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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
×
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
×
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
×
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
×
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges. Washington, DC: The National Academies Press. doi: 10.17226/26072.
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1-1 This document presents a guide for pedestrian and bicycle safety at alternative intersections and interchanges (A.I.I.), based on NCHRP Project 07-25. The guide uses a principles-based approach that applies to any A.I.I. or conventional intersection form, including new A.I.I. forms not yet developed. However, this guide also provides specific guidance for four common A.I.I. forms being built in the United States: the Diverging Diamond Interchange (DDI), Restricted Crossing U-Turn (RCUT), Median U-Turn (MUT), and Displaced Left-Turn (DLT). The techniques in this guide can be extended to other A.I.I. forms, including Quadrant Roadway (QR), Jughandle (JH), Continuous-T-Intersection (CT), and Single-Point Urban Interchange (SPUI). These A.I.I. designs may involve crossover movements of vehicular travel lanes, redirecting turning movements, or both. Pedestrian paths and bicycle facilities may cross through islands, take different routes than expected, and require attention to design techniques to promote safety for people walking or biking. The concern is acute for visually impaired pedestrians, who require information about the alignment of crosswalks, signal controls (if present), crossing times, the direction of traffic, and the intended path through islands. The concern is also acute for pedestrians and bicyclists who may be forced to operate within the motor vehicle travel lanes if adequate pedestrian and bicycle facilities are not provided. The intended audience for this report is practitioners, researchers, and policymakers who establish federal, state, and local guidelines for pedestrian and bicycle treatments at A.I.I.s. This introductory chapter presents the scope of the study, the organization of the guide, and an overview of A.I.I. concepts. 1.1 Objective and Scope of Guide The objective of this guidebook is to improve and integrate pedestrian and bicyclist safety considerations at A.I.I.s through planning, design, and operational treatments that (1) identify and evaluate current practices and emerging technologies and trends, in the United States and internationally; (2) describe current best practices for measuring the effectiveness of such A.I.I. treatments; (3) evaluate the safety and operational outcomes of specific A.I.I. treatments; and (4) identify and rank treatments for typical types of projects. The primary focus of the guide- book is roadway functional classifications of collector and above. The guide addresses a broad range of issues related to improved safety for people walking or biking at A.I.I.s such as, but not limited to, the following: 1. Describing new and emerging A.I.I. designs [e.g., DDI, DLT or Continuous Flow (CFI) inter- sections, RCUT intersections, MUT intersections, and other alternative intersections] and evaluating their effects on pedestrians and bicyclists; C H A P T E R 1 Introduction

1-2 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges 2. Documenting, for each A.I.I. type, key considerations such as for crossing, traversing, and wayfinding for people walking and biking, including special consideration for designing for people with disabilities (including those who are visually and/or hearing impaired); 3. Discussing the benefits and tradeoffs of pedestrian and bicycle A.I.I. design and operational treatments with consideration of delay and safety for pedestrians and bicyclists; 4. Developing a performance-based process for practitioners to evaluate pedestrian and bicycle design elements in a two-stage intersection control evaluation (ICE) process; and 5. Providing recommendations for intersection design concepts that focus on people walking and biking, and discussing specific treatments and countermeasures for each A.I.I. form. Although the guide directly applies to common A.I.I. designs, it also outlines decision-making processes and criteria that can assist agencies in identifying flexible solutions for new or hybrid intersection forms, and conventional intersection forms. 1.2 Overview of Alternative Intersections and Interchanges A.I.I.s are designs conceived to improve operations and safety for motorized traffic by strategically adjusting the geometric features at a location. A.I.I.s work on the general principle of redistributing motor vehicle demand at an intersection to limit the need to add capacity (i.e., adding lanes) to improve traffic flow. 1.2.1 Characteristics Alternative intersections have four key characteristics intended to improve traffic flow and safety: 1. Use of one-way street elements: When in a signalized system, one-way streets are simpler to coordinate than two-way streets. In addition, for unsignalized or permissive signalized move- ments, one-way street elements offer a simpler gap acceptance process for drivers and people walking and biking. 2. Breaking up a larger intersection/junction into a mini-network of smaller intersections: A mini-network that is strategically designed can spread out the demand of all movements into potentially optimal locations. Smaller intersections are more likely to reduce the exposure to conflicts and require shorter clearance times and crossing distances for all modes. A smaller intersection footprint with fewer legs generally increases safety and efficiency. 3. Applying more efficient signal phases (when signalized): Signal phases can have concurrent phases with higher volume movements that are impossible at a traditional intersection. At many A.I.I.s, there is also a reduction in the total number of phases, which allows a higher percentage of green time for all movements and less clearance time per cycle. 4. Reducing and spreading out potential conflicts: By spreading out potential conflicts, a user has fewer conflicts to consider at a particular location. Sometimes, crossing conflict points, including those most likely to be associated with crashes leading to severe injuries or fatalities, are reduced or even eliminated. For grade-separated junctions, alternative interchanges differ from other interchange forms in that the alternative interchange form incorporates more of the above characteristics than a standard diamond or cloverleaf form. For example, a DDI creates two opposing one-way street elements; the turning movements from the ramps are more spatially separated, creating a mini- network; only two phases are needed at the ramp terminals; and conflict points are reduced and spread out compared to a standard diamond interchange.

Introduction 1-3 1.2.2 Potential A.I.I. Benefits A.I.I.s reconfigure the geometric design of an intersection or interchange to improve opera- tions and safety. This reconfiguration is accomplished by concentrating on redistributing demand to improve traffic flow and minimize or eliminate the need for adding travel lanes. This reconfiguration may result in a significant reduction of costs and right-of-way needs compared to alternatives that can produce similar operational benefits (although some A.I.I.s may require more right-of-way). Therefore, A.I.I.s can often be constructed more quickly than alternative options, such as grade-separated intersections. Exhibit 1-1 conveys the safety, mobility, and value benefits of A.I.I.s. 1.2.3 Intersection Forms and Context Conceptually, a good way to reduce crashes from a simple geometric standpoint is to reduce conflicts and conflict points. Interchanges eliminate some of the most severe conflicts by grade- separating one or more movements. However, interchanges are expensive and not always appro- priate or practical, especially for nonmotorized users who may be exposed to high-speed and/or high-volume conflicts with motor vehicles. At-grade A.I.I.s can reduce conflict points at a much lower cost, in a form more appropriate for the context of the location. A.I.I.s can be applied to various contexts. In rural locations, the major road of an A.I.I. will tend to have high motor vehicle speeds and low volumes of motor vehicles, pedestrians, and bicyclists. In urban locations, A.I.I.s will generally have low speeds and higher volumes of motor vehicles, pedestrians, and bicyclists. In suburban locations, A.I.I.s may often have high speeds and high volumes of motor vehicles. In some suburban locations that would otherwise represent key active transportation routes or connections, underdeveloped facilities for people walking and biking may suppress demand. Providing safe and comfortable facilities can unlock latent demand and encourage an increase in biking or walking in a community. Another important context consideration of an A.I.I. is that the intersection can accom- modate the crossing of roadways with very different characteristics. As an example, one roadway at an A.I.I. could be a major arterial that forms a critical part of a motor vehicle network, while the intersecting roadway could be a minor collector that forms a critical part of a pedestrian and bicyclist network. Volumes and speeds for each mode could vary greatly between the two roadways, as may the treatments for each mode. Exhibit 1-1. Potential A.I.I. benefits.

1-4 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges 1.2.4 Pedestrians and Bicyclists at A.I.I.s Because many A.I.I.s are relatively new concepts developed to address issues for motorized vehicular traffic, the historical focus has been on the details of the geometric design for motor vehicle traffic to facilitate designs that are safe and intuitive for people driving. As a consequence, design for people walking and biking at many existing A.I.I.s has either been an afterthought or been incorporated too late in the design process, meaning insufficient design elements for biking and walking were incorporated into the remaining space within the planned right-of- way. Sometimes, these features have been left out altogether. These projects have resulted in constructed infrastructure that degrades active transportation safety, dis courages walking and biking, and can be expensive to retrofit. Exhibit 1-2 shows a version of status quo pedestrian and bicycle facility provisions across the four primary A.I.I. forms discussed in this guidebook. The pedestrian facilities are shown in orange, while bicycle facilities are shown in green. Other examples of pedestrian and bicycle provision at A.I.I.s are included as well as examples of status quo provision at conventional intersection forms. Of note is the DDI walkway, which may be located interior to the crossover portions of the interchange, and the DLT, which is a Exhibit 1-2. “Typical” pedestrian and bicycle provisions at four A.I.I. forms.

Introduction 1-5 partial DLT as presented (two displaced left-turns) rather than a full DLT (four displaced left- turns). An assessment of pedestrian and bicyclist safety and comfort for these concepts, and additional design concepts, will be introduced later in this guidebook. Theoretically, the following approaches that benefit vehicular traffic in A.I.I.s should also benefit people walking and biking: 1. Use one-way street element: Pedestrians and bicyclists may have fewer conflicts crossing one-way streets versus two-way streets. 2. Break up a larger intersection/junction into a mini-network of smaller intersections: A mini-network should provide more opportunities for crossings, and smaller intersections should reduce both exposure and delay. 3. Apply more efficient signal phases (when signalized): Signal efficiencies generally reduce pedestrian and bicycle delay more than they do for motorized vehicles. 4. Reduce and spread out conflicts: This benefit applies to conflict points between all road users, not just among motor vehicles. 5. Free up right-of-way availability: If an A.I.I. can be designed within the reduced right-of- way, the right-of-way footprint should theoretically be available to provide a higher quality facility for people walking and biking, such as wider sidewalks with a street buffer or a separated bike lane. Although the benefits above are significant, there are substantial challenges to maximiz- ing the potential safety and operational benefits for people walking and biking, including the following: 1. Out-of-direction travel: Unless designed per the recommendations in this guidebook, people biking may be redirected alongside vehicles, causing delay and potential for difficult cross- weaving maneuvers into a U-turn lane. Out-of-direction travel also increases travel time. 2. Channelized movements: Bicyclists sharing space with motorists in channelized turn lanes or crossover lanes can be a safety concern. 3. Unconventional paths: With redirected vehicle movements, the pedestrian walkway may be in unusual, unexpected, or uncomfortable locations. 4. Multiple crossings: With staged crossings come multiple conflict points and multiple delay points that can result in complex and long movements through intersections. 5. Free-flow movements: Where channelized movements are free-flowing, gap acceptance may be challenging in a motor vehicle traffic stream that is high volume, high speed, or both, resulting in potential safety and delay consequences for people crossing on foot or bike. 6. Accessibility: Unusual vehicular travel patterns and pedestrian travel paths can cause severe wayfinding and crossing challenges for pedestrians, especially those with vision disabilities. In a successful A.I.I. design, the challenges listed here should be recognized and mitigated. The opportunities and challenges of A.I.I.s are summarized in Exhibit 1-3. These design concerns and others are presented and evaluated in Chapter 4. To maximize the safety and operations of design for nonmotorized users, these considerations need to be included in the design process from an early stage, before the intersection type has been selected and before right-of-way has been established or acquired. Nonmotorized safety elements should therefore be considered throughout the design process. Providing safe and comfortable walking and biking facilities at A.I.I.s (or at any intersection or interchange form) may unlock latent demand for these travel modes by making them viable for a larger portion of the population. Providing such facilities in a developing area without a high level of walking or biking activity may position the facilities to provide a key connection if growth occurs or land use changes. This guide explains and illus- trates the challenges and design process to provide safe and comfortable facilities for non- motorized users.

1-6 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges 1.3 Design and Evaluation Process The process for design and evaluation in this guidebook is structured to support ICE using a performance-based design approach. This section discusses these two aspects as they relate to pedestrian and bicyclist design at A.I.I.s. 1.3.1 Intersection Control Evaluation ICE policies are encouraged by FHWA (1) and are increasingly being adopted by state and local agencies to choose between intersection designs, including roundabouts and A.I.I.s. ICE policies are intended to guide users through sequential steps in conducting the evaluation. Users are encouraged to consider the evaluation context of a project and adapt the ICE frame- work. This could result in sketch-level evaluations that support quick planning-level decisions early in the design process, while the framework is also set up to provide detailed and robust evaluation activities to address complex projects. ICE is intended to be flexible and adaptive by the user for a project context. ICE activities could be streamlined on some projects; other projects might require analyses that are more extensive. Ultimately, ICE policies are intended to foster thoughtful consideration of alternative intersection and interchange types and to ensure that a holistic, quantitative analysis is completed when selecting an intersection or interchange configuration. As documented by FHWA, the ICE process typically has two stages: Stage 1, Scoping Analysis, and Stage 2, Alternative Selection. The key features of each evaluation, along with the methods for integrating pedestrians and bicyclists into the process, are presented here. This guidebook is not intended to provide a comprehensive overview of all aspects of the ICE process; further detail can be found elsewhere (1). 1.3.2 Performance-Based, Context-Sensitive Design The design and evaluation process that supports an ICE process is a performance-based, context-sensitive process as documented in NCHRP Report 785: Performance-Based Analysis of Geometric Design of Highways and Streets (2). This is illustrated in Exhibit 1-4. The process centers on identifying intended outcomes (design objectives and principles), establishing geometric design decisions with those objectives in mind (developing the design based on those principles), evaluating how well a design meets those objectives (performance assessment), and iterating as needed to produce the desired design (balance tradeoffs). These tasks are discussed further in these sections. Exhibit 1-3. Challenges and opportunities for people walking and people biking at A.I.I.s.

Introduction 1-7 1.3.2.1 Identify Intended Outcomes At ICE Stage 1, the intended outcome for motor vehicle operational performance is usually based on the desired target of capacity, volume-to-capacity ratio, or level of service for motorists. Sometimes, rather than meeting the desired target, the screening process may simply identify those alternatives that get the best performance possible for motorists based on the constraints of the project regarding potential impacts, financial constraints, or other factors. Safety perfor- mance for motorists is typically based on whether an alternative improves, or at least maintains, existing safety performance. Existing ICE tools do not include techniques to evaluate pedestrian or bicyclists’ operations or safety. The following discussion provides strategies for evaluating pedestrian and bicyclist operations and safety in an ICE process. For pedestrians and bicyclists in Stage 1, the intended outcome may vary based on context, as shown in Exhibit 1-5. A possible objective for pedestrians and bicyclists may be to ensure all users can complete each origin-destination pattern safely and efficiently (i.e., to serve all users for all movements). The question of facility type selection needs to occur at Stage 1 before the intersection design form has been set and before right-of-way has been established or acquired. Chapter 3 provides specific guidance on bicycle facility selection. At ICE Stage 2, a more detailed design is completed for a smaller number of alternatives, and as a result, more quantitative and qualitative performance measures can be obtained. For motorists in Stage 2, performance measures include delay and queuing in addition to capacity and volume-to-capacity ratios. Crash data is evaluated on the geometric safety side for all users to help identify more specific causes of crashes so as to reduce the number of crashes. Deter- mination of travel time within an intersection, A.I.I., or system may also be necessary. The need for this will be based on coordination between signals, when necessary, and cycle lengths. For pedestrians and bicyclists in Stage 2, the assessment process is built around evaluating a set of design flags that address safety, access, operations, and comfort aspects of travel. It may be necessary or desirable to evaluate the operations proposed for each alternative at Stage 2 to understand the potential impacts on travel time and delay for bicyclists and pedestrians operating Exhibit 1-4. Performance-based design process. Source: NCHRP Report 785, Exhibit 1-1 (2).

1-8 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges through the intersection more accurately. At Stage 2, the designer may want to evaluate different geometric configurations to determine what is best for safety and operations for all users. For example, changes from an assumption of free-flow movements to controlled movements will have a large effect on the assumed operational efficiency of the movement for each mode and its related impact on safety. An understanding of the implications of geometric and traffic control decisions for each mode is necessary to produce a design that meets the needs of each mode. 1.3.2.2 Establish Geometric Design Decisions From the intended outcomes identified in the previous section, the designer can create an initial geometric design for all users. This establishes the number of motor vehicle lanes needed to meet desired operational outcomes. Equally important is for the designer to establish an initial geometry for pedestrians and bicyclists, including the number and location of pedestrian facili- ties and crossings, and the bicycle facilities (on-street, shared, or separated). Just as there is no “default” lane configuration for a particular intersection, the configuration of pedestrian and bicycle facilities should be specific to the pedestrian and bicyclist access and safety needs. For all modes—motor vehicles, pedestrians, and bicyclists—these initial geometric design decisions are anticipated to be refined as alternatives proceed through the ICE process. Design objectives and principles are used to achieve the desired outcomes for a design. These design objectives often compete with one another, requiring an evaluation of tradeoffs. Resolving these tradeoffs must be sensitive to the context of the intersection location and the users being served, with careful consideration of the unique vulnerabilities of people walking or bicycling. AASHTO presents design objectives for intersections in the 2018 edition of its Policy on Geometric Design of Highways and Streets. (the “Green Book”) (3). These design objectives are similar to those presented for roundabouts in NCHRP Report 672, which is also based on a context-sensitive, performance-based design process (4). The design objectives presented by AASHTO are as follows: • Reduce vehicle speeds through the intersection; • Provide the appropriate number of lanes and lane assignment to achieve adequate capacity, lane volume, and lane continuity; • Provide channelization that operates smoothly, is intuitive to drivers, and results in vehicles naturally using the intended lanes; Exhibit 1-5. Intended outcomes for two-stage ICE process.

Introduction 1-9 • Adequately accommodate the design vehicles; • Meet the needs of pedestrians and bicyclists; and • Provide appropriate sight distance and visibility. (3, 9-4). The fifth bullet, “Meet the needs of pedestrians and bicyclists,” is the focus of this guidebook and can be expanded into these major objectives: • Maximize safety • Provide access to traverse or cross the facility • Assure accessibility for pedestrians with disabilities • Manage delay and travel time • Provide comfort 1.3.2.3 Evaluate Performance Outcomes Evaluating performance outcomes uses quantitative and qualitative measures. Performance outcomes for motor vehicles are typically quantifiable using tools based on the Highway Capacity Manual (HCM) for operational performance (5) and the Highway Safety Manual (HSM) for safety performance (6). Tools for completing this assessment at a Stage 1-level of detail include FHWA’s CAP-X (7) and crash modification factors (CMFs) provided in the HSM and FHWA’s CMF Clearinghouse (8). For pedestrians and bicyclists, quantitative and qualitative performance measures are avail- able for Stage 1 and Stage 2 evaluations. Chapter 4 of this guide presents these performance measures in an ICE context. 1.3.2.4 Refine Decisions Based on Performance Outcomes The performance evaluation may reveal that refinements to an alternative can make it viable enough to be realistically advanced to the next stage. In these cases, the design decisions should be refined and the alternative evaluated to confirm whether it should be carried forward to the next stage. 1.3.2.5 Assess Financial Feasibility The designer will need to assess whether this project can move forward, given the likely financial impact considerations. Such analysis is outside the scope of this guidebook. 1.3.2.6 Select Project or Alternative For Stage 1, the selection will consist of a few alternatives to be carried forward into Stage 2. For Stage 2, the selection will be of the preferred alternative to be carried forward into the final design. 1.3.2.7 Final Design Although not specifically covered in this guidebook, the assessment process should continue in an iterative process through various milestones of the final design process. Certain design flags may develop during the final design process that will need to be addressed. 1.4 Organization of Guide This guide is organized as follows: • Chapters 2 through 4 present the principles of good pedestrian and bicycle design practice, followed by the means to assess the effectiveness of a design. – Chapter 2: “Pedestrians,” summarizes pedestrian characteristics and safety considerations with relevance to A.I.I.s.

1-10 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges – Chapter 3: “Bicycles,” summarizes bicyclist characteristics and safety considerations, with added consideration for facility selection based on FHWA guidance. – Chapter 4: “Assessment,” introduces and discusses the two-stage assessment framework for pedestrian and bicycle safety at A.I.I.s in an ICE context. • Chapters 5 through 9 provide specific design guidance for four A.I.I. forms: Diverging Diamond Interchanges (DDIs), Displaced Left-Turn (DLT) Intersections, Restricted Crossing U-Turn (RCUT) Intersections, and Median U-Turn (MUT) Intersections, along with more general guidance for other designs. These chapters include presentation and discussion of specific pedestrian and bicycle designs and operational methods unique to each design type. – Chapter 5: “Generalized Design Treatments,” discusses overarching multimodal design concepts that apply to multiple A.I.I. forms. – Chapter 6: “Median U-Turn (MUT) Intersections,” presents specific guidance for the MUT intersection form. – Chapter 7: “Restricted Crossing U-Turn (RCUT) Intersections,” presents specific guidance for the RCUT, or Superstreet, intersection form. – Chapter 8: “Displaced Left-Turn (DLT) Intersections,” presents specific guidance for the DLT or CFI intersection form. – Chapter 9: “Diverging Diamond Interchanges (DDIs)” presents specific guidance for the DDI. 1.5 References 1. FHWA. 2018. Primer on Intersection Control Evaluation (ICE). Report No. FHWA-SA-18-076. FHWA, Washington, DC. 2. Ray, B. L., E. M. Ferguson, J. K. Knudsen, R. J. Porter, and J. Mason. 2014. NCHRP Report 785: Performance- Based Analysis of Geometric Design of Highways and Streets. Transportation Research Board of the National Academies, Washington, DC. 3. AASHTO. 2018. A Policy on Geometric Design of Highways and Streets, 7th Edition. AASHTO, Washington, DC. 4. Rodegerdts, L., J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, R. B. Crown, B. Guichet, and A. O’Brien. 2010. NCHRP Report 672: Round- abouts: An Informational Guide. Second Edition. Transportation Research Board of the National Academies, Washington, DC. 5. TRB. 2016. Highway Capacity Manual, Sixth Edition. Transportation Research Board of the National Acad- emies, Washington, DC. 6. AASHTO. 2010. Highway Safety Manual, First Edition. AASHTO, Washington, DC. 7. FHWA. Capacity Analysis for Planning of Junctions (CAP-X) Tool. https://www.fhwa.dot.gov/software/ research/operations/cap-x/. Accessed March 24, 2019. 8. FHWA. Crash Modification Factors Clearinghouse. http://www.cmfclearinghouse.org/. Accessed March 24, 2019.

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Alternative Intersections and Interchanges (A.I.I.s) are designs that improve operations and safety for motorized traffic by strategically adjusting the geometric features at a given location, working on the general principle of redistributing motor vehicle demand at an intersection in an attempt to limit the need to add capacity with new lanes to improve traffic flow.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 948: Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges provides specific guidance for four common A.I.I.s: Diverging Diamond Interchange (DDI), Restricted Crossing U-Turn (RCUT), Median U-Turn (MUT), and Displaced Left-Turn (DLT).

These designs may involve reversing traffic lanes from their traditional directions, which may introduce confusion and create safety issues for pedestrians and bicyclists. In addition, pedestrian paths and bicycle facilities may cross through islands or take different routes than expected. These new designs are likely to require additional information for drivers, bicyclists, and pedestrians as well as better accommodations for pedestrians and bicyclists, including pedestrians with disabilities.

NCHRP 20-44(35) is the implementation project for NCHRP Research Report 948. The implementation project's objective is to share and disseminate the research results with public agencies and provide hands-on technology transfer assistance to these agencies. Find project outcomes, including webinars and training materials, on the implementation project page.

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