Roundabouts: An Informational Guide – Second Edition (2010) / Chapter Skim
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From page 43... ...
Chapter 3/Planning Page 3-1 Roundabouts: An Informational Guide CHAPTER 3 PLANNING CONTENTS 3.1 INTRODUCTION .
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Roundabouts: An Informational Guide Page 3-2 Chapter 3/Planning 3.7 ECONOMIC EVALUATION .
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Chapter 3/Planning Page 3-3 Roundabouts: An Informational Guide LIST OF EXHIBITS Exhibit 3-1 Planning Framework .
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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.
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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.
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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.
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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 influenced 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.
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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.
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Chapter 3/Planning Page 3-9 Roundabouts: An Informational Guide roundabout design. These factors should be taken into consideration when comparing alternatives and how well each balances the improvement objectives: • Physical or geometric complications may significantly influence a roundabout's design and may make a roundabout infeasible or uneconomical.
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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 roundabouts have been implemented in the region.
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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)
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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.
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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)
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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.
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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)
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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 intersection treatment (see Exhibit 3-7)
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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.
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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)
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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)
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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.
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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.
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Roundabouts: An Informational Guide Page 3-22 Chapter 3/Planning If the volumes fall within the ranges identified in Exhibit 3-12 where "additional analysis is needed," a single-lane or two-lane roundabout may still function 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.
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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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 terminating bicycle lanes and providing bicycle ramps at roundabouts can be found in Chapter 6.
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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 carefully considered)
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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)
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Chapter 3/Planning Page 3-33 Roundabouts: An Informational Guide anticipated to operate within its capacity and can be used to make some planninglevel comparisons of roundabout delay to signal delay. Exhibit 3-19 presents average delays per vehicle for signals and roundabouts.
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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.
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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 Alternative A is given in Equation 3-1.
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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.
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Chapter 3/Planning Page 3-37 Roundabouts: An Informational Guide week, first determine the weekday vehicle-hours of delay and then multiply by 52.1 (365 divided by 7)
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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 roundabouts and other forms of intersection control beyond the basic elements.
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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.
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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 definition, generation of design alternatives, selection of preferred alternative, detailed design, go/no-go decision, construction/opening, and landscape maintenance.
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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.
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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)
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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.
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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)
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Appendix B provides instructional material and model language for drivers, cyclists, and pedestrians that can be adapted to drivers manuals.
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Roundabouts: An Informational Guide Page 3-46 Chapter 3/Planning 14. City of Modesto, California.
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