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Roundabouts: An Informational Guide – Second Edition (2010)

Chapter: Chapter 3 - Planning

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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 3 - Planning." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Chapter 3/Planning Page 3-1 Roundabouts: An Informational Guide CHAPTER 3 PLANNING CONTENTS 3.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 3.2 PLANNING STEPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5 3.3 CONSIDERATIONS OF CONTEXT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6 3.3.1 Decision Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7 3.3.2 Site-Specific Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8 3.4 POTENTIAL APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10 3.4.1 New Residential Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11 3.4.2 Urban Centers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 3.4.3 Suburban Municipalities and Small Towns . . . . . . . . . . . . . . . . . . . 3-13 3.4.4 Rural Settings and Small Communities . . . . . . . . . . . . . . . . . . . . . . 3-14 3.4.5 Schools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15 3.4.6 Interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16 3.4.7 Gateway and Traffic Calming Treatments . . . . . . . . . . . . . . . . . . . . 3-16 3.4.8 Commercial Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 3.4.9 Unusual Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19 3.4.10 Closely Spaced Intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 3.5 PLANNING-LEVEL SIZING AND SPACE REQUIREMENTS . . . . . . . . 3-20 3.5.1 Planning Estimates of Lane Requirements . . . . . . . . . . . . . . . . . . . . 3-21 3.5.2 Mini-Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24 3.5.3 Space Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26 3.5.4 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27 3.6 COMPARING PERFORMANCE OF ALTERNATIVE INTERSECTION TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-30 3.6.1 Two-Way Stop-Control Alternative . . . . . . . . . . . . . . . . . . . . . . . . . 3-31 3.6.2 All-Way Stop-Control Alternative . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32 3.6.3 Signal Control Alternative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32

Roundabouts: An Informational Guide Page 3-2 Chapter 3/Planning 3.7 ECONOMIC EVALUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33 3.7.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-34 3.7.2 Estimating Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-35 3.7.3 Estimation of Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-37 3.8 PUBLIC INVOLVEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-38 3.8.1 Audience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-39 3.8.2 Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-39 3.8.3 Public Meetings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-39 3.8.4 Informational Brochures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-40 3.8.5 Websites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-41 3.8.6 Informational Videos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-43 3.8.7 Media Announcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-44 3.8.8 User Education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-44 3.9 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-45

Chapter 3/Planning Page 3-3 Roundabouts: An Informational Guide LIST OF EXHIBITS Exhibit 3-1 Planning Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5 Exhibit 3-2 Residential Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11 Exhibit 3-3 Urban Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 Exhibit 3-4 Small Town/Municipality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13 Exhibit 3-5 Rural Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14 Exhibit 3-6 Schools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15 Exhibit 3-7 Interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16 Exhibit 3-8 Gateway Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 Exhibit 3-9 Commercial Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18 Exhibit 3-10 Unusual Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19 Exhibit 3-11 Closely Spaced Intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 Exhibit 3-12 Planning-Level Daily Intersection Volumes . . . . . . . . . . . . . . . . . . 3-22 Exhibit 3-13 Traffic Flows at a Roundabout Entry . . . . . . . . . . . . . . . . . . . . . . . . 3-23 Exhibit 3-14 Volume Thresholds for Determining the Number of Entry Lanes Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23 Exhibit 3-15 Example Planning-Level Exercise for Determining Required Numbers of Lanes Using Turning-Movement Data . . . 3-24 Exhibit 3-16 Planning-Level Maximum Daily Service Volumes for Mini-Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25 Exhibit 3-17 Area Comparison: Single-Lane Roundabout versus Comparable Signalized Intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27 Exhibit 3-18 Area Comparison: Multilane Roundabout versus Comparable Signalized Intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28 Exhibit 3-19 Average Control Delay per Vehicle at the MUTCD Peak-Hour Signal Warrant Thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33 Exhibit 3-20 Estimated Costs for Crashes of Varying Levels of Severity . . . . . 3-36 Exhibit 3-21 Example of Public Information Poster . . . . . . . . . . . . . . . . . . . . . . . 3-40 Exhibit 3-22 Examples of Scale Roundabout Models for Public Involvement . 3-41 Exhibit 3-23 Examples of Project-Specific Informational Brochures . . . . . . . . . 3-42 Exhibit 3-24 Example of General Informational Brochure . . . . . . . . . . . . . . . . . 3-42 Exhibit 3-25 Examples of Roundabout Websites . . . . . . . . . . . . . . . . . . . . . . . . . 3-43

Roundabouts: An Informational Guide Page 3-4 Chapter 3/Planning 3.1 INTRODUCTION At the planning stage, there are a variety of possible reasons or goals for considering a roundabout at a particular intersection. In some states, consideration of a roundabout alternative is a requirement of all intersection analyses. Meanwhile, other locations may have a specific reason for evaluating a roundabout as an alternative, including improving safety or operations, improving aesthetics, assisting with access management, or promoting redevelopment. However, whatever the reasons for considering a roundabout, a number of common considerations should be addressed at the planning level: • Is a roundabout appropriate for this location? • How big should it be or how many lanes might be required? • What sort of impacts might be expected? • What public education and outreach might be appropriate? Chapter 1 presented a range of roundabout categories and suggested typical daily service volume thresholds below which four-leg roundabouts may be expected to operate, without requiring a detailed capacity analysis. Chapter 2 introduced roundabout performance characteristics, including comparisons with other intersection forms and control, which will be expanded upon in this chapter. This chapter covers the next steps that lead to the decision as to whether a roundabout is a feasible alternative. By confirming that there is good reason to believe that roundabout construction is feasible and that a roundabout is the best alternative, these planning activities avoid expending unnecessary effort required in more detailed steps. The initial steps in planning for a roundabout are to clarify the objectives and understand the context in which the roundabout is being considered. The next step is to specify a preliminary configuration. This identifies the minimum num- ber of lanes required on each approach and thus which type of roundabout is the most appropriate to use as a basis for design: mini, single-lane, or multilane. Given sufficient space, roundabouts can be designed to accommodate high traffic volumes. There are many additional levels of detail required in the design and analysis of a high-capacity, multilane roundabout that are beyond the scope of a planning-level procedure; these are given in later chapters. Therefore, this chapter focuses on the more common questions that can be answered using reasonable assumptions and approximations. Feasibility analysis requires an approximation of some of the design parameters and operational characteristics. Depending on the specific situation, it may be necessary to explore beyond base-level approximations with respect to one or more key attributes of the roundabout to ensure compatibility and feasibility. Consideration must also be given to the potential trade-offs between safety, operations, and design when planning for roundabouts. Particularly in the early stages of planning, these key aspects and their impacts on one another can help determine a roundabout’s feasibility. Some changes in these approximations may be necessary as the design evolves. A more detailed methodology for Planning determines whether a roundabout is even feasible, before expending the effort required for more detailed analysis and design.

Chapter 3/Planning Page 3-5 Roundabouts: An Informational Guide performing the operational evaluation and geometric design tasks is presented later in Chapters 4 and 6 of this guide, respectively. 3.2 PLANNING STEPS Exhibit 3-1 outlines many of the considerations that may need to be investigated prior to deciding whether to implement a roundabout at an intersection. Note that this is not intended to be all-encompassing, nor is it intended to reflect minimum Exhibit 3-1 Planning Framework Clarify the Objectives Clarify ing the objective for considering a roundabout at the beginning of the process may help to better guide the selection of an appropriate treatment and determine the need for additional information. • Is the improvement needed from an operational or safety perspective? Both? • Is the improvement desired to control vehicle speeds? • Is the improvement intended purely for aesthetic reasons? Etc. Consider the Context • Is this the first roundabout in a community or are roundabouts already well established? • Are there regional policy constraints that must be addressed? • Are there site-specific and/or community impact reasons why a roundabout of any size would not be a good choice? • What are the site constraints? • What is the potential for future growth within the vicinity? • What is the current or desired environment for non-motorized modes? Determine Preliminary Lane Numbers Based on Capacity Requirements (Section 3.5) Section 3.5 provides a useful methodology for obtaining a basic understanding of the required number of lanes. Chapter 4 provides additional detail on operational analysis. Determine the Space Requirements How big does it need to be and is there enough right-of-way to build it? This is a potential rejection point in some locations due to potential cost or the additional administrative complications caused by right-of-way acquisition. Section 3.5 provides additional information for evaluating the space requirements based upon the required number of lanes. Compare to Other Alternatives Make appropriate comparisons with alternative intersection treatments. Assess Other Impacts Are there other impacts that may occur from the roundabout, such as: • Utilities, • Existing buildings/structures, • Business access, and • Sensitive environmental areas. Assess Other Opportunities Does the roundabout offer any opportunities to improve existing conditions, such as: • Improve access management, • Stimulate redevelopment, • Improve safety, and • Improve oddly shaped intersection or other poor geometric condition. Is a roundabout feasible and/or a preferred alternative worthy of advancing for additional analysis and design?

Roundabouts: An Informational Guide Page 3-6 Chapter 3/Planning requirements. Rather, it is intended to provide a general framework for the steps typically necessary in identifying feasibility. The results of the steps above should be documented to some extent. The level of detail in the documentation will vary among agencies and will be influenced by the size and complexity of the roundabout. A roundabout feasibility study report may include the following elements: • It may identify why a roundabout is being considered as an improvement alternative at this intersection. • It may identify the current status of traffic operations and safety at the intersection for comparison with expected roundabout performance. • It may identify a conceptual roundabout configuration, which includes the number of lanes on each approach and the designation of those lanes. • It may demonstrate whether an appropriately sized and configured roundabout can be implemented feasibly. • It may identify all potential complicating factors, assess their relevance to the location, and identify any mitigation efforts that might be required. Where more complete or formal rationale is necessary, the roundabout feasi- bility study report may also include the following additional considerations: • It may demonstrate institutional and community support, indicating that key institutions (e.g., police, fire department, and schools) and key com- munity leaders have been consulted. • It may give detailed performance comparisons (including delay, capacity, emissions, and/or interaction effects with nearby intersections) of the roundabout with alternative control modes. • It may include an economic analysis indicating that a roundabout compares favorably with alternative control modes from a benefit–cost perspective. • It may include a detailed discussion about potential trade-offs between safety, operations, and design. • It may include detailed appendices containing traffic volume data, signal or all-way stop-control warrant analysis, and so on. 3.3 CONSIDERATIONS OF CONTEXT Adherence to sound planning and engineering principles will ensure that the decision to install a roundabout in a specific location is made appropriately. This guide focuses on principles, recognizing that each specific case or instance brings with it a myriad of unique opportunities and challenges. Suggested contents of a round- about feasibility study report.

Chapter 3/Planning Page 3-7 Roundabouts: An Informational Guide 3.3.1 DECISION ENVIRONMENTS The decision process for considering a roundabout can be significantly influ- enced by the environment in which the roundabout is being considered. While the same basic analysis tools and concepts apply to all environments, the relative importance of the various aspects and observations may differ, as may other policy decisions. At least three environments present unique opportunities and challenges for roundabout implementation: roundabouts in a new roadway system, the first roundabout in an area, and a retrofit of an existing intersection. A new roadway system. Fewer constraints are imposed if the location under consideration is not a part of an existing roadway system. Right-of-way is usually easier to acquire or commit. Other intersection forms also offer viable alternatives to roundabouts. There are often no field observations of site-specific problems that must be addressed. This situation is more commonly faced by private development than by public agencies, and thus coordination between private and public interests in the planning, analysis, and design of the roundabout becomes important. The first roundabout in an area. The first roundabout in any geographic area often faces significantly higher levels of public interest, if not apprehension, in the concept of a roundabout, and an early failure of the process could take years to recover from. This situation requires an implementing agency to be diligent regarding operational and design aspects of roundabouts, community impacts, user needs, and public acceptability, and to work interactively with the public and elected officials in communicating those aspects. On the other hand, a successfully implemented roundabout, especially one that solves a demonstrated problem, could be an important factor in gaining support for future roundabouts at appro- priate locations. Some important considerations for this decision environment include the following: • Efforts should be directed toward gaining community and institutional support for the selection of a site for the first roundabout in an area. Public acceptance, as for any complex project, requires agency staff to understand the potential issues and communicate these effectively with the impacted community. • An extensive justification effort may be necessary to gain the required support, accomplished through one or several of the techniques outlined in Section 3.8 (Public Involvement). • A cautious and conservative approach may be appropriate; careful con- sideration should be given to conditions that suggest that the benefits of a roundabout might not be fully realized. Collecting data on current users of the intersection can provide important insights regarding potential issues and design needs. • A single-lane roundabout in the near-term is more easily understood by most drivers and therefore may have a higher probability of acceptance by the motoring public. However, in several communities throughout the United States, multilane roundabouts have been quite successful as the first roundabouts within the area. A focus on good design and public education is important when considering a multilane roundabout. Will the roundabout be: • Part of a new roadway? • The first in an area? • A retrofit of an existing intersection? The first roundabout in an area may require greater education and justification efforts. Single-lane round- abouts will generally be more easily understood initially than multilane roundabouts.

Roundabouts: An Informational Guide Page 3-8 Chapter 3/Planning • The choice of design and analysis procedures could set a precedent for future roundabout implementation; therefore, the full range of design and analysis alternatives should be explored in consultation with other operating agencies in the region. • After the roundabout is constructed, evaluating and documenting its operation and the public response could support future installations. Many agencies that are contemplating the construction of their first round- about have a natural tendency to keep the roundabout as simple as possible. This typically means jurisdictions are reluctant to introduce multilane roundabouts until single-lane roundabouts have gained some success. It is also a common desire to avoid intersection designs that require additional right-of-way because of the effort and expense involved in right-of-way acquisition. Important ques- tions to be addressed in the planning phase are therefore: • Will a minimally configured roundabout (i.e., single-lane entrances and circulatory roadway) provide adequate capacity and performance for all users, or will additional lanes be required on some legs or at some future time? • Can the roundabout be constructed within the existing right-of-way, or will it be necessary to acquire additional space beyond the property lines? • If additional right-of-way is indeed required at the intersection to construct a roundabout, are there opportunities to reduce the overall cross section of the adjacent roads to offset the impact and provide a benefit to properties near the roundabout? • Can a single-lane roundabout be designed for economical future expan- sion to accommodate growth? Retrofit to an existing intersection in an area where roundabouts have already gained acceptance. This environment is one in which a solution to a site-specific problem is being sought. Communities with experience limited to single-lane roundabouts may now be comfortable pursuing opportunities to use higher-capacity multilane roundabouts. Within the region, design and evaluation procedures may also be better defined than in communities that are exploring their first roundabouts. The basic objectives of the selection process in this case are to demonstrate how the community will be affected and that a roundabout will function properly during the peak period within the capacity limits imposed by the space available, and to decide which one is the preferred alternative. If the required configuration involves additional right-of-way, a more detailed analysis will probably be necessary using the methodology described in Chapter 4. 3.3.2 SITE-SPECIFIC CONDITIONS Within the context of evaluating intersection alternatives, each individual location has its own unique characteristics, issues, and objectives for improvement. The optimal control choice will be the one that best balances those objectives. Roundabouts offer benefits under many circumstances; however, they may also be more complicated to implement in comparison to other control types. The following discussion identifies several site-related factors that may significantly influence a

Chapter 3/Planning Page 3-9 Roundabouts: An Informational Guide roundabout design. These factors should be taken into consideration when com- paring alternatives and how well each balances the improvement objectives: • Physical or geometric complications may significantly influence a round- about’s design and may make a roundabout infeasible or uneconomical. These could include right-of-way limitations, grades or unfavorable topog- raphy, utility conflicts, drainage problems, intersection skew, and so on. • Designated routes or proximity of generators of significant types of traffic may result in vehicles with difficulty negotiating the roundabout, such as oversized trucks (also known as “superloads”). At the planning stage, the evaluation of the space requirement may warrant the consid- eration of a larger footprint to accommodate high volumes of oversized vehicles. • Other nearby traffic control devices requiring preemption, such as at-grade rail crossings, could create queuing interactions with the roundabout that need to be addressed. • Nearby bottlenecks could routinely back up traffic into the roundabout, such as over-capacity signals or drawbridges. The successful operation of a roundabout depends on unimpeded flow on the circulatory roadway. If traffic on the circulatory roadway comes to a halt, momentary intersection gridlock can occur. In comparison, other intersection treatments may have fewer adverse effects under those conditions. • Intersections of a major arterial and a minor arterial or local road could create an unacceptable delay to the major road. Roundabouts delay and deflect all traffic entering the intersection and could introduce excessive delay or speed inconsistencies to flow on the major arterial. • Heavy pedestrian or bicycle movements could conflict with high motor vehicle traffic volumes. • In situations with intersections located on arterial streets within a coordi- nated signal network, the level of service on the arterial might be better with a signalized intersection operating in coordination to minimize arterial through movement delay. The existence of one or more of these conditions does not necessarily preclude the installation of a roundabout. Roundabouts have, in fact, been built at locations that exhibit nearly all of the conditions listed above. Such factors may be resolved in several ways: • They may be determined to be insignificant at the specific site; • They may be resolved by operational modeling or by adding specific design features; • They may be resolved through coordination with and support from other agencies, such as the local fire department, school district, and so forth; or • In some cases, specific design treatments may be required to address concerns.

Roundabouts: An Informational Guide Page 3-10 Chapter 3/Planning While not every complicating factor needs to be completely resolved prior to the choice of a roundabout as the preferred intersection alternative, each should have a reasonable certainty of resolution to ensure a successful project. The effect of a particular factor will often depend on the degree to which round- abouts have been implemented in the region. There are conditions that would not be expected to pose problems in areas in which roundabouts are an established form of intersection control that is accepted by the public. On the other hand, some conditions may suggest that the installation of a roundabout be deferred until this control mode has demonstrated regional acceptance. Most agencies have an under- standable reluctance to introduce complications at their first roundabout. 3.4 POTENTIAL APPLICATIONS Roundabouts serve as one potential tool within the toolbox of intersection control options and should be considered in a wide array of possible applications. There are numerous reasons for selecting a roundabout as a preferred alternative, with each reason carrying its own considerations and trade-offs. This section pro- vides a cursory overview of several example locations or situations where round- abouts are often considered. It also highlights situations where trade-offs may exist or certain aspects of the overall roundabout design may require further investigation to determine the feasibility of a roundabout and whether it is the preferred alternative. Strategies and methods to address potential issues associated with these and other applications with respect to operations, safety, and geometric design of roundabouts can be found in Chapters 4, 5, and 6, respectively. It may be easier to install the first roundabout in an area in a location with the fewest complications. On the other hand, a successful roundabout in a complicated area can often make subsequent roundabouts easier to install.

Chapter 3/Planning Page 3-11 Roundabouts: An Informational Guide 3.4.1 NEW RESIDENTIAL SUBDIVISION Developers have begun to use roundabouts in residential subdivisions with increasing frequency (see Exhibit 3-2). Roundabouts provide a variety of operational and aesthetic benefits and create a sense of place that is attractive to developers and homeowners. Exhibit 3-2 Residential Subdivision Denver, Colorado Benefits Considerations • Calming effect on traffic promotes lower travel speeds • Aesthetic benefits (community enhancement/gateway treatment) • Single-lane roundabout often appropriate given relatively low traffic volumes within neighborhoods • Pedestrian and bicycle friendly • Design vehicle (emergency/fire, garbage, large moving trucks) • Right-of-way needs • Driveway access to corner properties • Landscaping • Illumination

Roundabouts: An Informational Guide Page 3-12 Chapter 3/Planning 3.4.2 URBAN CENTERS Urban settings (see Exhibit 3-3) are active areas and typically have a mix of competing considerations and users—passenger cars, buses, emergency vehicles, trucks, pedestrians, and bicyclists—throughout the day, all in a constrained environment. Roundabouts may be considered an optimal choice in situations where existing or planned access-management strategies along a corridor facil- itate U-turn movements at nearby intersections. Roundabouts accommodate U-turns without requiring tight turning radii for vehicles or introducing significant amounts of delay to left-turning vehicles at conventional intersections. Exhibit 3-3 Urban Center Annapolis, Maryland Benefits Considerations • Promotes lower vehicular speeds and can reduce delay and emissions • Enhances pedestrian safety • Provides for aesthetic treatments (monuments, landscaping, etc.) • Low maintenance (no signals, detector loops) • Complementary to access management programs • Design vehicle • Right-of-way needs • Accessibility for pedestrians who are blind or have low vision • Emergency vehicle access/parking • Roadway system operations (e.g., interaction with adjacent signals) • Sight distance

Chapter 3/Planning Page 3-13 Roundabouts: An Informational Guide Brunswick, Maryland Benefits Considerations • May improve operations and decrease delay compared to two-way stop-control (TWSC) or signalized control • May provide a safer alternative to signalized control for locations where TWSC fails but minor street volumes remain relatively low • May address an existing safety deficiency • Lower speeds • Lower maintenance costs • Design vehicle • Pedestrian, bicycle, and transit access • Central island maintenance • Intersection visibility under high speed conditions 3.4.3 SUBURBAN MUNICIPALITIES AND SMALL TOWNS Smaller municipalities are often ideal locations to consider roundabouts (see Exhibit 3-4). Right-of-way is often less constrained, traffic volumes are lower, and the aesthetic opportunities for landscaping and gateway treatments are enticing. Existing operational and/or safety deficiencies can also often be addressed. Exhibit 3-4 Small Town/Municipality

Roundabouts: An Informational Guide Page 3-14 Chapter 3/Planning 3.4.4 RURAL SETTINGS AND SMALL COMMUNITIES Rural settings typically have different needs than urban centers or larger communities. Safety may often be the driving factor over capacity in making a roundabout an appealing choice. Within small communities along an extended highway, a roundabout is ideal for supporting speed reductions. Roundabouts have been demonstrated to be a particularly effective treatment in reducing fatali- ties and injuries at intersections on high-speed roadways. Roundabouts located on high speed roadways, particularly in rural settings (see Exhibit 3-5), may require additional design modifications to slow drivers in advance of the intersection. These can include geometric design features such as extended splitter islands and introducing horizontal curvature on high-speed approaches to slow drivers, using the physical alignment of the roadway rather than speed zones (signs) and other passive methods. Exhibit 3-5 Rural Setting Clackamas County, Oregon Benefits Considerations • May improve operations and decrease delay compared to TWSC or signalized control • May provide safer alternative to signalized control for locations where TWSC fails but minor street volumes remain relatively low • May address an existing safety deficiency • Lower speeds • Lower maintenance costs • Design vehicle • Pedestrian, bicycle and transit access • Central island maintenance • Intersection visibility under high speed conditions • Illumination

Chapter 3/Planning Page 3-15 Roundabouts: An Informational Guide 3.4.5 SCHOOLS Roundabouts may be an optimal choice for intersection control in the vicinity of schools (see Exhibit 3-6). One primary benefit is the reduction of vehicle speeds in and around the roundabout. Roundabouts improve pedestrian crossing opportunities, providing mid-block refuge and the ability for pedestrians to focus on one traffic stream at a time while crossing. Lower speeds also reduce the severity of vehicle–pedestrian crashes. Near schools, single-lane roundabouts are generally preferable to multilane roundabouts due to simpler crossings for children. However, if the traffic volume is sufficiently high, a multilane round- about may still be preferable to a large signalized intersection. Exhibit 3-6 Schools Clearwater, Florida Benefits Considerations • Lower vehicle speeds in and around intersection • Improved pedestrian and vehicle safety • Landscaping and gateway treatment • Design vehicle (school bus, emergency vehicles) • Right-of-way • User education and outreach • If crossing guards are used, the distance between crosswalks may require two crossing guards instead of one.

Roundabouts: An Informational Guide Page 3-16 Chapter 3/Planning Gig Harbor, Washington Benefits Considerations • Lower vehicle speeds and reduced speed differential through interchange area • Narrower bridge cross section— reduced cost • Landscaping and gateway treatments • Design vehicle (trucks, emergency vehicles) • Right-of-way • Signing and wayfinding • Driver familiarity 3.4.6 INTERCHANGES Interchange ramp terminals are potential candidates for roundabout inter- section treatment (see Exhibit 3-7). This is especially true if the subject interchange typically has a high proportion of left-turn flows from the off-ramps and to the on-ramps during certain peak periods, combined with limited queue storage space on the bridge crossing, off-ramps, or cross street approaches. Roundabouts at ramp terminals may also reduce the required width and/or length of bridges, providing a significant cost benefit. Exhibit 3-7 Interchanges The planning focus for commu- nity enhancement roundabouts should be to demonstrate that they will not create traffic prob- lems that do not now exist. 3.4.7 GATEWAY AND TRAFFIC CALMING TREATMENTS Roundabouts have been used as a part of a community enhancement project and not necessarily as a solution to capacity or safety problems. Such projects are often located in commercial and civic districts as a gateway treatment (see Exhibit 3-8) to convey a change of environment and to encourage traffic to slow down. A roundabout may also be appropriate as a traffic calming measure when the following conditions are present: • Documented observations of speeding, high traffic volumes, or careless driving activities;

Chapter 3/Planning Page 3-17 Roundabouts: An Informational Guide Mini-roundabouts can be appropriate for traffic calming purposes at local street intersections or intersections of minor collectors and local streets. Small, single-lane roundabouts are typically preferable for traffic calming purposes at intersections of two collector streets. Traffic volumes are typically well below the thresholds for single-lane roundabouts discussed in Section 3.5. 3.4.8 COMMERCIAL DEVELOPMENTS Roundabouts in commercial developments provide for a central focus point for a development and enhance aesthetic qualities (see Exhibit 3-9). They are also able to process high volumes of traffic when properly designed. • Inadequate space for roadside activities, or a need to provide slower, safer conditions for both vehicular and non-automobile users; or • New construction (road opening, traffic signal, new road, etc.) that would potentially increase the volumes of cut-through traffic. Roundabouts proposed as gateway treatments often require less rigorous analysis as a traffic control device. The main focus of roundabouts proposed as traffic calming features should be to demonstrate that they would not introduce traffic problems that do not currently exist. Particular attention should be given to any complications that could induce operational or safety problems. Conditions that traffic calming roundabouts may address. Exhibit 3-8 Gateway Treatment Naples, Florida Benefits Considerations • Central island provides ample space for aesthetic treatments • Minimal impact to traffic operations • Increases landscaping opportunities • Design vehicle (trucks, emergency vehicles) • Right-of-way

Roundabouts: An Informational Guide Page 3-18 Chapter 3/Planning Exhibit 3-9 Commercial Developments Westminster, Colorado Benefits Considerations • Introduce geometric delay to slow drivers • Improve safety of both vehicular and non-automobile users • Landscaping opportunities can enhance local neighborhoods • Where a series of roundabouts is used, the roundabouts allow for easy U-turn movements, so minor commercial driveways can easily be restricted to right-in, right-out, improving safety between intersections as well. • Design vehicle (emergency vehicles, moving trucks) • Right-of-way • Access to adjacent properties into or near the roundabout

Chapter 3/Planning Page 3-19 Roundabouts: An Informational Guide Exhibit 3-10 Unusual Geometry Colville, Washington Benefits Considerations • Effectively manage traffic flows in situations with unique geometric conditions • Reduced delay compared to signalized scenarios • Design vehicle (trucks, emergency vehicles) • Right-of-way • Entry path deflection and alignment 3.4.9 UNUSUAL GEOMETRY Intersections with unusual geometric configurations, intersection angles, or more than four legs are often difficult to manage operationally (see Exhibit 3-10). Roundabouts are a proven traffic control device in such situations, effectively managing traffic flows without the need for costly expenditures on unique signal controller equipment or unusual signal timing.

Roundabouts: An Informational Guide Page 3-20 Chapter 3/Planning 3.5 PLANNING-LEVEL SIZING AND SPACE REQUIREMENTS This section discusses planning-level techniques to determine the type of roundabout. Capacity and size are interrelated based on the number of lanes that will be required to accommodate the forecast traffic volumes. Section 3.5.1 provides a method for determining necessary lanes based on average annual daily traffic (AADT) volume data or a more refined method using turning-movement volumes. Planning-level capacity information for mini-roundabouts is provided in Section 3.5.2. Based upon the identified number of lanes required for the roundabout, the size and general footprint can be estimated using information provided in Section 3.5.3. Additional design considerations that correspond to the required size of the roundabout are provided in Section 3.5.4. Exhibit 3-11 Closely Spaced Intersections Numbers of lanes and space requirements are important planning analysis results. 3.4.10 CLOSELY SPACED INTERSECTIONS Roundabouts balance traffic flows and manage queue lengths between closely spaced intersections. The example shown in Exhibit 3-11 serves as the intersection of three streets configured into a pair of roundabouts. Buffalo, New York Benefits Considerations • Reduce queues and balance traffic flow • Accommodate range of access (public and private) • Capacity analysis needed to confirm operations • Right-of-way

Chapter 3/Planning Page 3-21 Roundabouts: An Informational Guide In general, single-lane roundabouts have a number of benefits over larger multilane roundabouts, including improved safety performance, simpler navigation for pedestrian and bicycle users, smaller footprints, and ease of use for motorists. Therefore, practitioners should reconsider the traditional transportation planning technique of using a 20-year traffic horizon for sizing a roundabout. If design-year traffic volumes indicate the need for a multilane roundabout but this need is not likely for several years, consideration should be given to phasing in the roundabout implementation so that it can be built initially as a single-lane roundabout. How- ever, it should also be designed to be readily expandable to a multilane roundabout if the traffic volumes actually increase as predicted. Chapter 6 provides additional discussions regarding the design of roundabouts for expandability. 3.5.1 PLANNING ESTIMATES OF LANE REQUIREMENTS A basic question that needs to be answered at the planning level is how many lanes are required throughout a roundabout to serve the traffic demand. The number of lanes not only affects the capacity of the roundabout but also its size. This section provides planning-level considerations for the purpose of the initial screening of roundabout feasibility. More detailed operational analyses (Chapter 4) may be required at later stages to confirm the planning level findings. Some assumptions and approximations have been necessary in this chapter to produce a planning-level approach. High-level planning often requires an initial screening of alternatives where turning-movement data may not be available but AADT volumes are known. Exhibit 3-12 presents ranges of AADT volumes to identify scenarios under which single- and two-lane roundabouts may perform adequately. A range of left turns from 0% to 40% of the total volume is an input to Exhibit 3-12 to improve the prediction of the potential capacity. The percentage of left turns on any given approach affects the conflicting volumes on other entries. Therefore, the potential capacity of the roundabout is reduced as the percentage of left turns increases. Within Exhibit 3-12, four general ranges of volumes are identified. These ranges represent volume thresholds where one-lane or two-lane roundabouts should operate acceptably and ranges of volumes over which more detailed analysis is required. This procedure is offered as a simple, conservative method for estimating roundabout lane requirements. As an example, if the twenty-four- hour volumes fall within the lowest range of volumes indicated in Exhibit 3-12, a single-lane roundabout should have no operational problems at any time of the day. This graph is applicable for the following conditions, with other conditions requiring more detailed analysis: • Ratio of peak-hour to daily traffic (K) of 0.09 to 0.10, • Direction distribution of traffic (D) of 0.52 to 0.58, • Ratio of minor street to total entering traffic of 0.33 to 0.50, and • Acceptable volume-to-capacity ratio of 0.85 to 1.00. The intermediate threshold for each type of roundabout (one-lane and two- lane) is based on the most conservative combination of the above factors; the upper threshold is based on the combination to produce the highest AADT

Roundabouts: An Informational Guide Page 3-22 Chapter 3/Planning If the volumes fall within the ranges identified in Exhibit 3-12 where “addi- tional analysis is needed,” a single-lane or two-lane roundabout may still func- tion quite well, but a closer look at the actual turning-movement volumes during the design hour is required. The procedure for such analysis is presented in Chapter 4. Where existing and/or projected turning-movement data is available at the planning level, an improved estimate of the required lane configurations can be identified. Even if future projections of turning movements are not available, estimating future turning movements using existing turning movements and a reasonable annual growth rate may provide a sufficient level of accuracy for this planning exercise. The procedure provided within this section is a simplification of the capacity estimates presented in Chapter 4. The capacity of a roundabout is generally driven by the amount of conflict- ing traffic (vehicles traveling along the circulatory roadway) that is present at each roundabout entry. High conflicting volumes reduce the number of oppor- tunities for vehicles to enter the roundabout and therefore reduce the capacity of a particular approach leg. Conversely, where low conflicting traffic volumes are present, the approach leg will have a higher capacity and allow for a higher number of vehicles to enter the roundabout. Each approach leg of the round- about is evaluated individually to determine the number of entering lanes that are required based upon the conflicting flow rates. The number of lanes within the circulatory roadway is then the number of lanes needed to provide lane continuity (e.g., K of 0.09, D of 0.52, minor street ratio of 0.50, and volume-to-capacity ratio of 1.00). It is suggested that a reasonable approximation of lane require- ments for a three-leg roundabout may be obtained using 75% of the service volumes shown on Exhibit 3-12. Exhibit 3-12 Planning-Level Daily Intersection Volumes 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000 0% 10% 20% 30% 40% Left-Turn Percentage A A D T Double-lane roundabout likely to operate acceptably Single-lane roundabout may be sufficient (additional analysis needed) Single-lane roundabout likely to operate acceptably Double-lane roundabout may be sufficient (additional analysis needed)

Chapter 3/Planning Page 3-23 Roundabouts: An Informational Guide through the intersection. More detailed lane assignments and refinements to the lane configurations can be determined later through a more formal operations analysis. The sum of the entering (ve) and conflicting (vc) traffic volumes, as illustrated in Exhibit 3-13, can be used to evaluate the number of lanes required on the entry (1). If the sum of the entering and conflicting volumes is less than 1,000 vehicles per hour (veh/h), then a single-lane entry can be reasonably assumed to operate within its capacity. Exhibit 3-14 provides additional planning-level lane requirements for various combinations of entering and circulating volumes, and Exhibit 3-15 gives an example of planning-level calculations. Exhibit 3-13 Traffic Flows at a Roundabout Entry Rule of Thumb: If the sum of the entering and circulating volumes for each approach is less than 1,000 veh/h, then a single-lane roundabout is likely to operate acceptably. Exhibit 3-14 Volume Thresholds for Determining the Number of Entry Lanes Required Volume Range (sum of entering and conflicting volumes) Number of Lanes Required 0 to 1,000 veh/h Single-lane entry likely to be sufficient 1,000 to 1,300 veh/h Two-lane entry may be needed Single-lane may be sufficient based upon more detailed analysis. 1,300 to 1,800 veh/h Two-lane entry likely to be sufficient Above 1,800 veh/h More than two entering lanes may be required A more detailed capacity evaluation should be conducted to verify lane numbers and arrangements. Source: New York State Department of Transportation

Roundabouts: An Informational Guide Page 3-24 Chapter 3/Planning 3.5.2 MINI-ROUNDABOUTS Mini-roundabouts are distinguished from traditional roundabouts primarily by their smaller size and more compact geometry. They are typically designed for negotiating speeds of 15 mph (25 km/h). Inscribed circle diameters generally vary from 45 to 80 ft (13 to 25 m). For most applications peak-period capacity is seldom an issue, and most mini-roundabouts operate on residential or collector streets at demand levels well below their capacity. It is important, however, to be able to assess the capacity of any proposed intersection design to ensure that the intersec- tion would function properly if constructed. At very small roundabouts, it is reasonable to assume that each quadrant of the circulatory roadway can accommodate only one vehicle at a time. In other words, a vehicle may not enter the circulatory roadway unless the quadrant on both sides of the approach is empty. Given a set of demand volumes for each of the 12 standard movements at a four-leg roundabout, it is possible to simulate the roundabout to estimate the maximum service volumes and delay for each Exhibit 3-15 Example Planning-Level Exercise for Determining Required Numbers of Lanes Using Turning- Movement Data Example: Estimating Number of Lanes Using Turning-Movement Volumes Question How many lanes are required to serve these design-year traffic volumes: Calculations Entering volume + Circulating volume = X Compare to Exhibit 3-14 250 + 617 = 867 <1,000 Single Lane OK 534 + 224 = 758 <1,000 Single Lane OK 317 + 534 = 851 <1,000 Single Lane OK 751 + 203 = 954 <1,000 Single Lane OK Conclusions Using the traffic data identified above and the volume ranges in Exhibit 3-14, a single-lane roundabout would be estimated to adequately handle the design-year traffic volumes for each individual approach. Therefore, a single-lane roundabout could be advanced forward for the rest of the planning-level evaluations, including estimates of the intersection footprint (right-of-way needs) and impacts.

Chapter 3/Planning Page 3-25 Roundabouts: An Informational Guide approach. By making assumptions about the proportion of left turns and the proportion of cross-street traffic, a general estimate of the total entry maximum service volumes of the roundabout can be made; an example is provided in Exhibit 3-16. AADT maximum service volumes are represented based on an assumed K value of 0.10. Note that these volumes range from slightly more than 12,000 to slightly less than 16,000 vehicles per day. The maximum through- put is achieved with an equal proportion of vehicles on the major and minor roads and with low proportions of left turns. Because of their mountable nature, mini-roundabouts do not provide the same degree of visibility and channelization provided by larger roundabouts with raised islands. As a result, mini-roundabouts have some notable limitations in application: • Mini-roundabouts are recommended primarily for areas in which all approaching roadways have an 85th-percentile speed of less than 30 mph (50 km/h) or less. Although some traffic calming may result from their use (and they could be integrated into a broader system of traffic calming measures), the mini-roundabout should be limited to use in lower speed environments. • Mini-roundabouts are not recommended in locations in which high U-turn traffic is expected, such as at the ends of street segments with access restrictions. However, the mini-roundabout should be designed to accommodate U-turns for passenger cars. Due to radius restrictions of the small inscribed circle diameter, larger vehicles may not be capable of making a U-turn movement. • Mini-roundabouts are not well suited for high volumes of trucks, as trucks will occupy most of the intersection when turning, significantly Exhibit 3-16 Planning-Level Maximum Daily Service Volumes for Mini-Roundabouts Mini-roundabouts are not recommended where approach speeds are greater than 30 mph (50 km/h), nor in locations with high U-turning volumes. 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 10% 30% 50% Percent Left Turns A A D T Ca pa ci ty (v eh icl es ) 25% Cross Traffic 50% Cross Traffic

Roundabouts: An Informational Guide Page 3-26 Chapter 3/Planning reducing the capacity of a mini-roundabout. Additionally, high volumes of trucks overrunning the central island may lead to rapid wear of the roadway markings. 3.5.3 SPACE REQUIREMENTS An initial estimate of the space (footprint) required for a roundabout is a com- mon question at the planning stage and may affect the feasibility of a roundabout at any given location. At this planning level, important questions may begin to be explored including: • Is sufficient space available to accommodate an appropriately sized roundabout? • What property impacts might be expected? • Is additional right-of-way likely to be required? • Are there physical constraints that may affect the location and design of the roundabout? Due to the need to accommodate large trucks (such as WB-50 or WB-67 tractor- trailer combinations) through the intersection, roundabouts typically require more space than conventional intersections. However, this may be offset by the space saved compared with turning lane requirements at alternative inter- section forms. The key indicator of the required space is the inscribed circle diameter. A detailed design is required to determine the space requirements at a specific site, especially if more than one lane is needed to accommodate the entering and circulating traffic. One important question is whether or not the proposed roundabout will fit within the existing property lines or whether additional right-of-way will be required. Exhibit 3-17 and Exhibit 3-18 illustrate that roundabouts typically require more area at the junction than conventional intersections. (Mini- roundabouts are not shown because they are assumed to be located within the footprint of a conventional intersection.) However, as capacity needs increase the size of the roundabout and comparable conventional (signalized) intersec- tion, the increase in space requirements is increasingly offset by a reduction in space requirements on the approaches. This is because the widening or flaring required for a roundabout can be accomplished in a shorter distance than is typically required to develop left-turn lanes and transition tapers at conven- tional intersections. Intersection skew can also affect the area impacts, and may require approach realignment or a large inscribed circle diameter to obtain appropriate geometry. Roundabouts often offer the potential for reducing special requirements on approaches compared to conventional intersections. This effect of providing capacity at the intersections while reducing lane requirements between inter- sections, known as the wide nodes, narrow roads concept, is discussed further in Chapter 2. Although roundabouts typically require more area at the junc- tion compared to conventional intersections, they may not need as much area on the approaches.

Chapter 3/Planning Page 3-27 Roundabouts: An Informational Guide 3.5.4 DESIGN CONSIDERATIONS Designing roundabouts involves trade-offs among safety, capacity, impacts, costs, and other factors. While a much more detailed discussion regarding roundabout geometric design is provided in Chapter 6, fundamental design considerations should be evaluated early on at a planning level to produce a better understanding of the size and potential impacts for the roundabout alternative. In the end, designing a roundabout involves determining the optimal balance between safety, operational performance, and accommodating appropriate design vehicles given the specific parameters and constraints for the site under evaluation. 3.5.4.1 Design Vehicle The choice of design vehicle will vary depending upon the approaching road- way types and the surrounding land use characteristics. The local or state agency with jurisdiction of the associated roadways should usually be consulted to iden- tify the design vehicle at each site. Appropriate design vehicle consideration will depend on road classification, input from jurisdictions and/or road authorities, and the surrounding environment. On larger statewide facilities, such as interstate freeway ramps or intersections with state highway facilities, it may be necessary to accommodate large WB-67 trucks or even oversized vehicles (superloads). Smaller design vehicles may often be chosen at local street intersections. The size of the design vehicle often has a direct effect on the size of the inscribed circle diameter required. In general, larger roundabouts are often used to accommodate large vehicles while maintaining low speeds for passenger vehicles. In some cases, land Exhibit 3-17 Area Comparison: Single- Lane Roundabout versus Comparable Signalized Intersection

Roundabouts: An Informational Guide Page 3-28 Chapter 3/Planning constraints also dictate the need for approach re-alignment to adequately accommodate large semi-trailer combinations while achieving appropriate deflection for small vehicles. In particular, at locations where a WB-67 is anticipated to be the design vehicle, a larger inscribed circle diameter should be planned for when estimating the space requirements of the roundabout. Design vehicles alone should not dictate roundabout designs or specific dimensions. It is often beneficial to engage local stakeholders to ensure that the proper design is developed. In the case of larger vehicles, it may be appropriate to choose another route entirely, negating the need to design the roundabout to accommodate these vehicles. In rural locations, a farm vehicle may be the most appropriate design vehicle and require special attention. 3.5.4.2 Speeds and Path Alignment Achieving appropriate vehicular speeds through the roundabout is a critical design objective that may affect safety. A well-designed roundabout reduces the relative speeds between conflicting traffic streams by requiring vehicles to negoti- ate the roundabout along a curved path. Any conceptual design(s) prepared at the Exhibit 3-18 Area Comparison: Multilane Roundabout versus Compara- ble Signalized Intersection

Chapter 3/Planning Page 3-29 Roundabouts: An Informational Guide planning level should depict reasonable entry deflection for speed control. Detailed procedures for evaluating the fastest path speeds through a roundabout are provided in Chapter 6 and may be used to verify reasonableness. In addition to evaluating vehicle speeds, the design of a multilane roundabout should naturally align entering lanes into their appropriate lane within the circu- latory roadway and then to the appropriate lanes on the exit. If the alignment of one lane interferes or overlaps with that of an adjacent lane, the roundabout may not operate as safely or efficiently as possible. At the planning level, any concep- tual designs prepared should be visually evaluated for reasonable alignment of the entry lanes to the corresponding lanes within the circulatory roadway. Designing to achieve both speed reductions and adequate path alignment may require offsetting of the approach alignment to the left of the existing road- way centerlines or other techniques that could affect the space required for the roundabout. As such, when evaluating the space availability for a roundabout, constraints along the approach roadways should also be identified. 3.5.4.3 Pedestrians In urban and suburban areas where pedestrians are expected, important design considerations include: • Minimizing the number of travel lanes to improve the simplicity and safety of roundabouts for pedestrians, • Designing for slow vehicle speeds, • Providing sidewalks that are set back from the circulatory roadway, • Providing well-defined and well-located crosswalks, and • Providing splitter islands with at least a width of 6 ft (1.8 m) at the crosswalks. Chapter 6 includes detailed information on providing these design considerations. 3.5.4.4 Bicyclists Safety and usability of roundabouts for bicyclists depends on the details of the roundabout design and special provisions for bicyclists. Since typical on-road bicyclist travel speeds are 12 to 20 mph (19 to 32 km/h), roundabouts that are designed to constrain the speeds of motor vehicles to similar values will minimize the relative speeds between bicyclists and motorists and thereby improve safety and usability for cyclists. Single-lane roundabouts are much simpler for cyclists than multilane roundabouts since they do not require cyclists to change lanes to make left-turn movements or otherwise select the appropriate lane for their direction of travel. Cyclists who have the knowledge and skills to ride effectively and safely on roadways can navigate low-speed single lane roundabouts without much difficulty. The primary design consideration for single-lane roundabouts is to terminate bicycle lanes prior to roundabouts and not include bicycle lanes on circulatory roadways.

Roundabouts: An Informational Guide Page 3-30 Chapter 3/Planning At multilane roundabouts and other roundabouts where typical on-road cyclists may not feel comfortable traversing some roundabouts in the same manner as other vehicles, bicycle ramps can be provided to allow access to the sidewalk or a shared-use path at the roundabout. More details about termi- nating bicycle lanes and providing bicycle ramps at roundabouts can be found in Chapter 6. 3.6 COMPARING PERFORMANCE OF ALTERNATIVE INTERSECTION TYPES A roundabout is often compared to other intersection types, usually either a stop- or signal-controlled intersection. Chapter 4 provides operational perform- ance evaluation models that may serve as a sound basis for comparison, but their application may require more effort and resources than an agency is prepared to devote in the planning stage. Similarly, Chapter 5 provides more detailed safety evaluation procedures, but those can require more data and effort than necessary for establishing roundabout feasibility. To simplify the planning process, the following generalized information is offered for a planning-level operational comparison of control modes: • A roundabout will always provide a higher capacity and lower delays than all-way stop-control (AWSC) operating with the same traffic volumes. • A roundabout is unlikely to offer better performance in terms of lower overall delays than TWSC at intersections with minor movements (including cross-street entry and major-street left turns) that are not experiencing, nor predicted to experience, operational problems under TWSC. • A single-lane roundabout may be assumed to operate within its capacity at any intersection that does not exceed the peak-hour volume warrant for signals. • A roundabout that operates within its capacity will generally produce lower delays than a signalized intersection operating with the same traffic volumes. Roundabouts offer significant benefits for improving safety and may easily be justified solely on the basis of crash reductions, particularly for reducing serious injury and fatal crashes. Recent research of roundabouts in the United States iden- tified crash reductions of approximately 35.4% for all crashes and 75.8% for injury crashes when an intersection was converted from a signal or stop control to a roundabout (2). Single-lane roundabouts generally offer greater safety benefits than multilane roundabouts due to fewer points of conflict. The decision to install a roundabout as a safety improvement should be based on a demonstrated safety problem of the type susceptible to correction by a roundabout. A review of crash reports and the type of crashes occurring is essential.

Chapter 3/Planning Page 3-31 Roundabouts: An Informational Guide Examples of safety problems that are potentially correctable by roundabouts include: • High rates of crashes involving right angle, head-on, left/through, and U-turn conflicts; • High crash severity (injury or fatality crashes); • Sight distance or visibility problems that reduce the effectiveness of stop sign control (in this case, landscaping of the roundabout needs to be care- fully considered); and • Inadequate separation of movements, especially on single-lane approaches. The remainder of this section provides additional planning-level guidance on operational and safety comparisons to other intersection control alternatives, including TWSC, AWSC, and signal control. 3.6.1 TWO-WAY STOP-CONTROL ALTERNATIVE The majority of intersections in the United States operate under TWSC, and most of these intersections operate with minimal delay. A roundabout is unlikely to offer better performance in terms of lower overall delays than TWSC at inter- sections with minor movements (including cross-street entry and major-street left turns) that are not experiencing, nor predicted to experience, operational prob- lems under TWSC. Therefore, the installation of a roundabout at a TWSC intersec- tion that is operating satisfactorily will be difficult to justify on the basis of operational performance improvement alone. From a safety perspective, roundabouts offer significant benefits over TWSC intersections. Research of U.S. roundabouts has identified that average reductions of 44.2% for all crashes and 81.8% for injury crashes have been observed when converting TWSC intersections to roundabouts (2). Injury reductions were found to range between 68% and 87%, depending on the setting (urban, rural, suburban) and whether the roundabout was single-lane or multilane. Higher crash reduc- tions were observed in rural settings, where total crash reductions were found to be 71.5% and injury crashes were reduced by 87.3%. The two most common operational problems at TWSC intersections are con- gestion on the minor street caused by a demand that exceeds capacity, and queues that form on the major street because of inadequate capacity for left-turning vehi- cles yielding to opposing traffic. Roundabouts may offer an effective solution to traffic problems at TWSC intersections with heavy left turns from the major route because they provide more favorable treatment to left turns than other control modes. T-intersections are especially good candidates in this category because they tend to have higher left turning volumes. On the other hand, the problems experienced by low-volume cross-street traf- fic at TWSC intersections with heavy through volumes on the major street are very difficult to solve by any traffic control measure. A roundabout may be a rea- sonable alternative even under situations where the minor street volume is low. However, when evaluating locations where the proportion of traffic on the major street is high, it is important to consider the context of the location when evaluat- ing the control alternatives. Roundabouts offer significant safety benefits over TWSC intersections. Roundabouts may offer an effective solution at TWSC intersections with heavy left turns from the major street.

Roundabouts: An Informational Guide Page 3-32 Chapter 3/Planning 3.6.2 ALL-WAY STOP-CONTROL ALTERNATIVE When cross-street traffic volumes are heavy enough to meet the Manual of Uniform Traffic Control Devices (MUTCD) (3) warrants for AWSC, roundabouts become an especially attractive solution because of their higher capacities and lower delays. Roundabouts can be expected to always offer better operational per- formance for vehicles than AWSC, given the same traffic conditions. Roundabouts that are proposed as alternatives to stop control would typically have single-lane approaches. A substantial part of the operational benefit of a roundabout compared to an all-way stop intersection is obtained during the off-peak periods because the restrictive stop control applies for the entire day. The MUTCD does not permit stop control on a part-time basis. The extent of the benefit will depend on the amount of traffic at the intersection and on the proportion of left turns. Left turns degrade the operation of all traffic control modes, but they have a smaller effect on roundabouts than stop signs or signals. From a safety perspective, U.S. research has identified that the conversion of an AWSC intersection to a roundabout results in an insignificant difference in safety performance, primarily due to the low volume conditions where an AWSC would be appropriate. Therefore, when comparing a roundabout to an AWSC alternative, the primary considerations should be operations and cost. Round- abouts may also offer other benefits to AWSC intersections, including use as a gateway treatment or for community enhancement. 3.6.3 SIGNAL CONTROL ALTERNATIVE When traffic volumes are heavy enough to warrant signalization, the selection process becomes somewhat more rigorous. The usual basis for selection is that a roundabout will provide better operational performance than a signal in terms of stops, delay, vehicle queues, fuel consumption, safety, and pollution emissions. For planning purposes, this may be assumed to be the case provided that the roundabout is operating within its capacity. A roundabout that operates within its capacity will generally produce lower delays than a signalized intersection oper- ating with the same traffic volumes and right-of-way limitations. The task then becomes to assess whether any roundabout configuration can be made to work satisfactorily. If not, a signalized intersection and a grade separation are com- monly the remaining alternatives. As in the case of stop control, intersections with heavy left turns are especially good roundabout candidates. Intersections with limited queue storage for major street left turns or minor street movements may also make a good candidate for a roundabout. Roundabouts are also an effective alternative to signalized control for closely spaced intersections since signal control can be difficult to manage vehicle queues between intersections. Unlike traffic signal control, there are no warrants for roundabouts currently included in the MUTCD. Each roundabout must be justified on its own merits as the most appropriate intersection treatment alternative. It is, however, useful to consider the case in which the traffic volumes just meet the MUTCD warrant thresholds for traffic signals. At these volume levels a single-lane roundabout is Roundabouts will always offer better operational performance for vehicles than AWSC. A substantial part of the delay- reduction benefit of round- abouts, compared to AWSC intersections, comes during off- peak periods.

Chapter 3/Planning Page 3-33 Roundabouts: An Informational Guide anticipated to operate within its capacity and can be used to make some planning- level comparisons of roundabout delay to signal delay. Exhibit 3-19 presents aver- age delays per vehicle for signals and roundabouts. These values represent the approach delay as perceived by the motorist. They do not include the geometric delay incurred within the roundabout. It is clear from this figure that roundabout control delays are substantially lower than signal delays, but in neither case are the delays excessive. Roundabouts offer significant safety benefits in comparison to signalized intersections. Roundabouts provide an overall reduction in vehicle speed, elimi- nate dangerous situations, such as red-light running, and remove some of the most serious conflict points including angle, left-turn, and head-on crashes. This results in observed safety benefits at U.S. roundabouts of 77.7% for injury crashes and 47.8% for all crash types and severities (2). 3.7 ECONOMIC EVALUATION Many factors influence the amount of economic investment justified for any type of intersection. Costs associated with roundabouts include construction costs, engineering and design fees, land acquisition, and maintenance costs. Bene- fits may include reduced crash rates and severity, as well as reduced delay, stops, fuel consumption, and emissions. When comparing costs, it is often difficult to separate the actual intersection costs from an overall improvement project. Accordingly, the reported costs of installing roundabouts have been shown to vary significantly from site to site. A roundabout may cost more or less than a traffic signal, depending on the amount of new pavement area and the extent of other roadway work required. At some existing unsignalized intersections, a traffic signal can be installed without signifi- cant modifications to the pavement area or curbs. In these instances, a roundabout Exhibit 3-19 Average Control Delay per Vehicle at the MUTCD Peak-Hour Signal Warrant Thresholds Roundabouts may require more pavement area at the intersection compared to a traffic signal, but less on the approaches and exits. 0 2 4 6 8 10 12 14 16 18 20 600 700 800 900 1000 1100 1200 1300 1400 1500 Total Major Street Volume (veh/h) A ve ra ge D el ay (s /ve h) Signal (10% left turns) Signal (50% left turns) Roundabout (10% left turns) Roundabout (50% left turns)

Roundabouts: An Informational Guide Page 3-34 Chapter 3/Planning is likely to be more costly to install than a traffic signal since the roundabout can rarely be constructed without significant pavement and curb modifications. However, at new sites and at signalized intersections that require widening on one or more approaches to provide additional turn lanes, a roundabout can be a comparable or less-expensive alternative. While roundabouts typically require more pavement area at the intersection, they may require less pavement width on the upstream approaches and downstream exits if multiple turn lanes associated with a signalized intersection can be avoided. The cost savings of reduced approach roadway widths is particularly advantageous at interchange ramp terminals and other intersections adjacent to grade separations where wider roads may result in larger bridge structures. In most cases, except potentially for a mini-roundabout, a roundabout is more expensive to construct than the two-way or all-way stop-controlled intersection alternatives. Higher costs are typically incurred when a substantial amount of realignment, grading, or drainage work is required. The cost of maintaining traffic during construction tends to be relatively high for retrofitting roundabouts. This expense is due mainly to the measures required to maintain existing traffic flow through the intersection while rebuilding it in stages. Other factors contributing to high roundabout costs are large amounts of landscaping in the central and splitter islands, extensive signing and lighting, and the provision of curbs on all outside pavement edges. Operating and maintenance costs of roundabouts are somewhat higher than for other unsignalized intersections but less than signalized intersections. In addi- tion, traffic signals consume electricity and require periodic service (e.g., bulb replacement, detector replacement, and periodic signal re-timing). For these rea- sons, operating costs over a design life of 20 years or longer should be considered when comparing between intersection treatments. Operating costs for a round- about are generally limited to the cost of illumination (similar to signalized alter- natives but typically more than is required for other unsignalized intersections). Maintenance includes regular re-striping and re-paving as necessary, as well as snow removal and storage in cold climates (these costs are also incurred by con- ventional intersections). Landscaping may require regular maintenance as well, including such things as pruning, mowing, and irrigation system maintenance. To the extent that roundabouts reduce crashes compared with conventional intersec- tions, they will reduce the number and severity of incidents that disrupt traffic flow and may require emergency service. The most appropriate method for evaluating public works projects of this type is usually the benefit–cost analysis method. The following sections discuss this method as it typically applies to roundabout evaluation, although it can be generalized for most transportation projects. 3.7.1 METHODOLOGY The benefit–cost method is explained in detail in a number of standard refer- ences, including the Transportation Planning Handbook (4) and various AASHO and AASHTO publications (5–6). The basic premise of this method of evaluation is to compare the incremental benefit between two alternatives to the incremental costs The cost of maintaining traffic during construction of a round- about retrofit can be relatively high.

Chapter 3/Planning Page 3-35 Roundabouts: An Informational Guide between the same alternatives. Assuming Alternatives A and B, the equation for calculating the incremental benefit–cost ratio of Alternative B relative to Alterna- tive A is given in Equation 3-1. Benefit–cost analysis typically takes two forms. For assessing the viability of a number of alternatives, each alternative is compared individually with a no-build alternative. If the analysis for Alternative A relative to the no-build alternative indicates a benefit–cost ratio exceeding 1.0, Alternative A has benefits that exceed its costs and is thus a viable project. For ranking alternatives, the incremental benefit–cost ratio analysis is used to compare the relative benefits and costs between alternatives. Projects should not be ranked based on their benefit–cost ratio relative to the no-build alternative. After eliminating any alternatives that are not viable as compared to the no-build alternative, alternatives are compared in a pair-wise fashion to establish the priority between projects. Since many of the input parameters may be estimated, a rigorous analysis should be considered of varying the parameter values of key assumptions to verify that the recommended alternative is robust, even under slightly varying assumptions, and under what circumstances it may no longer be preferred. 3.7.2 ESTIMATING BENEFITS Benefits for a public works project are generally composed of three elements: safety benefits, operational benefits, and environmental benefits. Each benefit is typically quantified on an annualized basis and so is readily usable in a benefit–cost analysis. The following sections discuss these in more detail. 3.7.2.1 Safety Benefits Safety benefits are defined as the assumed savings to the public due to a reduction in crashes within the project area. The procedure for determining safety benefits is as below. (Detail on the methodology can be found in Chapter 5.): 1. Quantify the existing safety history in the study area in terms of a crash rate for each level of severity (fatal, injury, property damage). This rate, expressed in terms of crashes per million entering vehicles, is computed by dividing the number of crashes of a given severity that occurred during the before period by the number of vehicles that entered the intersection during the same period. This results in a before crash rate for each level of severity. 2. Estimate the change in crashes of each level of severity that can be reason- ably expected due to the proposed improvements. As documented else- where in this guide, roundabouts tend to have proportionately greater reductions in fatal and injury crashes than property damage crashes. 3. Determine a new expected crash rate (an after crash rate) by using the procedures presented in Chapter 5. It is best to use local data to determine B C Benefits Benefits Costs Costs B A B A B A → = − − Equation 3-1 Rank alternatives based on their incremental benefit–cost ratio, not on their ratio relative to the no-build alternative. Projects may realize safety, operational, and environmental benefits.

Roundabouts: An Informational Guide Page 3-36 Chapter 3/Planning appropriate crash reduction factors due to geometric or traffic control changes, as well as the assumed costs of various severity levels of crashes. 4. Estimate the number of after crashes of each level of severity for the life of the project by multiplying the after crash rate by the expected number of entering vehicles over the life of the project. 5. Estimate a safety benefit by multiplying the expected number of after crashes of each level of severity by the average cost of each crash and then annualizing the result. The values in Exhibit 3-20 can provide a starting point, although local data should be used where available. 3.7.2.2 Operational Benefits The operational benefits of a project may be quantified in terms of the overall reduction in person-hours of delay to the public. Delay has a cost to the public in terms of lost productivity, and thus a value of time can typically be assigned to changes in estimated delay to quantify benefits associated with delay reduction. The calculation of annual person-hours of delay can be performed with vary- ing levels of detail, depending on the availability of data. For example, one method for computing the vehicle-hours of delay is as follows. 1. Estimate the delay per vehicle for each hour of the day. If turning move- ments are available for multiple hours, this estimate can be computed directly. If only the peak hour is available, the delay for an off-peak hour can be approximated by proportioning the peak-hour turning movements by total entering vehicles. 2. Determine the daily vehicle-hours of delay by multiplying the estimated delay per vehicle for a given hour by the total entering vehicles during that hour, and then aggregate the results over the entire day. If data is available, these calculations can be separated by day of week or by week- day, Saturday, and Sunday. In some cases it may be appropriate to assume that the daily vehicle-hours of delay are equal to a factor, say 10, times the delay during the peak hour. 3. Determine annual vehicle-hours of delay by multiplying the daily vehicle- hours of delay by 365. If separate values have been calculated by day of Exhibit 3-20 Estimated Costs for Crashes of Varying Levels of Severity Quantify operational benefits in terms of vehicle-hours of delay. Crash Severity Economic Cost per Crash (2008 dollars) Fatality $4,200,000 Class A (incapacitating injury) $214,200 Class B (non-incapacitating evident injury) $54,700 Class C (possible injury) $26,000 Property Damage Only (per crash) $2,400 Source: National Safety Council (7 )

Chapter 3/Planning Page 3-37 Roundabouts: An Informational Guide week, first determine the weekday vehicle-hours of delay and then mul- tiply by 52.1 (365 divided by 7). It may be appropriate to use fewer than 365 days per year because the operational benefits will not usually apply equally on all days. For example, to provide a conservative estimate of benefits, a value of 250 days per year could be used. 4. Convert the results to person-hours of delay using appropriate vehicle- occupancy factors (including transit), then add pedestrian delay if significant. 3.7.2.3 Environmental Benefits The environmental benefits of a project are most readily quantified in terms of reduced fuel consumption and improved air quality. Of these, reductions in fuel consumption and the benefits associated with those reductions are typically the simplest to determine. One way to determine fuel consumption is to use the same procedure for esti- mating delay as described previously. Fuel consumption is an output of several of the models in use today, although the user is cautioned to ensure that the model is appropriately calibrated for current U.S. conditions. Alternatively, one can esti- mate fuel consumption by using the estimate of annual vehicle-hours of delay and then multiplying that by an assumed fuel consumption rate during idling, expressed as gallons per hour (liters per hour) of idling. The resulting estimate can then be converted to a cost by assuming an average cost of fuel, expressed in dollars per gallon (dollars per liter). 3.7.3 ESTIMATION OF COSTS Costs for a public works project are generally composed of two elements: capitalized construction costs (including right-of-way) and operations and maintenance (O&M) costs. Although O&M costs are typically determined on an annualized basis, construction costs are typically a near-term activity that must be annualized. The following sections discuss these in more detail. 3.7.3.1 Construction Costs Construction costs for each alternative should be calculated using normal pre- liminary engineering cost-estimating techniques. These costs should include the costs of any necessary earthwork, paving, bridges and retaining walls, signing and striping, illumination, and signalization. To convert construction costs into an annualized value for use in the benefit–cost analysis, a capital recovery factor (CRF) should be used, shown in Equation 3-2. This converts a present-value cost into an annualized cost over a period of n years using an assumed discount rate of i percent. where i = discount rate n = number of periods (years) CRF i i i n n= +( ) +( ) − 1 1 1 Equation 3-2

Roundabouts: An Informational Guide Page 3-38 Chapter 3/Planning 3.7.3.2 Operation and Maintenance Costs Operation and maintenance (O&M) costs vary significantly between round- abouts and other forms of intersection control beyond the basic elements. Com- mon elements include signing and pavement marking maintenance and power for illumination, if provided. Roundabouts typically have slightly higher illumination power and mainte- nance costs compared to signalized or sign-controlled intersections due to a larger number of illumination poles. Roundabouts have slightly higher signing and pavement marking maintenance costs due to a higher number of signs and pave- ment markings. Roundabouts also introduce additional cost associated with the maintenance of any landscaping in and around the roundabout. Signalized intersections have considerable additional cost associated with power for the traffic signal and maintenance costs such as bulb replacement and detection maintenance. Power costs vary considerably from region to region and over time and should be verified locally. For general purposes, an annual cost of $3,000 for providing power to a signalized intersection is a reasonable approxima- tion. In addition, for optimal operation the signal timing for the intersection needs to be maintained. Signal timing maintenance requires a specialized workforce and equipment (including periodic collection of traffic count data), and often traffic signals are added to an agency’s responsibility without a commensurate increase in budget and workforce to accommodate this additional maintenance. Signal retiming has been documented to cost approximately $2,500 to $3,100 per signal and needs to be repeated every few years (8–9). 3.8 PUBLIC INVOLVEMENT Public acceptance of roundabouts has often been found to be one of the biggest challenges facing a jurisdiction that is planning to install its first roundabout. Without the benefit of explanation or first-hand experience and observation, the public is likely to incorrectly associate roundabouts with older, non-conforming traffic circles that they have either experienced or heard about. Equally likely, without adequate education, the public (and agencies alike) will often have a natural hesitation or resistance against changes in their driving behavior and driving environment. In such a situation, a proposal to install a roundabout may initially experience a negative public reaction. However, the history of roundabouts installed in the United States also indicates that public attitude toward roundabouts improves significantly after construction. Surveys conducted by the Insurance Institute for Highway Safety (IIHS) reported a significant negative public attitude toward roundabouts prior to construction (41% of the responses were strongly opposed) but a positive attitude after construction (63% of the responses were positive or very positive) (10). A wide variety of techniques have been used successfully in the United States to inform and educate the public about new roundabouts. Some of these include Roundabout O&M costs are typically slightly higher than signalized intersections for illumination, signing, pavement marking, and landscaping. Signalized intersections also have O&M costs for signal power, bulb replacement, and detection maintenance. Surveys find negative public attitudes toward roundabouts before construction, but positive attitudes following construction. Public meetings, videos and brochures, and media announcements are some of the ways to educate the public about new roundabouts.

Chapter 3/Planning Page 3-39 Roundabouts: An Informational Guide public meetings, informational brochures and videos, and announcements in the newspaper or on television and radio. A public involvement process should be initiated as soon as practical, preferably early in the planning stages of a project while other intersection forms are also being considered. 3.8.1 AUDIENCE The type of information presented and the way in which it is communicated is often dependent on the type of audience. Stakeholder audiences may include rep- resentatives from the police and fire departments, school district officials, transit operators, developers, business owners, and the freight industry. Audiences may also include public citizens such as nearby residents, seniors, teens, pedestrians with disabilities, and other representatives from the community. Identifying the target audience is one of the initial steps in developing a public involvement pro- gram. A roundabout may affect various stakeholders in different ways; therefore, all concerns or questions should be addressed. For example, representatives from the police and fire department are likely focused on ensuring that their emergency vehicles can navigate the intersection and that the roundabout does not significantly affect their response times. Parents in the community may be concerned about how their teen drivers will understand and make decisions as new drivers, or how comfortable they will be walking through the roundabout with their children. In some cases, it may be necessary to hold separate public involvement meetings for different audiences. Technical explanations of the design and operations may be appropriate for certain stakeholders, while more general educational discus- sions may be held with a group of citizens. The level of effort can vary consider- ably depending on whether this is the first roundabout in an area or if the local community has had a poor recent experience with roundabouts. 3.8.2 CONTENT The content that is presented to the public should be appropriate for the type of audience that is being targeted. For all audiences, the purpose of the informa- tion being presented or purpose of the meeting should be clearly communicated. In addition, introductory information about roundabouts should be presented, which may include highlighting the differences between roundabouts and other types of intersections, providing guidance on how to drive through a roundabout, and describing the overall advantages and disadvantages of roundabouts. For some public involvement purposes, that introductory material may be the scope of infor- mation presented. However, in other cases, more specific project information, stake- holder impacts, and specific community concerns and needs may be addressed. Public involvement information may be presented in many different ways with a variety of tools. Public meetings are often an effective way to communicate information and gather input from a specific group of individuals. In other cases, a general announcement such as a newspaper article, website, or other media venue may be used to inform a larger group of individuals. Specific tools used for each type of venue are described in the following sections. 3.8.3 PUBLIC MEETINGS Public meetings can be a useful forum for informing the public about round- abouts in their community and bringing the public into the design process. Engaging

Roundabouts: An Informational Guide Page 3-40 Chapter 3/Planning the public in the design process allows early identification of potential problems and helps to gain overall acceptance throughout the process. Public input may be useful at various stages in the planning process: data collection, problem defini- tion, generation of design alternatives, selection of preferred alternative, detailed design, go/no-go decision, construction/opening, and landscape maintenance. Many jurisdictions require or recommend public meetings with the affected neighborhood or businesses prior to approval of the project by elected officials. Even if such meetings are not required, they can be helpful in easing concerns about a new form of intersection for a community. Tools used in this type of public meeting may include project posters, aerial maps, and visually displayed project information. Exhibit 3-21 provides an example of a poster developed for a roundabout project. This poster highlights the project, stakeholder involvement, and the public information venues that were used throughout the project. Note that the sequence of conversations in this example aims at building consensus at key areas within a community (executive city staff, city council and county commission, and key community organizations) to help with approaching the community at large. Exhibit 3-21 Example of Public Information Poster Source: City of Springfield, Oregon (11) Other public meetings may be designed to teach the public about using roundabouts. For these types of meetings, it is often effective to bring large-scale roundabout models or simulation tools. Exhibit 3-22 illustrates roundabout mod- els that were developed by the Missouri Department of Transportation and the city of Overland Park, Kansas. The latter model was specifically designed to teach school-age children how to safely navigate a roundabout. 3.8.4 INFORMATIONAL BROCHURES Many agencies have used informational brochures to educate the public about roundabouts in their communities. Brochures have also been prepared for specific projects. Exhibit 3-23 shows examples from brochures prepared for specific projects. These brochures include drawings or photographic simula- tions of the proposed roundabout. The brochures also typically include general information on roundabouts (what roundabouts are, where they can be found, and the types of benefits that can be expected). Sometimes they also include instructions on how to use the roundabout as a motorist, bicyclist, and pedestrian.

Chapter 3/Planning Page 3-41 Roundabouts: An Informational Guide Exhibit 3-24 provides an example of general roundabout brochures that are commonly developed for many cities, counties, and states. These commonly provide detailed guidance for driving through roundabouts and clear illustrations of the signing and striping that drivers may expect to see at a roundabout. 3.8.5 WEBSITES Websites are an effective tool for educating the public about roundabouts in a specific area and directing the public to other informational websites about roundabouts. Agencies such as the Maryland State Highway Administration (12) and city of Sammamish, Washington (13), have developed roundabout demonstrations on their websites to teach motorists about using a roundabout. These demonstrations include a simulation tool that shows vehicles navigating through the intersection, as shown in Exhibit 3-25. In addition to the simulation tool, the website provides additional Web links and resources for the public to learn about more detailed information or even read about roundabouts in other parts of the county. Exhibit 3-22 Examples of Scale Round- about Models for Public Involvement (a) Missouri Department of Transportation (b) City of Overland Park, Kansas

Roundabouts: An Informational Guide Page 3-42 Chapter 3/Planning Exhibit 3-23 Examples of Project-Specific Informational Brochures Exhibit 3-24 Example of General Informational Brochure (a) Town of Vail, Colorado (b) City of Springfield, Oregon Minnesota Department of Transportation

Chapter 3/Planning Page 3-43 Roundabouts: An Informational Guide 3.8.6 INFORMATIONAL VIDEOS A number of agencies and consulting firms have prepared videos to inform the public about roundabouts. These videos are typically 10 to 15 minutes in length and include footage of existing roundabouts and narration about their operational and safety characteristics. These videos have been successfully used at public meetings as an effective means of introducing the public to roundabouts. Examples of these informational videos can be found at the city of Modesto, California (14), Washington State Department of Transportation (15), and other state, city, or county websites. Once developed, videos can also Exhibit 3-25 Examples of Roundabout Websites (a) Maryland State Highway Administration (b) City of Sammamish, Washington

Roundabouts: An Informational Guide Page 3-44 Chapter 3/Planning be shown at regular intervals on city or county government access television channels. 3.8.7 MEDIA ANNOUNCEMENTS Given the new nature of a roundabout in many communities, the local media (newspaper, radio, and television) is likely to become involved. Such interest often occurs early in the process and then again upon the opening of the roundabout. Radio reading services, telephone information services, and publications intended primarily for individuals with disabilities should be used to communicate with persons who are visually impaired when a roundabout is proposed and when it opens. 3.8.8 USER EDUCATION One of the important issues facing a state considering the implementation of roundabouts is the need to provide adequate driver, cyclist, and pedestrian edu- cation. To clarify the following tips and instructions, user education should begin by using simple exhibits such as Exhibit 1-1 from Chapter 1 to familiarize them with the basic physical features of a roundabout intersection. Users should also familiarize themselves with the instructions for all other modes so that they understand the expectations of each other. Many states in the United States have begun to implement roundabout driving instructions in the state driving manuals. This typically includes a brief introduction to roundabouts and detailed instructions for how to navigate and drive safely through this type of intersection. While states have made tremen- dous progress with implementing instructions for roundabouts into their driver’s manuals, many states do not provide sufficient information for teach- ing a driver about using turn signals and making decisions with pedestrians, bicycles, and emergency vehicles. The Kansas Driver’s Manual, however, does provide detailed steps of navigating a roundabout and considering all users and vehicle types. States may also consider implementing roundabout education programs within their community to educate all users of all ages about how to safely travel through a roundabout. The Virginia Department of Transportation (VDOT) has developed a website dedicated to educating users about roundabouts in Virginia. This information provides an overview of facts about roundabouts, step-by-step guidelines for using a roundabout, and information about considering pedestrians and older drivers at roundabouts. In addition, VDOT provides announcements of upcoming roundabout presentations and information about their state-wide pol- icy on roundabouts (16). The city of Bend, Oregon, has established a roundabout education program that is primarily focused on educating children about how to properly walk or bicycle through a roundabout. With a number of roundabouts within the Bend community, the city’s intent was to establish the knowledge at an early age with the hope that children would already understand this type of intersection when they reached the driving age and would also be able to share the valuable knowl- edge with their parents.

Appendix B provides instructional material and model language for drivers, cyclists, and pedestrians that can be adapted to drivers manuals. 3.9 REFERENCES 1. McCulloch, H. “The Roundabout Design Process—Simplified.” National Round- about Conference. Kansas City, Missouri, 2008. http://teachamerica.com/ RAB08/RAB08S3BMcCulloch/index.htm. Accessed July 30, 2009. 2. Rodegerdts, L., M. Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. Persaud, C. Lyon, D. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. Transportation Research Board of the National Academies, Washington, D.C., 2007. 3. Manual on Uniform Traffic Control Devices. FHWA, Washington, D.C., 2009. 4. Transportation Planning Handbook, 3rd ed. Institute of Transportation Engi- neers, Washington, D.C., 2009. 5. A Policy on Design of Urban Highways and Arterial Streets. AASHO, Washington, D.C., 1973. 6. A Manual on User Benefit Analysis of Highway and Bus Transit Improvements. AASHTO, Washington, D.C., 1977. 7. National Safety Council. “Estimating the Cost of Unintentional Injuries.” www.nsc.org/news_resources/injury_and_death_statistics/Pages/Estimating theCostsofUnintentionalInjuries.aspx. Accessed March 2010. 8. Intelligent Transportation Systems for Traffic Signal Control, Deployment Benefits and Lessons Learned. Report No. FHWA-JPO-07-004. U.S. Department of Transportation, FHWA, Washington, D.C., 2007. 9. Koonce, P., L. Rodegerdts, K. Lee, S. Quayle, S. Beaird, C. Braud, J. Bonneson, P. Tarnoff, and T. Urbanik. Traffic Signal Timing Manual. Report No. FHWA- HOP-08-024. FHWA, Washington, D.C., 2008. 10. Status Report, Volume 36, Number 7. Insurance Institute of Highway Safety, Arlington, Virginia, 2001. 11. Barnet, B. F. and city of Springfield, Oregon. “Anatomy of Education and Outreach to Inform Elected Officials, Community Leaders, and Citizens.” Poster from Transportation Research Board National Roundabout Confer- ence, Kansas City, Missouri, 2008. 12. Maryland State Highway Administration. “Traveling Maryland’s Round- abouts.” www.sha.state.md.us/Safety/oots/Roundabouts/info.asp. Accessed November 2008. 13. City of Sammamish, Washington. “Roundabout Demonstration.” www.ci. sammamish.wa.us/RoundaboutDemo.aspx?Show=Main. Accessed November 2008. Chapter 3/Planning Page 3-45 Roundabouts: An Informational Guide

Roundabouts: An Informational Guide Page 3-46 Chapter 3/Planning 14. City of Modesto, California. “Roundabouts.” Traffic Engineering Division. www.ci.modesto.ca.us/PWD/traffic/roundabouts/videos.asp. Accessed November 2008. 15. Washington State Department of Transportation, city of Lacey, and city of Olympia. “Driving Modern Roundabouts.” www.wsdot.wa.gov/eesc/CAE/ designvisualization/video/portfolio/Modern_Roundabouts/mpg_index.htm. Accessed November 2008. 16. Virginia Department of Transportation. Roundabouts in Virginia. www. virginiadot.org/info/faq-roundabouts.asp. Accessed March 2009.

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Roundabouts: An Informational Guide – Second Edition Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 672: Roundabouts: An Informational Guide – Second Edition explores the planning, design, construction, maintenance, and operation of roundabouts. The report also addresses issues that may be useful in helping to explain the trade-offs associated with roundabouts.

This report updates the U.S. Federal Highway Administration’s Roundabouts: An Informational Guide, based on experience gained in the United States since that guide was published in 2000.

Errata

Equation 6-3 on page 6-58 incorrectly contains an addition sign (+) as an operator. The correct operator should be a subtraction sign (-).

Errata #2

Exhibit 5-23 on page 5-29 of NCHRP Report 672 contains an incorrect calculation of estimated injury crashes.

There is a summary document, Paths to Practice, available.

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