overlaps using between two 

and four rings. The interaction between the two DDI traffic signals is critical, so using one traffic 

controller is appealing to maintain progression regardless of traffic volume or communication 

reliability. For example, late at night when coordination with adjacent signals major is not 

desirable, the interchange signals can run in free mode because the single controller will keep 

movements within the interchange synced. A single controller also prevents coordination 

problems due to early gap outs. 

Using two signal controllers, on the other hand, allows more flexibility and design transparency. 

By using two controllers, the offset between the two intersections can be modified easily 

throughout the day to adjust priorities for progression. The more straightforward phasing design 

for the two-controller DDI simplifies the job of developing timing plans for the interchange as 

well as programming the signal controllers and wiring the cabinets. This makes the design more
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151 

understandable to operators and technicians, and may better accommodate use of older 

controllers. Some of the downsides of a two controller design can also be mitigated using pretimed operations and/or including backup means such as GPS units to prevent clocks from 

drifting.

A sample single-controller phasing diagram is shown in Exhibit 8-3. In this four ring design, 

rings 1 and 3 are each used to control one of the crossovers; rings 2 and 4 are used to control the 

two signalized pedestrian crossings across the left-turn movements onto the freeway (pedestrian 

facilities on the outside). 

Exhibit 8-3. Example of a single controller with four rings servicing protected pedestrian 

and vehicle movements on all approaches.

A sample phasing diagram for a DDI with two controllers is shown in Exhibit 8-4. The more 

simplistic phasing design for the two-controller DDI simplifies the job of developing timing 

plans for the interchange as well as programming the signal controllers and wiring the cabinets. 

Exhibit 8-4. Example of two controllers with two rings servicing protected pedestrian and 

vehicle movements on all approaches. 

For one existing DDI in Utah, the decision to use two controllers was driven by a voltage loss 

that would be too high due to the length of the wiring from one side of the DDI to the other. 

While this impact may seem trivial, an important goal for any signal designer should be to make 
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152 

the design as understandable as possible to operators and technicians. The more standard the 

signal design, the more quickly technicians can respond to and resolve field issues. 

Signal Equipment Location

This section discusses guidance for signalization of the DDI, including an overview signal head 

placement, mast arm configurations, detector placement, and pedestrian signal provisions. The 

focus of the section is on the technical aspects of traffic signals at a DDI; a discussion of signal 

operations is provided above and in Chapter 5. 

Signal Head Placement 

Stopping sight distance must be provided at all signalized and yield-controlled approaches to the 

DDI. The visibility of the signal heads and the distance to clearly view traffic signal heads must 

be considered for all movements. This is especially true at the crossovers where the small curve 

radii leading into the curve can impede visibility of the signal head. The crossover signals are 

typically placed on the opposing side of the intersection to allow for the necessary set-back from 

the stop bar. 

Signals placed on the opposite side of the crossover provide motorists an additional visual cue to 

guide into the appropriate travel lanes. The trade-off of this downstream placement is that it may 

be difficult to see the signal heads from a distance due to the crossover curvature. When 

adequate visibility of the signals is not provided, supplemental near-side signals can be used to 

provide advanced warning to drivers. 

A less common alternative strategy is to use a single mast arm with signal heads in both 

directions. This strategy does not provide the same guidance through the intersection; however, 

green arrows in the bulb assembly can be used to assist with proper direction. A single mast arm 

installation generally provides drivers better visibility of the signal heads when approaching the 

crossover. 

Mast arms should not impede the visibility of signals. This blocked view is most frequent with 

the mast arm assembly of one approach impeding visibility of the other approach. The mast arms 

for ramp turning movements can have a similar effect. An example of these visibility effects for 

an outbound DDI movement approaching the crossover is shown in Exhibit 8-5. 
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Exhibit 8-5. Restricted signal head visibility from opposing movement mast arm.(26)

153 

Supplemental Signals

Supplemental signal heads are recommended when the visibility of the overhead-mounted signal 

heads at the crossover are not provided for approaching traffic due to the placement of mast arms 

and the configuration of the crossover. Other geometric elements such as the vertical alignment 

of bridge structures may further impede visibility of the signals. Supplemental advanced signals 

may help improve visibility of the signals in these cases. Examples of supplemental signals at 

DDIs are shown in Exhibits 8-6, 8-7, and 8-8. 

Exhibit 8-6. Supplemental signal on right for an inbound movement at a DDI.

(26)
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Exhibit 8-7. Supplemental signal on left for an outbound movement at a DDI.

(26)

Exhibit 8-8. Supplemental signal for a DDI exit ramp from freeway.

(26)

For the outbound movement, the supplemental signal head is typically installed on the left-hand 

side of the street as illustrated in Exhibit 8-9. Supplemental signals may also be required for the 

freeway exit ramp movements as shown in Exhibit 8-10. For exit ramps, supplemental signals 

are placed on the right side of the road for the left-turning movement, and on the left side of the 

road for right-turning movements. Notice that the right-turn signal in Exhibit 8-10 is 

supplemented with a sign to convey the nature of the signal to drivers: “Right Turn Signal.” 

Types of Signal Heads 

The following discussion distinguishes three types of signal heads: 

• Type 1: red, yellow, and green balls 

• Type 2: red ball, yellow and green arrows

• Type 3: red, yellow, and green arrows 
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155 

For through traffic at the crossover, either a type 1 or type 2 signal head may be used for each 

lane, with the type 2 signal heads showing an angular arrow to denote a through movement. Type 

2 signal heads may help convey and confirm to drivers that they are supposed to not turn at 

intersections (as seen in Exhibit 8-9). 

For turning movements, Type 3 signals should be used to clearly identify that the signals are 

intended for left- or right-turning traffic. However, in some states, a type 3 signal does not allow 

vehicles to turn on red; these situations may require the use of a type 2 signal for those 

movements. In states where the RTOR rule is controlled through supplemental signage, either 

configuration may be acceptable. Also, while most states allow RTOR, several states do not 

allow for a LTOR. Adequate signage should supplement the signal heads to promote conformity 

with state laws. 

Exhibit 8-9. Type 2 crossover signals with a type 1 supplemental signal on left.

(26)

Pedestrian Signals 

Anywhere pedestrian signals are provided at a DDI, Accessible Pedestrian Signals and 

Detectable Warning Surfaces need to be installed to comply with the American with Disabilities 

Act (ADA). While no specific ADA regulation exists for the DDI, the proposed Public Rights of 

Way Accessibility Guidelines (PROWAG) can be applied to all intersections and interchanges, 

including the DDI.(23) The 2009 MUTCD provides specifications for pushbutton locations and 

for APS in sections 4E.08 – 4E.13.(10)

An Accessible Pedestrian Signal (APS) consists of a push-button with a push-button locator tone 

to assist pedestrians who are blind to find the device, as well as a tactile arrow and an audible 

message. The audible message communicates to the blind traveler when the walk interval phase 

is active. The audible message can be in the form of a rapid ticking or beeping/chirp sound, or 

can be a speech message. The latter is required for two APS devices that are separated by less 

than 10 feet, while a rapid tick message is required for APS that are more than 10 feet apart. The 

APS devices for different crossings should be installed on two separate poles if at all possible. 
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156 

At the DDI, the provision of APS may pose a challenge on the median island for pedestrian 

facilities located in the center. Exhibit 8-10 shows an example of an undesirable pedestrian pushbutton installation with the push-button for the two directions on the same pole. The lack of 

separation may make it difficult for pedestrians to distinguish, which push-button is intended for 

which crossing. The example shown further does not provide APS devices or any audible 

information about the crossing. 

Exhibit 8-10. Undesirable use of single pole with two pedestrian push-buttons, no APS, and 

insufficient separation of the two detectable warning surfaces.

(26)

Given that the “point” of the median island does not provide adequate room to allow the

pedestrian push-buttons to be on separate poles and sufficiently separated, it is recommended 

that the poles are located on the opposite side of the waiting area where the island is generally 

wider. This is shown in Exhibit 8-11. Alternatively, pedestrian push-buttons could be separated 

diagonally, with the push-buttons being consistently to the right of the pedestrian (in direction of 

travel), as shown in Exhibit 8-12. 

In general, wider islands are strongly recommended to provide a true refuge area for pedestrians 

of at least six feet in length. This assures a minimum of two feet between the detectable warnings 

for the two directions, as well as adequate storage for wheelchair users. 
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Exhibit 8-11. DDI splitter island with pedestrian signals on same side.

Exhibit 8-12. DDI splitter island with diagonal pedestrian signals.

For both options above, the splitter has to be at least 6 feet wide in the direction of pedestrian 

travel to provide sufficient waiting area for a wheelchair user. A wider island may be desired to 

provide additional storage, passing ability for multiple wheelchair users, and a 10-foot separation 

of pedestrian push-buttons. 
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158 

If the two APS devices are less than 10 feet apart, speech messages are required with customized 

wording specific to the DDI. One potential for such wording after activating the push-button (the 

push-button information message, see MUTCD 4E.13, par 9 & 10) may be: "Wait to cross 

eastbound lanes Airport Rd. at Highway 26. Traffic coming from your left."(10) During the walk 

interval, the message then would be: “Eastbound lanes Airport Rd, walk sign is on to cross 

eastbound lanes Airport Rd.” An expert in accessibility installations may need to be consulted

for specialized applications and signal installations at a DDI to assure that the crossings are 

accessible to and usable by all users as required by ADA. 

If possible, the crosswalk may be moved back from the crossover by some small distance to 

result in a wider splitter island, and provide the necessary space for two APS devices on separate 

poles without bleed over of the unit sounds. However, this would require the vehicle stop bar to 

be moved back accordingly, which can have other implications for visibility. 

The placement of pedestrian signals is more straightforward for the crossing towards the median 

island and the crossing of a right- or left- turn lane. In both cases, only a single pole and single 

APS is needed. Reach ranges for wheelchair users need to be carefully measured and considered 

in installation. An example of a pedestrian signal crossing toward the median island is shown in 

Exhibit 8-13; an example of a pedestrian crossing for a left turn onto the freeway is shown in 

Exhibit 8-14. For both examples, the cut-through widths as shown could be increased further to 

enhance the pedestrian experience and to allow opposing pedestrian movements (including 

wheelchairs) to pass, especially given the additional right-of-way already provided in the 

channelization island. 

Exhibit 8-13. Pedestrian signal and push-button on outside of crossover (pedestrian 

facilities in center).(14)
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Exhibit 8-14. Pedestrian signal for left-turn onto freeway (pedestrian facilities on 

outside).(14)

Bicycle Signals

There are no known installations of bicycle signals at DDIs at this time. If bicycles remain in the 

street on bicycle lanes, they are controlled by vehicular signals and bicycle signals are 

unnecessary. If bicycles use a shared-use path such as a center walkway, bicycle signals could 

supplement pedestrian signals and reinforce the intended route of travel for bicyclists. 

SIGNING 

This section covers signing conventions for numerous sign types used at several DDIs across the 

United States. Proper signing can be an effective aid in moving drivers through the DDI 

correctly. Errors in signing or sign clustering can cause driver confusion, so careful attention 

should be paid when designing the signing for DDIs. 

The types of signs covered in this section include regulatory, warning, and guide signs for 

vehicular movements. The unique design of the DDI has led to several techniques for signing the 

interchange for vehicles, but pedestrian and bicycle signage is generally not unique to DDIs and 

is therefore not covered. Signing and pavement marking plans from several locations are 

provided in the Appendix. Agencies should consult the MUTCD for further details related to 

placement and installation of signs.(10)

Regulatory Signs 

Regulatory signs are traffic signs intended to instruct road users on what they must or should do 

(or not do) under a given set of circumstances. The signs covered in this section include: 

• No Right Turn (R3-1), No Left Turn (R3-2), No U-Turn (R3-4) 
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• Stay Right of Divider (R4-7), Stay Left of Divider (R4-8), Keep Right (R4-17), Keep 

Left (R4-18), and supplemental object marker signs 

• Do Not Enter (R5-1) 

• Wrong Way (R5-1a) 

• One Way (R6-1) 

Exhibit 8-15 to 8-18 provides several options for regulatory signs at DDIs based on the MUTCD 

and designs constructed to date.(3) Signing of DDIs is an evolving practice and not explicitly 

addressed in the MUTCD. Options presented here sometimes overlap. 

The combination No Left and No Right turn sign assembly at the crossover is consistently used 

at DDIs, as shown in Exhibit 8-15. The use of No Left and No Right turn signs at ramps has 

increased since the first DDIs opened in Missouri. 

Exhibit 8-15. No Left and No Right turn signs.(26)

The Stay Left and Stay Right signs are used consistently at DDI crossovers, as shown in Exhibit 

8-16. Some sites use this sign at the splitter island upstream of the crossover also. Supplemental

sign options include: 

• Keep Right or Keep Left plaque 
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• Object Marker signs OM1-1 or OM3-R and L 

OM1-1 OM3-R and L 

Exhibit 8-16. Stay Right and Stay Left signs.(26)

Do Not Enter signs are used at some DDIs to supplement the No Right and Left Turn signs and 

Stay Left and Stay Right signs installed at the crossovers. Wrong-Way signs can also be installed 

to supplement Do Not Enter signs related to the crossovers and exit ramps, as shown in Exhibit 

8-17. In some instances, the Do Not Enter sign was installed with a One-Way sign at the exit 

ramp left turn. Per the MUTCD, Wrong-Way signs are supplemental to Do Not Enter signs. 
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Exhibit 8-17. Do not Enter and Wrong Way signs.(26)

One Way signs are often used at ramp merge and diverge locations as shown in Exhibit 8-18. 

Sometimes, the One Way sign is not used if ramp entry angles are such that a vehicle would not 

be likely to make a wrong way movement.
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Exhibit 8-18. One Way signs.(26)

Warning Signs

Warning signs call attention to conditions on, or adjacent to, a highway or street that are 

hazardous to traffic operations. These signs are used particularly when the hazard is not obvious 

or cannot be seen by the motorist. The signs covered in this section include: 

• Advance traffic control, Yield Ahead (W3-2) and Advance Signal Ahead (W3-3) 

• Reverse Curve (W1-4) 

• Lane Split (W12-1) 

Yield Ahead warning signs are consistent used for yield-controlled movements. Some DDIs use 

Advanced Signal Ahead signs, and one known sites uses the Reverse Curve Ahead sign. These

signs are shown in Exhibit 8-19. 
Diverging Diamond Interchange Informational Guide

Exhibit 8-19. Yield Ahead, Advanced Signal Ahead and Reverse Curve Ahead signs.(26)

164 

Double Arrow signs are used at diverge points at DDIs, as shown in Exhibit 8-20. 
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Exhibit 8-20. Double Arrow signs.(26)

Guide Signs

Guide signs show route, street or city designations, directions, distances, services, and more. 

Guide signs should be located in areas with unobstructed views that can be read from the proper 

distance to allow drivers to make safe driving decisions prior to entering the DDI. The specifics 

of guide sign types are not covered in this section. Instead, guide signs used along the vehicle 

routes from the crossroad to the limited access facility are provided as their installation 

techniques and locations are unique at various sites. Overhead lane use control sign installation 

techniques are also covered.

The MUTCD provides examples of guide signs that can be used at five specific locations of a 

diamond interchange with multilane approaches.(10) The first two sign locations are the junction 

and destination signs well upstream of the DDI, which are no different than a conventional 

diamond interchange. However, the last three guide signs are somewhat unique to the DDI as a 

driver approaches the unique ramp terminals which do differ from the conventional diamond 

interchange.

The first location (of the last three signs) is just prior to the first crossover where drivers must 

make a decision to take a right onto the exit ramp or continue through the crossover. Ideally, the 

signage provides information to drivers well upstream of any queues allowing for good lane 

discipline through the DDI. Signage is usually provided overhead using a gantry or mast arm 

system, but in some locations use ground mounted signs. 
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The second location is just past the first crossover. The signs are most often ground mounted 

with crossroad overpass designs and mounted overhead for underpass designs. The signs remind 

drivers about the lane assignments for merging onto the freeway, and in some cases signage is 

provided for through movements as well. Guide signs were not provided at one site location. 

The third location provides guide signs at the ramp diverge point. Drivers should not be making 

last minute decisions at this point in time, but instead are reminded where the left-turn movement 

takes place.

Lane Control Signs 

Exhibit 8-21 shows the lane control signing options used at signal controlled movements. 

Crossover movements consistently used Thru Only (R3-5a) signs. The one site that signalized 

the pedestrian movement on the ramp used no lane control signing as this ramp was a single turn 

lane and post mounted signals were used. Two signing options were deployed for signalized left 

and right turns, supplementing signals with Left and Right Turn Only (R3-5L/R) or Left and 

Right Turn Signal (R10-10L/R).

Exhibit 8-21. Lane control signing for signalized movements.(26)

PAVEMENT MARKINGS

In general, striping for a DDI should be standardized to maintain consistency and efficiency. 

Typical pavement markings for DDIs delineate the vehicle entry and exit approaches of ramps 

and crossovers and provide direction for bicycle and pedestrian movements when in the 

pavement ROW. This section discusses the application of pavement markings in the interchange 
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influence area that are unique and may vary between sites constructed at this time. Examples of 

pavement marking layouts used at several DDIs are provided in the Appendix. 

Centerlines and Edge Lines

Travel on the left side of the road at a DDI creates an unusual situation for the application of 

longitudinal pavement markings. The MUTCD, which not does explicitly address DDIs, states 

that white line markings shall delineate the separation of traffic flows in the same direction or the 

right-hand edge of the street, and that yellow line markings shall delineate the separation of 

traffic travelling in opposite directions or the left-hand edge of the streets of divided highways 

and one-way streets or ramps. The pavement marking designs employed along the crossed-over 

sections of street are quite different across states and even within states in some cases. A 

pavement marking design example is provided in Exhibit 8-22 based on practices to date. This 

example does not reflect a standard. 

The traffic driving on the left side of the road is unique to the DDI (shown in blue). At most 

DDIs, yellow lines have been used on left side of travel lanes in these areas and white lines have 

been used on the right side of travel lanes, as if each direction of travel is a one-way street. 

However, if treated as a single, two-way facility, this configuration places white rather than 

yellow lines between the two (wrong-way) directions of travel. The channelizing turn islands 

have three gore locations, each where a merge or diverge occurs. Most agencies choose to 

change pavement marking color at the gore point, as shown in Exhibit 8-22; however, some 

agencies have chosen to transition white pavement marking on both sides of the gore for several 

feet.
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Exhibit 8-22. Example centerline and edge line pavement markings at a DDI.(26)

Lane Lines

As per the MUTCD, lane lines should be used at multilane approaches. For DDIs, solid lane 

lines are, at a minimum, recommended to discourage lane changing in the immediate vicinity of 

the crossover and ramp terminal approaches, as shown in Exhibit 8-23. 
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Exhibit 8-23. Lane line markings at crossover and ramp terminal approaches.(26)

169 

There are several benefits to providing solid lane lines, including: 

• Reducing the likelihood of side-swipe crashes due to last minute lane changes at an 

approach

• Discouraging lane changing immediately before crosswalks, which reduces the likelihood 

of multiple threat crashes between vehicles and pedestrians

Although they are not often currently used at DDIs, solid lane lines are recommended on the 

cross-street at the crossover departures. These will extend through crossover curves but not to 

any merge or departure maneuvers at the entry and exit ramps. The addition of solid lane lines at 

the departure with solid lines at the approach can discourage drivers from cutting across multiple 

lanes to obtain a faster path through the reverse curves of the intersection crossover. 
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Line extensions can be helpful for directing turning and crossover vehicles. Exhibit 8-22 shows 

line extensions used in the crossover to show the alignment to receiving lanes, which is similar to 

those used at many conventional intersection configurations. The aerial shows line extensions 

along the outer portion of the outermost lanes in the crossover, which is not always used in 

design, but is recommended to provide additional channelization. Another potential location for 

line extensions is for dual left-turn lanes from the exit ramp, also shown in Exhibit 8-23. 

Short skip lane lines are sometimes used to delineate an exclusive left-turn lane onto the limited 

access facility, as shown in Exhibit 8-24. 

Exhibit 8-24. Use of short skip lines to delineate an exclusive left turn lane onto a limited 

access facility.(26)

White channelizing lines are often used at exit and entry ramp movements with multiple lanes. 

Many agencies use channelizing islands to create painted islands between multiple entry lanes of 

a single approach, called “vane islands.” These painted islands are thought to provide additional 

deflection and typically use chevron markings in the neutral area of the island. An example is 

provided in Exhibit 8-25. 
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Exhibit 8-25. Channelizing lines and vane islands at the exit ramp.(26)

Lane Use Arrows

Lane use arrows, along with intersection lane control signs, provide the best opportunity to guide 

motorists correctly through the interchange. Lane use arrows are standard application with the 

exception of the crossover intersections which use one of three through pavement arrow 

configurations, as shown in Exhibit 8-26. 

Exhibit 8-26. Three variations of lane use arrows at the crossover intersection.(26)
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The most basic use of through pavement arrows is a typical arrow with 12-inch striping as shown 

in Part A of Exhibit 8-26. An extension of this pavement marking use provides another 12-inch 

through pavement arrow in the receiving lane to provide additional guidance, as is shown in Part 

B. Another less frequently used option is the use of a wrong-way arrow at the entry approach 

with a traditional through pavement arrow in the downstream receiving lane, as shown in Part C. 

Stop Bars and Yield Lines

For DDIs, stop bars will be used at signalized intersection approaches of crossovers and ramps, 

whereas yield lines are used at unsignalized exit and entry ramp movements without dedicated 

receiving lanes. The stop bar and yield line defines where vehicles should stop in response to a 

signal or yield sign, respectively. 

Two stop bar lane configurations are used at the DDI approaches, staggered and straight, as 

shown in Exhibit 8-27. Staggered stop bars are recommended, especially at ramp terminals with 

RTOR or LTOR operations to allow drivers to see past the vehicle in the inside lane of a multilane approach. Since the crossovers are always signalized, staggered versus straight has no 

immediate benefit. 

Exhibit 8-27. Stop bar locations at the intersection crossover and exit ramp terminal.(26)

Yield controlled approaches are often used on exit and entry ramp movements. The MUTCD 

describes yield lines as isosceles triangles pointing toward approaching vehicles.(10) These yield 

lines should provide a stopping point in response to a yield sign, as shown in Part A of Exhibit 8-

28. Part B of Exhibit 8-28 shows another yield controlled movement at a ramp merge, but several 
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173 

constructed have DDIs failed to provide yield markings prior to pedestrian crossings at entry 

ramp right and left turns. 

Exhibit 8-28. Yield line locations.(26)

Pedestrian Crosswalk Markings

Pedestrian crosswalk markings should be installed at all pedestrian crossing locations at DDIs in 

urban and suburban locations. Crosswalk markings provide guidance to pedestrians who are 

navigating a DDI while also providing a visual cue to drivers of where pedestrians may be 

present in the street. Two basic scenarios for pedestrian crosswalks are described in the 

MUTCD.(10) Use of either marking type is acceptable and choice is usually dependent on state 

agency preference.

Where pedestrian crossings are expected, markings must be used to legally establish a crosswalk. 

Specific design details can be found in the MUTCD. 

Transverse crosswalk markings are most commonly installed at DDIs because they are less likely 

to be confused among other unique pavement markings (especially with center crosswalk 

designs), and they are less expensive to install than longitudinal markings. An example of a DDI 

with transverse markings is provided in the Part A of Exhibit 8-29; note the wear on several of 

the crossings. Crosswalk markings that are longitudinal to vehicular traffic flow are generally 

more visible. These marking types are known as “Zebra” or “Continental” crosswalk markings. 

An example of longitudinal crosswalks at a DDI is provided in Part B of Exhibit 8-29. 
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Exhibit 8-29. Transverse (A) and longitudinal (B) crosswalk markings at a DDI.(26)

174 

Look Left 

Regardless of whether pedestrian facilities are provided along the outside or in the center 

median, a unique issue with pedestrian crossings at DDIs is the propensity to look the wrong 

direction for gaps in traffic. One treatment that could be considered in addition to supplemental 

signing and/or speech messages used with APS devices, is an embedded pavement marking such 

as the one shown in Exhibit 8-30. This treatment, although unusual, could be helpful to 

pedestrians and is relatively inexpensive to install and maintain. The installation process requires 

that a small section of pavement be removed prior to marking installation. This provides 

protection against snow plows and wheel friction, which reduces the maintenance needs for the 

marking. 
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Exhibit 8-30. Engraved pedestrian pavement marking – “Look Left.”(26)

Bicycle Lanes

Guidance on the placement of bicycle lanes is provided in Chapter 3. In general, DDIs do not 

have unique requirements for bicycle pavement markings. 

LIGHTING

This section provides basic guidance for street lighting designers along with state and local 

agencies regarding the design and application of street lighting. It is not intended to be a detailed 

guide, as lighting designs often vary depending on agency internal policies, though they should 

meet minimum guidelines found in various resource documents. As such, this section is provided 

as a resource for policy makers and the design and construction community to evaluate the need, 

potential benefits, and even applicable references when considering street lighting at DDIs. The 

2005 AASHTO Street Lighting Design Guide and Illuminating Engineering Society’s (IES) The 

Lighting Handbook, 10th edition offer more detailed recommendations on lighting levels and 

configurations that should be used at interchange influence areas.(44, 45) In addition, agencyspecific street lighting policies should be followed when necessary. As the DDI is a unique 

interchange design, agency policies regarding street lighting at these facilities may need to 

include specifications specific to this interchange design. 

The FHWA Highway Design Handbook for Older Drivers and Pedestrians provides 

recommendations for fixed lighting installations and certain interchange configurations.(46) 

Although this handbook does not constitute a new standard of practice, it provides considerations 

for fixed lighting installations by practitioners with respect to the decision-making processes for 

the elderly and pedestrians. The possible considerations for where to add fixed lighting include: 
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• Where an increased potential for wrong-way movements is expected either through crash 

identification or engineering judgment

• Where shifts in lane assignment, turn-only assignment, or pavement width forces a pathfollowing adjustment at or near the intersection

• Where twilight or nighttime pedestrian volumes are high

Although there is limited crash data available with respect to wrong-way movements at DDIs, 

there is presumably an increased chance that these movements will take place given the unusual 

crossover design. Findings on wrong-way maneuvers, presented in Chapter 4, suggest this is the 

case. In addition, reverse curve designs with limited tangents through the crossover, unusual lane 

assignment patterns at approaches, and even changes in lane width through the crossovers 

provide significant opportunities for last minute adjustments to lane assignment. These two 

considerations alone provide reasonable concern with respect to the decision-making process of 

some drivers and an obvious need for fixed lighting at the DDI.(46) 

Based on these concerns, the handbook makes the following recommendation at interchanges:(46) 

Complete interchange lighting (CIL) is the preferred practice, but where a CIL system is 

not feasible to implement, a partial interchange lighting (PIL) system comprised of two 

high-mast installations (e.g. 60 to 150 feet) with 2 to 12 luminaires per structure per 

ramp is recommended, with one fixture located on the inner ramp curve near the gore 

and one fixture located on the outer curve of the ramp midway through the controlling 

curvature.

AASHTO provides warrants for CIL and PIL systems.(44) Although these warrants do not 

represent a requirement to provide lighting, they do provide a mechanism for designers to make 

sound design decisions on how to provide adequate lighting. Warrants for CIL and PIL systems 

are based on conditions including traffic volume, interchange spacing, adjacent light use, and 

night-to-day crash ratios. Generally speaking, it will be desirable to provide PIL systems at rural 

DDI locations as a minimum requirement, but consideration should be given to CIL designs. CIL 

lighting will be desirable on bridge designs in urban and suburban areas. 

Pole spacing options at DDI crossroads will be highly variable based on several factors such as: 

• The type of system deployed (CIL or PIL) 

• The types of poles being considered (high mast poles, mast arms, post-top, etc.) 

• Interchange area being considered (e.g. inbound approaches, ramps, between the 

crossover, etc.)

• Surrounding environment 

• Width of the bridge(s) 

• Clear zone considerations 
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At sites that are a part of a CIL, lighting should be used on the entry and exit ramps for 

continuity. If continuous freeway lighting is not provided, such as the case with PIL systems, it is 

recommended that lighting be provided on the ramps to transition into the interchange given the 

unique design. Most often, mast arm and davit-style lighting will be used along the ramps. 

When pedestrian walkways are provided along the outside of the crossroad, high mast lighting 

will usually be more effective than mast arm, truss, or davit-style lighting. This provides more 

options for installing high mast arms as sufficient clear zone outside the crossroad and the 

median are optimal. This option requires fewer poles than other methods, though their 

maintenance costs may be higher in some cases. An example of a CIL system with high mast 

lighting (six luminaires each) at the crossovers is provided in Exhibit 8-31. In this example, 

crosswalks are not present in the median and high mast lighting is used on the outside. If 

sufficient clear zone were available, it is possible to install high mast poles in the median also. 

Note: Transparent circular and triangular shapes only describe basic lighting characteristics 

and not necessarily the zone being lighted. 

Exhibit 8-31. Continuous interchange lighting in an urban setting.(26)

Sites with pedestrian facilities in the median will typically have mast arm or davit-style lighting 

installed along the cross road; however, it is possible to install high mast along the outside as 

well provided there is sufficient clear zone. This pole type requires more poles than high mast 

types, though they are typically easier to maintain. An example of a PIL system is provided in 

Exhibit 8-32 where lighting is provided on the ramps to transition off and onto the dark limited 

access facility. The lighting at the ramps show key merge and diverge points at the entry and exit 
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ramps. Post-top lighting is used along the bridge for aesthetics. An example of a PIL system with 

no lighting on the ramps is provided in Exhibit 8-33. 

Note: Transparent circular and triangular shapes only describe basic lighting characteristics 

and not necessarily the zone being lighted. 

Exhibit 8-32. Partial interchange lighting in a suburban setting. Lighting on the ramps is 

provided as a transition.(26)
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Note: Transparent triangular shapes only describe basic lighting characteristics and not 

necessarily the zone being lighted. 

Exhibit 8-33. Partial interchange lighting in a suburban setting. No lighting is provided on 

the ramps for transition.(26)

Where pedestrian crosswalks are provided in the medians, such as in Exhibit 8-34, recessed 

lighting is recommended for reasons described in Chapter 3. 

Pedestrian Lighting

The lighting design for the pedestrian facilities at a DDI should follow the same considerations 

as at other interchanges. For arterial underpasses, these considerations include adequate 

illumination of the pedestrian facilities under the bridge structure, as well as lighting on 

pedestrian walkways and crossing points. 

For center walkway locations, recessed LED lighting as shown in Exhibit 8-34 provides an 

opportunity to illuminate the walkway without the need for additional street lights. Street lights

may be difficult to place in a constrained right-of-way and should not be placed within the 

walkway where it would form an obstacle for pedestrians. The recessed lighting feature can

provide adequate illumination without introducing such obstacles, but illumination levels should 

always be checked against the appropriate federal, state, or local standards. Illumination levels of 

pedestrian pathways should take into account low-vision users, especially in cases where 

ambient light may be blocked through structures or retaining walls. 
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Exhibit 8-34. Recessed lighting in DDI center walkway.(14)
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CHAPTER 9— CONSTRUCTION AND MAINTENANCE

This chapter provides an overview of the general considerations of constructing and maintaining 

a DDI. It also discusses law enforcement considerations. While there are differences in 

constructing, maintaining, and providing law enforcement at a DDI compared to a conventional 

diamond interchange, none of the differences is likely large enough to overshadow the safety and 

operational effects discussed in previous chapters. 

CONSTRUCTION

Implementing a DDI poses some challenges in maintaining traffic flow during construction; 

however, the thought processes are really no different than with other designs. Sequencing of 

construction can use several strategies depending on factors such as whether the facility is an 

overpass or underpass, if an overpass how many structures are being built and are any existing 

structures already in place, nearby detour routes, and the traffic demands of the facility, to name 

a few. Each of these factors also has significant effects on the costs of implementing a DDI 

design. As there are many staging options that could be considered, this section will provide 

some options that could be considered based on the design constraints of the site. 

Construction Staging

Compared to other interchange forms such as partial cloverleaf designs, tight diamond 

interchanges (TDI), and single point diamond interchanges (SPDI), the DDI typically takes much 

less time to construct, particularly for upgrades of existing diamonds. This is especially true if 

the design is a retrofit that allows for the use of the existing bridge structure. MoDOT conducted 

constructability analysis of a TDI and SPDI versus a DDI, and it was concluded that DDI 

construction would last a single season and TDI or SPDI construction would last two seasons or 

more.(16) This is consistent with findings from other agencies.

This section will provide some examples of how staging could be implemented based on a few 

interchanges constructed across the United States. The primary consideration for construction 

staging is new construction, additional structures, or true retrofit designs. Sites with retrofit 

designs are most common to date. Questions to consider include: 

• Can the interchange be closed?

• Is the existing pavement going to be used or replaced? 

• Is additional cross-section necessary to accommodate future traffic?

• When are the best times to switch traffic between various stages of the project?

Sites constructed in locations where no interchange currently exists will cause the least 

disruption to traffic. Sites that can be fully closed by detouring traffic to nearby interchanges 

provide the best accommodations for construction agencies but at a cost to drivers. For most 

agencies, DDIs will be constructed while accommodating most of the existing traffic demands 

already in place. 
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If original pavement will be used in place, switching traffic to a DDI configuration early can be 

accommodated. Oftentimes, simply resurfacing can facilitate the traffic switch. Switching traffic 

early will usually reduce the amount of temporary pavement needed during construction. 

Switching traffic later is most often done when considerable pavement replacement is necessary. 

The latter method is most commonly implemented and usually uses half-at-a-time construction 

phasing; an example is provided in Exhibit 9-1. No different than normal interchange design 

considerations, portions of the available cross-section must be cordoned off using raised barriers 

to allow construction activities to take place. 
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Existing diamond interchange lane configuration 

Stage 1. Partial interchange lane configuration, EB lanes under construction 

Stage 2. Partial interchange lane configuration, pedestrian center crosswalk under construction

Stage 3. Partial interchange lane configuration, WB lanes under construction

Stage 4. Full interchange lane configuration, median barrier in ROW

Stage 5. Final interchange lane configuration prior to opening of DDI

Exhibit 9-1. Example of construction staging using an existing structure.(47)

183 
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Where new structures must be completed, the design is usually constructed with less interference 

to normal traffic operations. Structures can be built in place or constructed off-site and driven in 

using a self-propelled modular transporter (SPMT). An example of a pre-constructed concrete 

girder system being driven to the site is provided in Exhibit 9-2. This technique has been used 

with great success for many bridge projects in Utah.

(48) Although these bridge design techniques 

will usually require more upfront costs, UDOT is seeing cost come down as contractors become 

more comfortable with the technique. Recent transport and installation of a parallel bridge took 

less than six hours to complete.

Exhibit 9-2. Self-propelled modular transporter (SPMT) brings superstructure to DDI 

location.(48)

Exhibit 9-3 shows the construction staging used at one of the UDOT sites where a prefabricated 

bridge was rolled into place. The middle picture shows the short transition where the structure 

was installed in less than one day. 
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Exhibit 9-3. Construction staging using pre-cast construction methods.

(48)

The staging of newly constructed bridge designs is not dependent on the technique used to build 

the structure. In fact, the illustration provided in Exhibit 9-3 could easily represent a more typical 

design that constructs the bridge in place. 

Staging techniques just prior to opening the DDI are also dependent on whether the site is a 

retrofit or new construction. For retrofit designs, the entire interchange will likely need to be 

closed for a short period of time. In some designs, the right-turn movements on and off the 

limited access facility are allowed while the crossovers are tied in and striped. This is usually 

done in a period of two to three days over a weekend. A different tie-in method used may include 

one of the crossovers being closed at a time, allowing one of the left-turn movements from the 

limited access facility to still take place. When the first crossover is complete, the other 

crossover is tied in using a similar technique. This method can be employed over a short period 

of time also, usually a weekend. For designs with new structures, a common time frame is 

needed to do the tie-ins. 

COSTS

One of the primary advantages of the DDI is the reduced costs associated with the design and 

construction compared to other typical interchange designs. In fact, this is the primary reason 

DDIs have taken a strong foothold in the transportation community in the past five years. In 

addition, the footprint of the DDI can often fit within the existing right-of-way and on an existing 

bridge, making it less expensive and faster to construct compared to other interchange forms 

previously noted in this chapter. Structural costs are the primary driving factor in how much 
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interchanges cost, making the DDI particularly attractive if being considered as a retrofit of an 

existing structure.

The actual costs of designing and constructing a DDI are highly variable based on the factors 

described above and other site-specific elements, particularly if a design is newly constructed 

versus a retrofit. Construction costs at several facilities constructed to date are provided in 

Exhibit 9-4. 

Exhibit 9-4. Construction cost estimates.(14)

Among several retrofit DDIs constructed to date, costs ranged from approximately $3 to $8.5 

million. The more expensive retrofit design at National Avenue incorporated an underpass 

facility at the adjacent intersection, while the DDI at Timpanogos Highway added an additional 

bridge to the interchange.

Among several new DDIs constructed to date, costs ranged from $14 to $22 million. 

MAINTENANCE

General maintenance considerations for a DDI are not significantly different than other 

interchange forms, though the following should be considered: 

• Lighting: High-mast systems are often more expensive to deploy, yet easier to maintain 

as they are usually installed in locations with easy access and outside the lane lines. 

Maintenance is done by lowering the luminaire ring from the mast head to the base using 

a winch and motor, making them accessible at the ground or using a small cherry picker. 

• Pavement markings: Pavement marking will need to be inspected and maintained more 

frequently than with normal interchange designs. The unique crossover design should be 

well marked at all times to make sure the lanes are easily seen to help prevent path 

overlap. Pavement marking wear will take place much faster as the full lane widths are 

often used when negotiating the reverse curves. 

Snow Removal 

Consistent for all types of interchanges, snow removal strategies focus on systematically pushing 

snow to the outside of the street. The DDI designer needs to work collaboratively with snowplow 

Interchange Location

Open to 

Traffic

Construction 

Cost Retrofit

Bessemer St. and US 129 Alcoa, TN 2010 $2.9 million Yes

MO 13 and I-44 Springfield, MO 2009 $3.2 million Yes

Winton Rd. and I-590 Rochester, NY 2012 $4.5 million Yes

National Ave. and US-60 Springfield, MO 2012 $8.2 million Yes

Timpanogos Hwy. and I-15 Lehi, UT 2011 $8.5 million Yes

Mid Rivers and I-70 St. Peters, MO 2013 $14 million No

CR 120 and Hwy 15 St. Cloud, MN 2013 $17.5 million No

Pioneer Crossing and I-15 American Fork, UT 2010 $22 million No
Diverging Diamond Interchange Informational Guide

187 

operators to provide end treatments that delineate curb locations (i.e., surface mounted 

delineators). Snowplow operators will need to become familiar with the DDI configuration and 

develop a sequence for plowing the different travel paths. Through lanes are typically plowed as 

part of a corridor. 

LAW ENFORCEMENT NEEDS 

The channelization of vehicle movements at a DDI creates a relatively “self-enforcing” 

interchange. Unlike many other alternative intersections and interchanges, there are no desired 

movements prohibited with signing and pavement marking alone. 

Unique law enforcement needs of a DDI are primarily related to the opening period. 

Enforcement during this period could help drivers become familiar with the crossed-over nature 

of the arterial and reduce unintentional wrong-way maneuvers. Enforcement may be most 

beneficial during low volume, nighttime hours when there is a decreased likelihood of opposing 

vehicles that would naturally help drivers avoid a wrong-way maneuver. 
Diverging Diamond Interchange Informational Guide

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REFERENCES

1. Utah Department of Transportation (UDOT). Diverging Diamond Interchange web page. 

http://www.udot.utah.gov/1100southddi/diverging-diamond-interchange.php

2. Georgia Department of Transportation. Diverging Diamond Interchange web page. 

http://www.perimetercid.org/canyouddi/what-is-the-ddi.html 

3. Minnesota Department of Transportation (MnDOT). Diverging Diamond Interchange web 

page. http://www.dot.state.mn.us/d3/hwy15ddi/Driving.html 

4. Idaho Transportation Department (ITD). Photo Credit. 

5. Rodegerdts, L. Photo Credit. 

6. Daleiden, A. Photo Credit. 

7. Missouri Department of Transportation. Diverging Diamond Interchange web page. 

http://www.modot.org/southwest/major_projects/Greene/documents/DDIExplanation-Trifold.pdf 

8. American Association of State Highway and Transportation Officials (AASHTO). A Policy on 

the Geometric Design of Highways and Streets. Washington, DC: AASHTO, 2011. 

9. Transportation Research Board (TRB). Highway Capacity Manual (HCM). Washington, DC: 

TRB, National Research Council (NRC), 2010. 

10. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. 

Washington, DC: U.S. Department of Transportation (USDOT), 2009. 

11. AASHTO. Highway Safety Manual, 1st Edition. Washington, DC: AASHTO, 2010. 

12. FHWA. Techbrief: Double Crossover Diamonds, Report No. FHWA-HRT-09-054, October 

2009. 

13. FHWA. Alternative Intersection/Interchanges: Informational Report (AIIR), Report No. 

FHWA-HRT-09-060. Washington, DC: USDOT, 2010. 

14. FHWA, Field Evaluation of Double Crossover Diamond Interchanges. Contractor’s draft 

submittal. Project No. DTFH61‐10‐C‐00029. March, 2014

15. FHWA, HCM Chapters/Guidance for Alternative Intersections/Interchanges, New HCM 

methodologies for DDI interchange. Saxton Transportation Lab Project 13-002. To be published. 

16. Missouri Department of Transportation (MoDOT). Missouri’s Experience with the Diverging 

Diamond Interchange. MoDOT Report No. OR10- 02. Jefferson City, MO: MoDOT, 2010. 

17. Chilukuri, V., et al. MoDOT Diverging Diamond Interchange Performance Evaluation (I-44 

& Route 13). MoDOT Report No. OR11-012. Jefferson City, MO: MoDOT, 2011. 

18. HNTB. Kentucky Transportation Cabinet DDI Evaluation Study, Presentation. 2009 ACECKY/FHWA/KYTC Partnering Conference. http://transportation.ky.gov/HighwayDesign/Conference%20Presentation/HNTB%20Divering%20Diamond%20Interchanges.pdf. 
Diverging Diamond Interchange Informational Guide

190 

19. FHWA, Techbrief: Driver Evaluation of the Diverging Diamond Interchange, Report No. 

FHWA-HRT-07-048, April 2012. 

20. Nevada Department of Transportation. Diverging Diamond Interchange web page. 

https://www.nevadadot.com/Traveler_Info/Safety/Diverging_Diamond_Interchange.aspx

21. FHWA, USDOT FHWA Information Videos web page. 

https://www.youtube.com/user/USDOTFHWA

22. FHWA, Safety Program Informational Brochures, web page. http://safety.fhwa.dot.gov. 

23. ADA.gov. “Information and Technical Assistance on the Americans with Disabilities Act.” 

http://www.ada.gov/2010ADAstandards_index.htm 

24. Ray, B. et al., NCHRP 15-34A: Performance-Based Analysis of Geometric Design of 

Highways and Streets, Contractor Final Report. Washington, DC: Transportation Research 

Board of the National Academies, 2014. 

25. FHWA and AASHTO. Results of Joint AASHTO/FHWA Context Sensitive Solutions 

Strategic Planning Process, Summary Report. Raleigh, NC.: Center for Transportation and the 

Environment, North Carolina State University, 2007. 

26. Google Inc. Google Earth. Accessed July 2014. 

28. National Association of City Transportation Officials (NACTO). Urban Bikeway Design 

Guide. New York, NY: NACTO, 2012.

27. AASHTO. Guide for the Development of Bicycle Facilities, 4th Edition. Washington, DC: 

AASHTO, 2012. 

29. MAC & Kimley-Horn & Associates, Inc. Photo Credit. 

30. New York State Department of Transportation (NYSDOT). Traffic Data.

31. Institute of Transportation Engineers. Manual of Transportation Engineering Studies, 2nd

Edition. Washington, DC: ITE, 2010. 

32. FHWA. Crash Modifications Factors (CMF) Clearinghouse. 

http://www.cmfclearinghouse.org/

33. FHWA. Freeway Management and Operations Handbook. FHWA Report No.: FHWA-OP04-003. Washington, DC, 2003. 

34. FHWA. Ramp Management and Control Handbook. FHWA Report No.: FHWA-HOP-06-

001. Washington, DC, 2006 

35. Messer, C., et al., NCHRP Synthesis 345, “Single Point Urban Interchange Design and 

Operations Analysis.” Washington, DC: Transportation Research Board of the National 

Academies, 1991. 

36. Chlewicki, Gilbert. New Interchange and Intersection Designs: The Synchronized SplitPhasing Intersection and the Diverging Diamond Interchange. Anaheim, CA : 2nd Urban Street 

Symposium, 2003. 
Diverging Diamond Interchange Informational Guide

191 

37. Bared, J. G., Edara, P. K. and Jagannathan, R. Design and Operational Performance of 

Double Crossover Interchange and Diverging Diamond Interchange. In Transportation Research 

Record 1912 (2005). 

38. FHWA. Traffic Analysis Tools web page. 

http://www.ops.fhwa.dot.gov/trafficanalysistools/index.htm 

39. Federal Highway Administration, University of Central Florida. CAP-X web page. 

http://tsi.cecs.ucf.edu/index.php/cap-x/release-info. 

40. Xu, H., Liu, H., Tian, Z., and Zhang, W. Control Delay Calculation at Diverging Diamond 

Interchanges. In Transportation Research Record 2257, (2011). 

41. Institute of Transportation Engineers (ITE). Freeways and Interchange Geometric Design 

Handbook. Washington, DC: ITE, 2005. 

42. Rodegerdts, L., et al., NCHRP Report 672, Roundabouts: An Informational Guide, Second 

Edition. Washington, DC: Transportation Research Board of the National Academies, 2010. 

43. UDOT Diverging Diamond Interchange (DDI) Observations and Experiences. Prepared by I15 CORE Traffic Team, Eric Rasband, UDOT, and Taylor Forbush and Kelly Ash, Horrocks 

Engineers. Report No. UT-12.05. April 2012.

44. AASHTO. Roadway Lighting Design Guide. Washington, DC: AASHTO, 2005. 

45.Illuminating Engineering Society of North America (IESNA). Recommended Practices for 

Roadway Lighting. New York, NY: IESNA, 2000. 

46. Staplin, L., K. Lococo, S. Byington, and D. Harkey. Highway Design Handbook for Older 

Driversand Pedestrians. Report No. FHWA-RD-01-103. Washington, DC: USDOT, 2001. 

47. Burgess and Nipple (B&N). Diverging Diamond Interchange, Presentation. 

http://www.in.gov/indot/files/FTWD_DivergingDiamondPP1_110711.pdf

48. RoadsBridges.com, True Pioneer web page. 

http://www.roadsbridges.com/sites/default/files/22_DDI%20in%20Utah.indd.pdf
Diverging Diamond Interchange Informational Guide

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Appendix A CATALOG OF ALL KNOWN INSTALLATIONS IN THE 

UNITED STATES

DDIs have been implemented in many different locations with a variety of design features. This 

appendix shows an inventory of 36 DDIs constructed to date at the time of this publication.

Exhibit A-1 presents location information for all known installations of DDIs in the United 

States. 

Exhibit A-1. Known installations of DDIs in the United States

Interchange Location Built URL

I-70 & US 6/US 50 Grand Junction. 

CO 2014 http://www.coloradodot.info/projects/i70exit26grandjunctionddi 

I-285 & Ashford 

Dunwoody Road Atlanta, GA 2012 

http://www.perimetercid.org/factsheets/A

shford_Dunwoody_Diverging_Diamond.

pdf

I-85 & Pleasant Hill Road Duluth, GA 2013 

http://duluth.patch.com/articles/bridge-ati-85-and-pleasant-hill-road-reopeningsunday-at-noon

U.S. 91/Yellowstone 

Highway & Interstate 86 Chubbock, ID 2013 http://itd.idaho.gov/projects/d5/Chubbuck

Interchange/ddInterchangeVideo.html 

I-35 & Homestead Lane Edgerton, KS 2013 http://i35swjohnsoncointerchange.ksdot.o

rg/ 

Harrodsburg Road & New 

Circle Road Lexington, KY 2011 

http://www.kentucky.com/2011/08/16/18

46182/motorists-navigate-newharrodsburg.html 

MD 295 & Arundel Mills 

Boulevard Hanover, MD 2012 http://www.roads.maryland.gov/pages/ddi

.aspx?PageId=397 

I-494 & 34th Street Bloomington, MN 2013 

http://bloomingtonmn.gov/cityhall/dept/p

ubworks/engineer/streets/curr_proj/20143

01/2014301.htm

Highway 15 & County 

Road 120 St. Cloud, MN 2013 http://www.dot.state.mn.us/d3/hwy15ddi/ 

US 52 & New Olmstead 

County Road 12 Oronoco, MN 2013 http://www.dot.state.mn.us/d6/newsrels/1

3/H52/9-3elkrun.html 

I-70 & Woods Chapel 

Road Blue Springs, MO 2014 

http://www.modot.org/kansascity/major_p

rojects/Woods_Chapel_Diverging_Diamo

nd.htm 

US-65 & MO-248 Branson, MO 2011 

http://bransontrilakesnews.com/news_free

/article_901ecb74-123f-11e1-ad73-

001cc4c002e0.html
Diverging Diamond Interchange Informational Guide

194 

Interchange Location Built URL

I-70 & Stadium Boulevard Columbia, MO 2013 http://www.modot.org/central/major_proj

ects/DivergingDiamond.htm 

US-67 & Columbia Street Farmington, MO 2012 

http://dailyjournalonline.com/farmingtonpress/news/local/new-diverging-diamondoverpass-opens/article_469bdaa0-f790-

11e1-9164-001a4bcf887a.html

I-44 & Range Line Road Joplin, MO 2013 http://www.modot.org/southwest/major_p

rojects/RangeLineRoadI-44DDI.htm 

I-435 & Front Street Kansas City, MO 2012 

http://www.kansascity.com/2012/05/24/3

626389/new-diverging-diamondinterchange.html

MO-150 & Botts Road Kansas City, MO 2012 http://www.modot.org/kansascity/major_p

rojects/Botts_Road.htm

I-44 & MO-13 Springfield, MO 2009 http://www.modot.org/southwest/major_p

rojects/greene/i-44androute13.html

US 60 & MO 13 Springfield, MO 2013 http://www.modot.org/southwest/major_p

rojects/Greene/I-44andRoute13.html 

US-60 & National Avenue Springfield, MO 2010 

http://douglascountyherald.com/2010/07/

15/traffic-changes-in-springfield-nationalavenue-james-river-freeway/

US-65 & East Chestnut 

Expressway Springfield, MO 2012 http://ozarksfirst.com/fulltext?nxd_id=71

9172 

I-70 & Mid River’s Mall 

Drive St Peter’s, MO 2013 

http://www.modot.org/stlouis/i70andmidriversmalldrivedivergingdiamon

dinterchange.htm 

I-270 & Dorsett Road St Louis, MO 2010 http://www.modot.org/stlouis/links/Diver

gingDiamondInterchange.htm

I-580/US-395 & Moana 

Lane Reno, NV 2012 

http://www.rgj.com/article/20121121/NE

WS/121121013/Freeway-interchangeMoana-Reno-now-open-traffic

I-590 & Winton Road Rochester, NY 2012 https://www.dot.ny.gov/590winton

I-270 & Roberts Road Columbus, Ohio 2013 https://www.dot.state.oh.us/districts/D06/

projects/270Roberts/Pages/default.aspx 

I-95 & Thurbers Avenue Providence, RI 2013 http://en.wikipedia.org/wiki/Diverging_di

amond_interchange 

US-129 & Bessemer Street Alcoa, TN 2010 

http://www.gspnet.com/News/Projects/Di

verging-Diamond-Interchange-DDIDesign-Completed 
Diverging Diamond Interchange Informational Guide

195 

Interchange Location Built URL

I-15 & Pioneer 

Crossing/Main Street 

American Fork, 

UT 2010 http://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523 

I-15 & S 500 E Street American Fork, 

UT 2011 http://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523 

I-15 & SR-92 (Timpanogos 

Highway) 

American Fork, 

UT 2011 http://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523 

SR-201 & Bangerter 

Highway 

West Valley City, 

UT 2011 http://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523

I-15/84 & US-91 Salt Lake City, 

UT 2013 http://www.udot.utah.gov/1100southddi/ 

I-15 & St. George 

Boulevard St. George, UT 2013 

http://www.udot.utah.gov/projects/f?p=25

0:2007:0::NO:2007:P2007_EPM_PROJ_

XREF_NO,P2007_PROJECT_TYPE_IN

D_FLAG:8431,A 

I-64 & US 15 Zion Crossroads, 

VA 2014 http://www.virginiadot.org/projects/culpeper/zioncrossroads.asp 

I-25 & College Drive Cheyenne, WY 2013 http://www.wyomingnews.com/articles/2

013/11/17/news/01top_11-17-13.txt 
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Appendix B SUPPLEMENTAL OPERATIONAL AND SAFETY

DETAILS

This appendix highlights the wrong-way maneuver trends of the findings from a six-month 

monitoring effort of wrong-way maneuvers at five DDI locations. Exhibit 4-8 illustrates the 

trends associated with inbound versus outbound movements, time-of-day, vehicle type, weather, 

and peak versus off-peak travel times.

Inbound versus Outbound Time of Day

Peak versus Off-Peak Vehicle Type

Weather

Exhibit B-1. Wrong-way maneuver trends.
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Appendix C MARKETING AND OUTREACH MATERIALS

This appendix provides some examples of DDI Public Outreach Material Examples. 

FHWA has created alternative intersection and interchange informational videos and video case 

studies, which can be viewed on the FHWA YouTube channel 

(https://www.youtube.com/user/USDOTFHWA). Exhibit C-1 is an example of the type of 

information provided in the video for the Diverging Diamond Interchange. 

 

Exhibit C-1. FHWA Diverging Diamond Interchange Informational Video.
Diverging Diamond Interchange Informational Guide

200 

In addition, FHWA has developed alternative intersection brochures that can be found on the 

FHWA website ( http://safety.fhwa.dot.gov). An example of the Diverging Diamond Interchange 

brochure is shown in Exhibit C-2. 

Exhibit C-2. FHWA Diverging Diamond Interchange Brochure.

Several examples from state and local agencies are provided below, although various others are 

available online for additional information and guidance. 

Educational Videos 

Several agencies have developed educational videos as part their outreach with DDIs. Examples 

weblinks are provided below for access to these videos. 

• Route 13 and I-44 DDI in Springfield, MO - 

http://www.youtube.com/watch?v=B5JtZMPTNAY 

• I-270 and Dorsett Road DDI in St. Louis, MO - 

http://www.youtube.com/watch?v=JIypFKBz4YI

• I-70 and Mid Rivers Mall Drive DDI in St. Peters, MO 

http://www.youtube.com/watch?v=yH72Q504nPs 

• Chubbock, ID DDI - 

http://itd.idaho.gov/projects/d5/ChubbuckInterchange/ddInterchangeVideo.html 
Diverging Diamond Interchange Informational Guide

201 

• North Carolina DOT - http://www.youtube.com/watch?v=HD-0QnUlLOQ

• UDOT DDIs - 

https://www.youtube.com/watch?v=LqE1Z77ccwQ&index=3&list=PLB029E1D3EB77E

979

https://www.youtube.com/watch?v=v20qJmnDPE&index=2&list=PLB029E1D3EB77E979 

http://www.youtube.com/watch?v=xWHEi8baCPE 

Brochures and Fact Sheets

• Exhibit C-3 illustrates a fact sheet from the MoDOT I-270 & Dorsett Road DDI project 

in Maryland Heights, MO. 

• Exhibit C-4 illustrates a fact sheet from the MoDOT Kansas Expressway & James River 

Freeway DDI project in Springfield, MO.

• Exhibit C-5 illustrates a project website for DDIs in Gwinnett County, GA.

• Exhibit C-6 illustrates a project website for the I-590 & Winton Road DDI project in 

Rochester, NY.

• Exhibit C-7 illustrates page one of three pages of a newsletter for the I-590 & Winton 

Road DDI project in Rochester, NY.

• Exhibit C-8 illustrates page two of three pages of a newsletter for the I-590 & Winton 

Road DDI project in Rochester, NY.

• Exhibit C-9 illustrates page three of three pages of a newsletter for the I-590 & Winton 

Road DDI project in Rochester, NY.

• Exhibit C-10 illustrates page one of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.

• Exhibit C-11 illustrates page two of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.

• Exhibit C-12 illustrates page three of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.

• Exhibit C-13 illustrates page four of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.

• Exhibit C-14 illustrates page five of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.
Diverging Diamond Interchange Informational Guide

202 

• Exhibit C-15 illustrates page six of a six-slide public presentation for the Union Cross 

DDI project in Winston-Salem, NC.

• Exhibit C-16 illustrates a project website and explanation of “how to drive through the 

DDI” for the I-270 & Roberts Road DDI project in Ohio.

• Exhibit C-17 illustrates page one of a two-page brochure from the Wisconsin DOT DDI 

project near Janesville, WI.

• Exhibit C-18 illustrates page two of a two-page brochure from the Wisconsin DOT DDI 

project near Janesville, WI.
Diverging Diamond Interchange Informational Guide

203 

Exhibit C-3. Fact sheet from the MoDOT I-270 & Dorsett Road DDI project in Maryland 

Heights, MO. 
Diverging Diamond Interchange Informational Guide

204 

Exhibit C-4. Fact sheet from the MoDOT Kansas Expressway & James River Freeway

DDI project in Springfield, MO. 
Diverging Diamond Interchange Informational Guide

205 

Exhibit C-5. Project website for DDIs in Gwinnett County, GA. 
Diverging Diamond Interchange Informational Guide

206 

Exhibit C-6. Project website for the I-590 & Winton Road DDI project in Rochester, NY.
Diverging Diamond Interchange Informational Guide

Exhibit C-7. Page one of a three-page newsletter for the I-590 & Winton Road DDI project 

in Rochester, NY.

207
Diverging Diamond Interchange Informational Guide

Exhibit C-8. Page two of a three-page newsletter for the I-590 & Winton Road DDI project 

in Rochester, NY.

208
Diverging Diamond Interchange Informational Guide

209

Exhibit C-9. Page three of a three-page newsletter for the I-590 & Winton Road DDI 

project in Rochester, NY.
Diverging Diamond Interchange Informational Guide

210

Exhibit C-10. Page one of a six-slide public presentation for the Union Cross DDI project in 

Winston-Salem, NC. 

Exhibit C-11. Page two of a six-slide public presentation for the Union Cross DDI project in 

Winston-Salem, NC. 
Diverging Diamond Interchange Informational Guide

211

Exhibit C-12. Page three of a six-slide public presentation for the Union Cross DDI project 

in Winston-Salem, NC. 

Exhibit C-13. Page four of a six-slide public presentation for the Union Cross DDI project 

in Winston-Salem, NC. 
Diverging Diamond Interchange Informational Guide

212

Exhibit C-14. Page five of a six-slide public presentation for the Union Cross DDI project in 

Winston-Salem, NC. 

Exhibit C-15. Page six of a six-slide public presentation for the Union Cross DDI project in 

Winston-Salem, NC. 
Diverging Diamond Interchange Informational Guide

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Exhibit C-16. Project website and explanation of “how to drive through the DDI” for the I270 & Roberts Road DDI project in Ohio. 
Diverging Diamond Interchange Informational Guide

214

Exhibit C-17. Page one of a two-page brochure from the Wisconsin DOT DDI project near 

Janesville, WI. 

Exhibit C-18. Page two of a two-page brochure from the Wisconsin DOT DDI project near 

Janesville, WI. 
Diverging Diamond Interchange Informational Guide

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safety.fhwa.dot.gov

Publication Number FHWA-SA-14-067

Front and back cover photos property of Federal Highway Administration