Diverging Diamond Interchange Informational Guide, Second Edition (2021)

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1 1.1 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 sub- stantive resources needed for planning, analysis, design, and public outreach and education are limited compared to those for traditional intersections. 1.2 Intersection Control Evaluations and Considerations The term “intersection” refers to the junction of two or more street facilities. A number of state and city transportation 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. In the context of an interchange, this intersection control evaluation refers to an evaluation of ramp terminal intersections. Approximately 10 states currently 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 objectives generally include but are not limited to the following: • Understanding the intended context and how operations, safety and geometry fit that con- text for each intersection or corridor including intended users [e.g., pedestrians, bicyclists, passenger cars, transit vehicles, freight, emergency responders, and oversize/overweight (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. • Comparing engineering and economic analysis results of practical alternatives that consider implementation costs, performance benefits and impacts (e.g., safety, multimodal, operations, environment, etc.), and the estimated service life of alternatives. C H A P T E R 1 Introduction

2 Diverging Diamond Interchange Informational Guide 1.3 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 diverging diamond interchanges (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 alter- native interchanges in general and DDIs in particular. Considerations discussed include poli- cies, 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 research efforts conducted as part of the development of this guide. Conflict points, wrong-way maneuvers, and emergency services at DDIs are discussed in this chapter as well. Chapter 5: Conceptual Operations—This chapter provides information on the unique operational characteristics of DDIs and how they affect elements such as traffic signal phasing and coordination. It provides considerations for developing the lane configuration and traffic signal phasing scheme of a DDI. Chapter 6: Geometric Design—This chapter describes the typical DDI design approach and provides guidance for geometric features. Chapter 7: Traffic Control Devices and Illumination Applications—This chapter presents information relating to the design and placement of signals, signs, pavement markings, and intersection lighting at DDIs. Chapter 8: Construction and Implementation—This chapter focuses on the constructability and maintenance of a DDI. The appendix included at the end of this guide presents detailed information on the safety study that developed the crash modification factors presented in Chapter 4. 1.4 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 furthest 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 tech- niques, evaluation procedures for assessing safety and operational performance, design guide- lines, 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 plan- ning topics and considerations when investigating intersection control forms. 1.5 DDI Overview The 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 dif- ference between a DDI and a conventional diamond interchange is the design of directional

Introduction 3 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 been 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 reduced-speed 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. 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 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. 1.6 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 environ- ments and design features of the constructed DDI. Exhibits 1-2 to 1-5 show several of the DDIs recently constructed in the United States. Exhibits 1-6 to 1-11 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. Exhibit 1-1. Key characteristics of a DDI. RTOR = right turn on red.

Exhibit 1-2. First constructed DDI in Utah at Pioneer Crossing and Interstate 15 (American Fork, Utah) (1). Exhibit 1-3. First constructed DDI in Georgia at Ashford Dunwoody Road near Perimeter Market (2). Exhibit 1-4. First constructed DDI in Minnesota at Highway 15 (3).

Introduction 5 Exhibit 1-5. First constructed DDI in Idaho at Interstate 86 and US-91/Yellowstone Highway (Pocatello-Chubbuck, Idaho) (4). Exhibit 1-6. Crossover location at the DDI located at SR-92 and Interstate 15 (Utah County, Utah) (5). Exhibit 1-7. Overhead signing at the DDI located at SR-92 and Interstate 15 (Utah County, Utah) (5).

6 Diverging Diamond Interchange Informational Guide Exhibit 1-8. Pedestrian crossing at the right-turn lane of the exit located at Pioneer Crossing and Interstate 15 DDI (American Fork, Utah) (6). Exhibit 1-9. Sidewalk and recessed lighting located on the bridge at the Pioneer Crossing and Interstate 15 DDI (American Fork, Utah) (6). Exhibit 1-10. DDI (overpass) at 500 East American Fork (American Fork, Utah) (1).

Introduction 7 Exhibit 1-11. DDI (underpass) at Dorsett Road and Interstate 270 (St. Louis, Missouri) (7). 1.7 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 move- ments 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 perfor- mance, 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 main- taining 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 off-tracking. These and other trade-offs of DDI geometric design are discussed in detail in Chapter 6. Exhibit 1-10 and Exhibit 1-11 illustrate typical designs for an overpass and underpass at a DDI. 1.8 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 (AASHTO “Green Book”) (8). • Highway Capacity Manual (HCM6) (9). • Manual on Uniform Traffic Control Devices (MUTCD) (10).

8 Diverging Diamond Interchange Informational Guide • 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 Federal Highway Administration (FHWA) publications. The following supplemental resource documents related to the DDI are available: • FHWA Techbrief on double crossover diamonds (FHWA-HRT-09-054) (12). • FHWA Project DTFH61-10-C-00029: Field Evaluation of Double Crossover Diamond Inter- changes. Contractor’s Draft Submittal (13). • Missouri Department of Transportation (MoDOT) Report: Missouri’s Experience with the Diverging Diamond Interchange (14). • Utah Department of Transportation (UDOT): DDI Guideline: A UDOT Guide to Diverging Diamond Interchanges (15). 1.9 References 1. Utah Department of Transportation. “Diverging Diamond Interchange” web page. http://www.udot.utah. gov/1100southddi/diverging-diamond-interchange.php. Accessed August 1, 2014. 2. Georgia Department of Transportation. “Diverging Diamond Interchange” web page. http://www.perimetercid. org/canyouddi/what-is-the-ddi.html. Accessed August 1, 2014. 3. Minnesota Department of Transportation. “Diverging Diamond Interchange” web page. http://www.dot. state.mn.us/d3/hwy15ddi/Driving.html. Accessed August 1, 2014. 4. Idaho Transportation Department. 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. Accessed August 1, 2014. 8. A Policy on Geometric Design of Highways and Streets, 7th ed. AASHTO, Washington, D.C., 2018. 9. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 10. Manual on Uniform Traffic Control Devices. FHWA, U.S. Department of Transportation, 2009. 11. Highway Safety Manual. AASHTO, Washington, D.C., 2010. 12. Techbrief: Double Crossover Diamonds. Report No. FHWA-HRT-09-054. FHWA, U.S. Department of Trans- portation, Washington D.C., October 2009. 13. Cunningham, C., B. Schroeder, J. Hummer, C. Vaughan, C. Yeom, K. Salamati, D. Findley, J. Chang, N. Rouphail, S. Bharadwaj, C. Jagadish, K. Hovey, and M. Corwin. Field Evaluation of Double Crossover Diamond Interchanges. Contractor’s Draft Submittal. FHWA, Project No. DTFH61-10-C-00029, 2014. 14. Missouri’s Experience with the Diverging Diamond Interchange. MoDOT Report No. OR10-021. Missouri Department of Transportation, Jefferson City, MO, 2010. 15. DDI Guideline: A UDOT Guide to Diverging Diamond Interchanges. Utah Department of Transportation, Salt Lake City, UT, June 2014. https://udot.utah.gov/main/uconowner.gf?n=14769524027177477.