Roundabouts: An Informational Guide – Second Edition (2010)

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Chapter 1/Introduction Page 1-1 Roundabouts: An Informational Guide CHAPTER 1 INTRODUCTION CONTENTS 1.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3 1.2 DISTINGUISHING CHARACTERISTICS OF A ROUNDABOUT . . . . . 1-3 1.2.1 Other Types of Circular Intersections . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 1.2.2 Comparison of Features between Roundabouts and Other Circular Intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8 1.2.3 Additional Design Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8 1.3 CATEGORIES OF ROUNDABOUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10 1.3.1 Mini-Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12 1.3.2 Single-Lane Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-13 1.3.3 Multilane Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-13 1.4 SCOPE OF THE GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-17 1.5 ORGANIZATION OF THE GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-17 1.6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-19

Roundabouts: An Informational Guide Page 1-2 Chapter 1/Introduction LIST OF EXHIBITS Exhibit 1-1 Key Roundabout Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3 Exhibit 1-2 Description of Key Roundabout Features . . . . . . . . . . . . . . . . . . . . . . 1-4 Exhibit 1-3 Example of a Rotary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5 Exhibit 1-4 Example of a Signalized Traffic Circle . . . . . . . . . . . . . . . . . . . . . . . . . 1-6 Exhibit 1-5 Example of Neighborhood Traffic Circles . . . . . . . . . . . . . . . . . . . . . . 1-7 Exhibit 1-6 Conversions of Rotaries to Roundabouts . . . . . . . . . . . . . . . . . . . . . . 1-8 Exhibit 1-7 Comparison of Roundabouts with Traffic Circles . . . . . . . . . . . . . . . 1-9 Exhibit 1-8 Common Roundabout Design Features . . . . . . . . . . . . . . . . . . . . . . 1-10 Exhibit 1-9 Roundabout Category Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12 Exhibit 1-10 Features of Typical Mini-Roundabout . . . . . . . . . . . . . . . . . . . . . . . 1-12 Exhibit 1-11 Example of Mini-Roundabout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-13 Exhibit 1-12 Features of Typical Single-Lane Roundabout . . . . . . . . . . . . . . . . . 1-14 Exhibit 1-13 Examples of Single-Lane Roundabouts . . . . . . . . . . . . . . . . . . . . . . 1-14 Exhibit 1-14 Features of Typical Two-Lane Roundabout . . . . . . . . . . . . . . . . . . 1-15 Exhibit 1-15 Features of Typical Three-Lane Roundabout . . . . . . . . . . . . . . . . . 1-15 Exhibit 1-16 Examples of Multilane Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . 1-16

Chapter 1/Introduction Page 1-3 Roundabouts: An Informational Guide 1.1 INTRODUCTION A roundabout is a form of circular intersection in which traffic travels counterclockwise (in the United States and other right-hand traffic countries) around a central island and in which entering traffic must yield to circulating traffic. Exhibit 1-1 is a drawing of a typical roundabout, annotated to identify the key characteristics. Exhibit 1-2 provides a description of each of the key features. 1.2 DISTINGUISHING CHARACTERISTICS OF A ROUNDABOUT Traffic circles have been part of the transportation system in the United States since at least 1905 when one of the first circles, known as the Columbus Circle in New York City, was designed by William Phelps Eno. Subsequently, many large circles or rotaries were built in the United States. The prevailing designs enabled high-speed merging and weaving of vehicles. Priority was given to entering vehicles, facilitating high-speed entries. Yet, high crash experience and congestion in the circles led to rotaries falling out of favor in America after the mid-1950s. Internationally, the experience with traffic circles was equally negative, with many countries experiencing circles that locked up as traffic volumes increased. The modern roundabout was developed in the United Kingdom to rectify problems associated with these traffic circles. In 1966, the United Kingdom adopted a rule at all circular intersections that required entering traffic to give Exhibit 1-1 Key Roundabout Characteristics Key roundabout features include a generally circular shape, yield control of entering traffic, and geometric curvature and features to induce desirable vehicular speeds. Splitter islands have multiple roles: separate entering and exiting traffic, deflect and slow entering traffic, and provide a pedestrian refuge. The modern roundabout was developed in the United Kingdom in the 1960s.

Roundabouts: An Informational Guide Page 1-4 Chapter 1/Introduction way, or yield, to circulating traffic. This rule prevented circular intersections from locking up by not allowing vehicles to enter the intersection until there were suffi- cient gaps in circulating traffic. In addition, smaller circular intersections were proposed that required adequate horizontal curvature of vehicle paths to achieve slower entry and circulating speeds. These changes improved the safety characteristics of the circular intersections by reducing the number and the severity of crashes. The modern roundabout rep- resents a significant improvement, in terms of both operations and safety, when compared with older rotaries and traffic circles (1–3). Therefore, many countries have adopted the modern roundabout as a common intersection form, and some have developed extensive design guides and methods to evaluate the operational performance of modern roundabouts. 1.2.1 OTHER TYPES OF CIRCULAR INTERSECTIONS Roundabouts are but one type of circular intersection. In fact, there are at least four distinct types: 1. Roundabouts are a subset of circular intersections with specific design and traffic control features. These features include yield control of all entering traffic, channelized approaches, and geometric curvature and features to induce desirable vehicular speeds. Exhibit 1-2 Description of Key Roundabout Features Modern roundabouts provide substantially better operational and safety characteristics than older traffic circles and rotaries. Types of circular intersections. Feature Description Central island The central island is the raised area in the center of a roundabout around which traffic circulates. The central island does not necessarily need to be circular in shape. In the case of mini-roundabouts the central island is traversable. Splitter island A splitter island is a raised or painted area on an approach used to separate entering from exiting traffic, deflect and slow entering traffic, and allow pedestrians to cross the road in two stages. Circulatory roadway The circulatory roadway is the curved path used by vehicles to travel in a counterclockwise fashion around the central island. Apron An apron is the traversable portion of the central island adjacent to the circulatory roadway that may be needed to accommodate the wheel tracking of large vehicles. An apron is sometimes provided on the outside of the circulatory roadway. Entrance line The entrance line marks the point of entry into the circulatory roadway. This line is physically an extension of the circulatory roadway edge line but functions as a yield or give-way line in the absence of a separate yield line. Entering vehicles must yield to any circulating traffic coming from the left before crossing this line into the circulatory roadway. Accessible pedestrian crossings For roundabouts designed with pedestrian pathways, the crossing location is typically set back from the entrance line, and the splitter island is typically cut to allow pedestrians, wheelchairs, strollers, and bicycles to pass through. The pedestrian crossings must be accessible with detectable warnings and appropriate slopes in accordance with ADA requirements. Landscape strip Landscape strips separate vehicular and pedestrian traffic and assist with guiding pedestrians to the designated crossing locations. This feature is particularly important as a wayfinding cue for individuals who are visually impaired. Landscape strips can also significantly improve the aesthetics of the intersection.

Chapter 1/Introduction Page 1-5 Roundabouts: An Informational Guide 2. Rotaries (see Exhibit 1-3), an old-style circular intersection common to the United States prior to the 1960s, are characterized by a large diameter [often greater than 300 ft (100 m)]. The diameter of a rotary is primarily a conse- quence of the length of the weaving section required between intersection legs. Unlike the modern roundabout, lane changes are typically required within a rotary for some movements. In addition, some rotaries operate with circulating traffic yielding to entering traffic, which can create congestion on the circulatory roadway. Circulating speeds are high due to the large diameter, making maneuvers within the circle more challenging. Exhibit 1-3 Example of a Rotary Fort Worth, Texas 3. Signalized traffic circles are old-style circular intersections used in some cities in the United States where traffic signals are used to control one or more entry–circulating point. As a result, signalized traffic circles have distinctly different operational characteristics from yield- controlled roundabouts, with queue storage within the circulatory roadway and progression of signals required. Exhibit 1-4 provides an example of a signalized traffic circle. Note that signalized traffic circles are distinct from roundabouts with pedestrian signals, as the entry–circulating point at a roundabout is still governed by a yield sign. 4. Neighborhood traffic circles are typically built at the intersections of local streets for reasons of traffic calming and/or aesthetics. The intersection approaches may be uncontrolled or stop-controlled. They do not typically

Roundabouts: An Informational Guide Page 1-6 Chapter 1/Introduction include raised channelization to guide the approaching driver onto the circulatory roadway. At some traffic circles, left-turning movements for larger vehicles are allowed to occur in front of the central island, potentially conflicting with other circulating traffic. Exhibit 1-5 shows examples of typical neighborhood traffic circles. The example in Portland, Oregon, is an all-way stop-controlled intersection; the example in Seattle, Washington, is uncontrolled. There are cases in which a rotary or traffic circle has been successfully retrofit- ted with a modern roundabout design. While it may be difficult to incorporate all of the design features and characteristics of a modern roundabout, if the primary design principles are achieved, the retrofitted intersection may still operate efficiently and safely as a roundabout. Exhibit 1-4 Example of a Signalized Traffic Circle (a) Hollywood, Florida (b) Cape Town, Western Cape, South Africa

Chapter 1/Introduction Page 1-7 Roundabouts: An Informational Guide Exhibit 1-6 provides two examples of intersections that were converted to modern roundabouts from older rotary designs. The Long Beach, California, example retains the original diameter of the rotary but improves the design of the entries. The Kingston, New York, example has a new roundabout built inside the old rotary; the photograph was taken partway through the conver- sion process. Since the purpose of this guide is to assist in the planning, design, and perfor- mance evaluation of roundabouts, not other circular intersections, it is important to be able to distinguish between them. These distinctions may not always be obvi- ous, and rotaries or neighborhood traffic circles (hereafter referred to as “traffic circles”) may be mistaken for a roundabout by the public or even technical staff. Exhibit 1-5 Example of Neighborhood Traffic Circles Circular intersections that do not conform to the character- istics of modern roundabouts are called “traffic circles” in this guide. (a) Portland, Oregon (b) Seattle, Washington

Roundabouts: An Informational Guide Page 1-8 Chapter 1/Introduction Therefore, the ability to carefully distinguish roundabouts from other circular intersections is important. 1.2.2 COMPARISON OF FEATURES BETWEEN ROUNDABOUTS AND OTHER CIRCULAR INTERSECTIONS Exhibit 1-7 identifies some of the major characteristics of roundabouts and contrasts them with other circular intersections. Note that all circular intersections should have counterclockwise rotation in the United States and other countries with right-hand traffic, except in specific instances where larger trucks need to turn left in front of the central island. Some of the traffic circles shown have many of the features associated with roundabouts but are different in one or more critical areas. Note also that these characteristics apply to yield-controlled roundabouts; signalized roundabouts are a special case discussed in later chapters. 1.2.3 ADDITIONAL DESIGN FEATURES In addition to the design characteristics identified in the previous section, roundabouts often include one or more additional design features intended to enhance the safety and/or capacity of the intersection. However, their absence does not necessarily preclude an intersection from operating as a roundabout. These additional features are identified in Exhibit 1-8. Exhibit 1-6 Conversions of Rotaries to Roundabouts (a) Long Beach, California (b) Kingston, New York

Chapter 1/Introduction Page 1-9 Roundabouts: An Informational Guide Roundabouts Traffic Circles Traffic Control Yield control is used on all entries. The circulatory roadway has no control. (a) Santa Barbara, California Some traffic circles use stop control, or no control, on one or more entries. (b) Howard County, Maryland Priority to Circulating Vehicles Circulating vehicles have the right-of-way. (c) Juneau, Alaska Some traffic circles require circulating traffic to yield to entering traffic. (d) Paris, France Direction of Circulation All vehicles circulate counterclockwise and pass to the right of the central island. (e) Sherwood, Oregon Some neighborhood traffic circles are so small that large vehicles may need to pass to the left of the central island. (f) Portland, Oregon Exhibit 1-7 Comparison of Roundabouts with Traffic Circles

Roundabouts: An Informational Guide Page 1-10 Chapter 1/Introduction 1.3 CATEGORIES OF ROUNDABOUTS For the purposes of this guide, roundabouts have been separated into three basic categories according to size and number of lanes to facilitate discussion of specific performance or design issues: mini-roundabouts, single-lane roundabouts, and multilane roundabouts. Note that separate categories have not been explicitly identified for rural, urban, and suburban areas. Roundabouts in urban areas may require smaller Adequate Speed Reduction Good roundabout design requires entering vehicles to negotiate the roundabout at slow speeds. Once within the circulatory roadway, vehicle paths are further deflected by the central island. (a) Ladera Ranch, California Some roundabouts allow high-speed entries for major movements. This increases the risk for more severe crashes for vehicles, bicycles, and pedestrians. (b) Bradenton Beach, Florida Design Vehicle Good roundabout design makes accommodation for the appropriate design vehicle. This may require the use of an apron. (c) Lothian, Maryland Some roundabouts may not be designed to accommodate large vehicles that periodically approach the intersection. (d) Naples, Florida Exhibit 1-8 Common Roundabout Design Features

Chapter 1/Introduction Page 1-11 Roundabouts: An Informational Guide inscribed circle diameters due to smaller design vehicles and existing right-of-way constraints. They may also include more extensive pedestrian and bicycle features. Roundabouts in rural areas typically have higher approach speeds and thus may need special attention to visibility, approach alignment, and cross-sectional details. Suburban roundabouts may combine features of both urban and rural environments. Exhibit 1-9 summarizes and compares some fundamental design and opera- tional elements for each of the three roundabout categories. The following sections provide a qualitative discussion of each category. Exhibit 1-8 (cont.) Common Roundabout Design Features Entry Flare Splitter Island Flare on an entry to a roundabout is the widening of an approach to multiple lanes to provide additional capacity and storage at the entrance line. (e) Long Beach, California All but some mini-roundabouts have raised splitter islands. These are designed to separate traffic moving in opposite directions, deflect entering traffic, and to provide opportunities for pedestrians to cross in two stages. Mini-roundabouts may have splitter islands defined only by pavement markings. (f) Lawrence, Kansas Pedestrian Crossing Locations Parking Pedestrian crossings are located only across the legs of the roundabout, typically separated from the circulatory roadway by at least one vehicle length. (g) Coralville, Iowa No parking is allowed within the circulatory roadway or at the entries. Parking maneuvers within the intersection, as is the case at some traffic circles, interfere with circulatory flow and present a potential safety hazard. (h) Orange, California

Roundabouts: An Informational Guide Page 1-12 Chapter 1/Introduction In most cases, roundabouts in all three categories are designed with pedes- trian and bicycle facilities; however, in some instances a jurisdiction may choose to not provide these features if these types of users are not anticipated or can be better served in another location. 1.3.1 MINI-ROUNDABOUTS Mini-roundabouts are small roundabouts with a fully traversable central island. They are most commonly used in low-speed urban environments with average operating speeds of 30 mph (50 km/h) or less. Exhibit 1-10 shows the features of typical mini-roundabouts, and Exhibit 1-11 provides an example. They can be useful in such environments where conventional roundabout design Exhibit 1-9 Roundabout Category Comparison Design characteristics of the three roundabout categories. Mini-roundabouts can be useful in low-speed urban environments with right-of-way constraints. Exhibit 1-10 Features of Typical Mini-Roundabout Design Element Mini-Roundabout Single-Lane Roundabout Multilane Roundabout Desirable maximum entry design speed 15 to 20 mph (25 to 30 km/h) 20 to 25 mph (30 to 40 km/h) 25 to 30 mph (40 to 50 km/h) Maximum number of entering lanes per approach 1 1 2+ Typical inscribed circle diameter 45 to 90 ft (13 to 27 m) 90 to 180 ft (27 to 55 m) 150 to 300 ft (46 to 91 m) Central island treatment Fully traversable Raised (may have traversable apron) Raised (may have traversable apron) Typical daily service volumes on 4-leg roundabout below which may be expected to operate without requiring a detailed capacity analysis (veh/day)* Up to approximately 15,000 Up to approximately 25,000 Up to approximately 45,000 for two-lane roundabout *Operational analysis needed to verify upper limit for specific applications or for roundabouts with more than two lanes or four legs.

Chapter 1/Introduction Page 1-13 Roundabouts: An Informational Guide is precluded by right-of-way constraints. In retrofit applications, mini-roundabouts are relatively inexpensive because they typically require minimal additional pavement at the intersecting roads and minor widening at the corner curbs. They are mostly recommended when there is insufficient right-of-way to accommodate the design vehicle with a traditional single-lane roundabout. Because they are small, mini-roundabouts are perceived as pedestrian-friendly with short crossing distances and very low vehicle speeds on approaches and exits. A fully traversable central island is provided to accommodate large vehicles and serves one of the distinguishing features of a mini-roundabout. The mini- roundabout is designed to accommodate passenger cars without requiring them to traverse over the central island. The overall design of a mini-roundabout should align vehicles at entry to guide drivers to the intended path and minimize running over of the central island to the extent possible. 1.3.2 SINGLE-LANE ROUNDABOUTS This type of roundabout is characterized as having a single-lane entry at all legs and one circulatory lane. Exhibit 1-12 shows the features of typical single-lane roundabouts, and Exhibit 1-13 provides examples. They are distinguished from mini-roundabouts by their larger inscribed circle diameters and non-traversable central islands. Their design allows slightly higher speeds at the entry, on the circulatory roadway, and at the exit. The geometric design typically includes raised splitter islands, a non-traversable central island, crosswalks, and a truck apron. The size of the roundabout is largely influenced by the choice of design vehicle and available right-of-way. 1.3.3 MULTILANE ROUNDABOUTS Multilane roundabouts have at least one entry with two or more lanes. In some cases, the roundabout may have a different number of lanes on one or Exhibit 1-11 Example of Mini-Roundabout Dimondale, Michigan

Roundabouts: An Informational Guide Page 1-14 Chapter 1/Introduction Exhibit 1-12 Features of Typical Single- Lane Roundabout Exhibit 1-13 Examples of Single-Lane Roundabouts (a) Dublin, Ohio (b) Skagit County, Washington

Chapter 1/Introduction Page 1-15 Roundabouts: An Informational Guide more approaches (e.g., two-lane entries on the major street and one-lane entries on the minor street). They also include roundabouts with entries on one or more approaches that flare from one to two or more lanes. These require wider circulatory roadways to accommodate more than one vehicle traveling side by side. Exhibit 1-14 through Exhibit 1-16 provide examples of typical multilane roundabouts. The speeds at the entry, on the circulatory roadway, and at the exit are similar or may be slightly higher than those for the single- lane roundabouts. The geometric design will include raised splitter islands, truck apron, a non-traversable central island, and appropriate entry path deflection. Exhibit 1-14 Features of Typical Two-Lane Roundabout Exhibit 1-15 Features of Typical Three-Lane Roundabout

Roundabouts: An Informational Guide Page 1-16 Chapter 1/Introduction (a) Bend, Oregon (b) Carmel, Indiana (c) Wisconsin Rapids, Wisconsin Exhibit 1-16 Examples of Multilane Roundabouts

Chapter 1/Introduction Page 1-17 Roundabouts: An Informational Guide 1.4 SCOPE OF THE GUIDE This guide provides information and guidance on roundabouts, resulting in designs that are suitable for a variety of typical conditions in the United States. The scope of this guide is to provide general information, planning techniques, evaluation procedures for assessing operational and safety performance, design guidelines for roundabouts, and principles to be considered for selecting and designing roundabouts. The most important principles will be highlighted in the margins throughout this document. This guide has been developed with the input of transportation practitioners and researchers from around the world. In many cases, items from national and international practice and research indicate considerable consensus, and these items have been included in this guide. However, other items have generated con- siderable differences of opinion (e.g., methods of estimating capacity), and some practices vary considerably from country to country (e.g., marking of the circula- tory roadway in multilane roundabouts). Where international consensus is not apparent, a reasoned approach is presented that the authors believe is currently most appropriate for the United States. As more roundabouts are built, the opportunity to conduct research to refine or develop better methods will enable future editions of this guide to improve. Despite the comprehensive nature of this document, it cannot discuss every issue related to roundabouts. In particular, it does not cover the following topics: • Non-traversable traffic calming circles. These are small traffic circles with raised central islands. They are typically used on local streets for speed and volume control. They are typically not designed to accommodate large vehicles, and often left-turning traffic is required to turn left in front of the circle. Mini-roundabouts, which are covered, may be an appropriate substitute. Additionally, there may be some advantage to using round- about principles (e.g., yield on entry, mountable or painted splitter islands, etc.) at these traffic calming circles. • Specific legal or policy requirements and language. The legal information that is provided in this guide is intended only to make the reader aware of potential issues. The reader is encouraged to consult with an attorney before adopting any of the recommendations contained herein on specific legal issues of concern. Similarly, regarding policy information, the guide refers to or encompasses applicable policies, such as those of the American Association of State Highway and Transportation Officials (AASHTO) (4). It does not, however, establish any new policies. 1.5 ORGANIZATION OF THE GUIDE This guide has been structured to address the needs of a variety of readers, including the general public, policy makers, transportation planners, operations and safety analysts, and conceptual and detailed designers. This chapter distinguishes Topics not discussed in this guide.

Roundabouts: An Informational Guide Page 1-18 Chapter 1/Introduction roundabouts from other circular intersections and defines the types of round- abouts addressed in the remainder of the guide. The remaining chapters in this guide increase in the level of detail provided. Chapter 2—Roundabout Considerations: This chapter provides a broad overview of the performance characteristics of roundabouts and discusses the various trade-offs of installing roundabouts versus other types of intersections. Legal issues and public involvement techniques are also discussed. Chapter 3—Planning: This chapter provides guidelines for identifying appropriate intersection control options given daily traffic volumes and identi- fies procedures for evaluating the feasibility of a roundabout at a given location. This chapter provides sufficient detail for a transportation engineer or planner to decide under what circumstances roundabouts are likely to be appropriate and how they compare to alternatives at a specific location. Public involvement tools and techniques are also discussed in this chapter. Chapter 4—Operational Analysis: This chapter identifies methods for analyzing the operational performance of each category of roundabout in terms of capacity, delay, and queuing. Chapter 5—Safety: This chapter discusses the expected safety performance of roundabouts and methods for analyzing safety performance. Chapter 6—Geometric Design: This chapter presents geometric design prin- ciples, design elements for each category of roundabout, and design applications. Chapter 7—Application of Traffic Control Devices: This chapter discusses a number of traffic design aspects, including pavement markings, signs, and traffic signals. Chapter 8—Illumination: This chapter discusses principles and recommenda- tions regarding illumination, along with recommended lighting levels and potential equipment types. Chapter 9—Landscaping: This chapter presents recommendations for land- scaping at roundabouts. Discussions include the relationship to visibility and sight distance requirements, types of landscaping and fixed objects appropriate for the central island and external areas, and other relevant items. A brief discussion of the use of art and other aesthetics in the vicinity of roundabouts is also provided. Chapter 10—Construction and Maintenance: This chapter focuses on constructability and maintenance of a roundabout. Appendices: Appendices are provided to expand upon topics in certain chapters. Several typographical devices have been used to enhance the readability of the guide. Margin notes, such as the note next to this paragraph, highlight important points or identify cross-references to other chapters of the guide. References have been listed at the end of each chapter and have been indicated in the text using italic numbers in parentheses, such as: (3). New terms are presented in italics and are defined in the glossary at the end of the document. Margin notes have been used to highlight important points.

Chapter 1/Introduction Page 1-19 Roundabouts: An Informational Guide 1.6 REFERENCES 1. Brown, M. TRL State of the Art Review: The Design of Roundabouts. HMSO, London, 1995. 2. Todd, K. “A History of Roundabouts in Britain.” Transportation Quarterly, Vol. 45, No. 1, January 1991. 3. Jacquemart, G. Synthesis of Highway Practice 264: Modern Roundabout Practice in the United States. TRB, National Research Council, Washington, D.C., 1998. 4. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 2006.