Diverging Diamond Interchange Informational Guide, Second Edition (2021)

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9 This chapter contains guidance on how to consider alternative intersections and interchanges in general and diverging diamond interchanges (DDIs) in particular. This chapter summarizes policy and planning considerations related to a DDI. The subsequent 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. 2.1 Planning Considerations for Alternative Intersections and Interchanges Alternative intersection evaluations may vary depending on the stage of the project develop- ment process. Each project stage can affect how each of the policy and technical considerations is assessed. In many states, intersection control evaluation policies and processes described in Chapter 1 guide this evaluation. 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. 2.1.1 Serving Pedestrians and Bicycles A DDI offers an excellent opportunity to integrate multimodal facilities into an interchange. Almost all DDIs constructed to date include some combination of pedestrian and/or bicycle facilities. The reduced number of signal phases can make it easier to serve nonmotorized movements compared to a multi-phase signal. In most cases, the two-phase DDI signal 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 accep- tance process and reduces the risk for pedestrian-vehicle conflicts, provided pedestrians under- stand 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 C H A P T E R 2 Policy and Planning

10 Diverging Diamond Interchange Informational Guide date have been constructed with bicycle lanes through the crossovers, providing dedicated right- of-way 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 consid- erations and provides recommendations for how to achieve safe and efficient provisions for multimodal users of a DDI. 2.1.2 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. The DDI is an alternative to the conventional diamond interchange, high-capacity diamond forms such as the tight diamond and single-point diamond, as well as other interchange forms like a partial cloverleaf. The primary difference between a DDI and a conventional diamond inter- change 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. Exhibit 2-1. Relationship between volume and interchange type.

Policy and Planning 11 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. The DDI design has been shown to improve the operations of turning movements to and from the freeway facility and significantly reduce the number of vehicle-to-vehicle conflict points compared to a conven- tional diamond interchange. 2.2 Stakeholder Outreach Similar to other transportation projects, stakeholder outreach is a critical part of the overall planning process. Successful implementation of 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 consider- ations. 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 inter- change 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 UDOT and Nevada DOT project websites. Exhibit 2-4 is an example of a fact sheet of how Exhibit 2-2. Example video screen captures from UDOT of how to travel through a DDI (1).

12 Diverging Diamond Interchange Informational Guide Exhibit 2-3. Example video screen capture from Nevada DOT of how to travel through a DDI (2). Exhibit 2-4. Fact sheet from Minnesota DOT on how various users travel through a DDI (3).

Policy and Planning 13 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. 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, monitor- ing driver behavior and using law enforcement as necessary to promote proper use of the new form can aid driver acclimation. Exhibit 2-6 is an example of a DDI branding campaign used by Georgia Department of Transportation (GDOT) and a local improvement district. The branding campaign included a website, a unique logo, and a slogan titled, “Can You DDI? Arrive-Crossover-Drive.” This branding effort provides the public with an easy, identifiable look to this planned, urban DDI. FHWA has created alternative intersection and interchange informational videos and video case studies, which can be viewed on the FHWA YouTube channel (6). In addition, FHWA has developed alternative intersection brochures that can be found on the FHWA website (7). 2.3 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 Exhibit 2-5. Public outreach brochure used by MoDOT (4).

14 Diverging Diamond Interchange Informational Guide Exhibit 2-6. Example public outreach brochure used by GDOT (5).

Policy and Planning 15 asso ciated 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 Rehabilita- tion Act) (8). • Bicycle facilities. • Managed lane scenarios, including ramp metering. • Snow removal and storage. • Design vehicle. • Incident management. • Emergency response needs. 2.3.1 Access Management The subject of adjacent intersections has been one of the biggest concerns noted by prac- titioners building and operating DDIs and by academics who study the effects of DDIs on operations and safety. Adjacent intersections are problematic for many interchange configu- rations 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 motorists turning right from an exit ramp and weaving across traffic to make a left at the adjacent intersection, especially if right-turn-on-red operations are allowed from the exit ramps. 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. • Using grade separation to eliminate one or more signal phases at the intersection. • Using alternative intersection designs could be considered to reduce the number of necessary signal phases at adjacent intersections along the corridor. This treatment has been used along Poplar Tent Road in Charlotte, North Carolina, and at several locations in Utah. 2.4 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.

16 Diverging Diamond Interchange Informational Guide 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 identify key stakeholders for a particular project might include: – What is the purpose and function of the existing or planned road facilities? – What are the existing and planned land uses adjacent to and in the vicinity of the road facilities? – Who will likely desire to use the road facilities given the existing and planned land uses? – 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? – What are the perceived or actual shortcomings of the existing road facilities? – Who has jurisdiction over the facility? – Where is capital funding for the project originating (or expected to originate)? – 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 facili- ties. 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/overweight (OSOW) vehicles. 2.5 Planning Challenges The following are several challenges associated with planning DDIs: • Driver education—Successful implementations of DDIs are often preceded by public outreach and education campaigns, which are typically not conducted for conventional inter- section 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 configura- tion must consider and serve the various users who currently or may be expected to use the facilities. This should always include pedestrians and bicyclists, understanding that the exact provisions may necessarily vary from site to site. 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. • Complex signal timing and phasing—The opposing directions of the crossroad cross each other. • 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.

Policy and Planning 17 2.6 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 (9). 2.6.1 Accessibility Chapter 3 of this guide describes accessibility as it relates to special consideration given to pedestrians with disabilities including accommodating pedestrians with vision, mobility, or physical 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 (8). With respect to considering applicable inter section 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. 2.6.2 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. 2.6.3 Quality of Service Quality of service is defined as the perceived quality of travel by a road user. It is used in the HCM 6th edition 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. 2.6.4 Reliability Reliability is defined as the consistency of performance over a series of time periods (e.g., hour to hour, day to day, year to year). 2.6.5 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, includ- ing whether or not a crash involves a nonmotorized 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 crashes, it may be necessary to review a longer period of time to gain a more accurate understanding.

18 Diverging Diamond Interchange Informational Guide 2.7 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. 2.7.1 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. 2.7.2 Alternatives Identification and Evaluation The project needs identified in prior planning studies inform concept identification, develop- ment, 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 colla- borative, interdisciplinary approach that involves all stakeholders in providing a transporta- tion facility that fits its setting” (10). In considering the concept of “context sensitive design/ solutions,” this stage calls for meaningful and continuous stakeholder engagement to progress through the project development process. 2.7.3 Preliminary Design Concepts advancing from the previous stage are further refined and screened during pre- liminary design. For more complex, detailed, or impactful projects, the preliminary design (typically 30% 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 environ- mental 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, and • Utilities. Exhibit 2-7. Project development process.

Policy and Planning 19 2.7.4 Final Design The design elements are advanced and refined in final design. Typical review periods include 60%, 90%, and 100% plans before completing the final plans, specifications, and estimate. 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. 2.7.5 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. 2.8 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. Nevada Department of Transportation. “Diverging Diamond Interchange” web page. https://www. nevadadot.com/Traveler_Info/Safety/Diverging_Diamond_Interchange.aspx. 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. Missouri Department of Transportation. Diverging Diamond Interchange. https://www.modot.org/sites/ default/files/documents/southwest_district/ddiexplanation-trifold.pdf. 5. Georgia Department of Transportation. “Diverging Diamond Interchange” web page. http://www.perimetercid. org/canyouddi/what-is-the-ddi.html. Accessed August 1, 2014. 6. FHWA, U.S. Department of Transportation, FHWA Informational Videos, https://www.youtube.com/user/ USDOTFHWA. 7. FHWA, Safety Program Informational Brochures, web page. http://safety.fhwa.dot.gov. 8. Americans with Disabilities Act. “Information and Technical Assistance on the Americans with Disabilities Act.” http://www.ada.gov/2010ADAstandards_index.htm. 9. Ray, B., E. M. Ferguson, J. K. Knudsen, R. J. Porter, and J. Mason. NCHRP Report 785: Performance-Based Analysis of Geometric Design of Highways and Streets. Transportation Research Board of the National Academies, Washington, D.C., 2014. 10. Center for Transportation and the Environment, North Carolina State University. Results of Joint AASHTO/ FHWA Context Sensitive Solutions Strategic Planning Process, Summary Report. FHWA and AASHTO, Washington, D.C., 2007.