Roundabouts: An Informational Guide – Second Edition (2010) / Chapter Skim
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From page 144... ...
Chapter 6/Geometric Design Page 6-1 Roundabouts: An Informational Guide CHAPTER 6 GEOMETRIC DESIGN CONTENTS 6.1 INTRODUCTION .
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Roundabouts: An Informational Guide Page 6-2 Chapter 6/Geometric Design 6.6 MINI-ROUNDABOUTS .
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Chapter 6/Geometric Design Page 6-3 Roundabouts: An Informational Guide LIST OF EXHIBITS Exhibit 6-1 General Design Process .
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Roundabouts: An Informational Guide Page 6-4 Chapter 6/Geometric Design Exhibit 6-32 Example of a Partial Three Lane Roundabout with an Offset Approach Alignment .
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Chapter 6/Geometric Design Page 6-5 Roundabouts: An Informational Guide Exhibit 6-62 Roundabout with Realigned Ramp Terminal Approach to Provide Better Angle of Visibility to the Left .
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Roundabouts: An Informational Guide Page 6-6 Chapter 6/Geometric Design 6.1 INTRODUCTION The geometric design of a roundabout requires the balancing of competing design objectives. Roundabouts operate most safely when their geometry forces traffic to enter and circulate at slow speeds.
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Chapter 6/Geometric Design Page 6-7 Roundabouts: An Informational Guide Operational Analysis (From Chapter 4) Identify Lane Numbers/Arrangements Identify Initial Design Elements: • Size • Location • Alignment • Sidewalk and buffer widths • Crosswalk location and alignment Section 6.7: Performance Checks • Fastest path • Natural path • Design vehicle • Sight distance and visibility Section 6.8: Design Details • Pedestrian design • Bicycle design • Vertical design • Curb, apron, and pavement design Applications • Closely spaced roundabouts (Section 6.9)
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From page 151... ...
Roundabouts: An Informational Guide Page 6-8 Chapter 6/Geometric Design This chapter is organized such that the design principles common among all roundabout types are presented first. Even at the concept level, engineers are encouraged to develop designs that are consistent with the design principles in order to depict realistic impacts and to better define the required geometry.
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Chapter 6/Geometric Design Page 6-9 Roundabouts: An Informational Guide 6.2.1 SPEED MANAGEMENT Achieving appropriate vehicular speeds for entering and traveling through the roundabout is a critical design objective as it has profound impacts on safety of all users; it also makes roundabouts easier to use and more comfortable for pedestrians and bicyclists. A well-designed roundabout reduces vehicle speeds upon entry and achieves consistency in the relative speeds between conflicting traffic streams by requiring vehicles to negotiate the roundabout along a curved path.
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From page 153... ...
Roundabouts: An Informational Guide Page 6-10 Chapter 6/Geometric Design International studies have shown that reducing the vehicle path radius at the entry (i.e., deflecting the vehicle path) decreases the relative speed between entering and circulating vehicles and thus results in lower entering–circulating vehicle crash rates.
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From page 154... ...
Chapter 6/Geometric Design Page 6-11 Roundabouts: An Informational Guide The allowed movements assigned to each entering lane are key to the overall design. Basic pavement marking layouts should be considered integral to the preliminary design process to ensure that lane continuity is being provided.
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Roundabouts: An Informational Guide Page 6-12 Chapter 6/Geometric Design executing left or right turn movements. The same is true when drivers negotiate a two-lane loop ramp at an interchange.
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Chapter 6/Geometric Design Page 6-13 Roundabouts: An Informational Guide upon the exit from the roundabout where the exit radii are too small or the overall exit geometry does not adequately align the vehicle paths into the appropriate lane. Additional information on entry and exit design at multilane roundabouts is provided in Section 6.5.
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Roundabouts: An Informational Guide Page 6-14 Chapter 6/Geometric Design of Highways and Streets provides the dimensions and turning path requirements for a variety of common highway vehicles (4)
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Chapter 6/Geometric Design Page 6-15 Roundabouts: An Informational Guide recommended in areas where pedestrians and cyclists are common. Second, as described elsewhere in this document, one-lane roundabouts are generally easier and safer for non-motorized users than multilane roundabouts.
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Roundabouts: An Informational Guide Page 6-16 Chapter 6/Geometric Design sight distance is measured for vehicles entering the roundabout, with conflicting vehicles along the circulatory roadway and entering from the immediate upstream entry taken into account. International evidence suggests that it is advantageous to provide no more than the minimum required intersection sight distance on each approach (6)
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Chapter 6/Geometric Design Page 6-17 Roundabouts: An Informational Guide possible alternatives could be developed by varying the size of the inscribed circle diameter. Each of the alternatives shown in Exhibit 6-8 results in different impacts to the adjacent properties.
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Roundabouts: An Informational Guide Page 6-18 Chapter 6/Geometric Design alignment (7)
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Chapter 6/Geometric Design Page 6-19 Roundabouts: An Informational Guide Entry Alignment Question Should the approach alignment run through the center of the inscribed circle? Or is it acceptable to offset the approach centerline to one side?
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Roundabouts: An Informational Guide Page 6-20 Chapter 6/Geometric Design A common starting point in design is to center the roundabout so that the centerline of each leg passes through the center of the inscribed circle (radial alignment)
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Chapter 6/Geometric Design Page 6-21 Roundabouts: An Informational Guide possible. Y-shaped intersection alignments have the potential for higher speeds than desired.
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Roundabouts: An Informational Guide Page 6-22 Chapter 6/Geometric Design 6.4 SINGLE-LANE ROUNDABOUTS This section presents specific parameters and guidelines for the design of individual geometric elements at a single-lane roundabout. Many of these same principles also apply to the design of multilane roundabouts; however, there are some additional complexities to the design of multilane roundabouts that are described in detail in Section 6.5.
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Chapter 6/Geometric Design Page 6-23 Roundabouts: An Informational Guide Exhibit 6-12 shows the minimum dimensions for a splitter island at a singlelane roundabout, including the location of the pedestrian crossing. While the above diagram provides minimum dimensions for splitter islands, there are benefits to providing larger islands.
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Roundabouts: An Informational Guide Page 6-24 Chapter 6/Geometric Design 6.4.2 ENTRY WIDTH Entry width is measured from the point where the entrance line intersects the left edge of traveled way to the right edge of the traveled way, along a line perpendicular to the right curb line. The width of each entry is dictated by the needs of the entering traffic stream, principally the design vehicle.
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Chapter 6/Geometric Design Page 6-25 Roundabouts: An Informational Guide throughout the roundabout (9)
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Roundabouts: An Informational Guide Page 6-26 Chapter 6/Geometric Design Landscaping and other treatments within the central island are discussed in Chapter 8. 6.4.5 ENTRY DESIGN As shown in Exhibit 6-14, the entry is bounded by a curb or edge of pavement consisting of one or more curves leading into the circulatory roadway.
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Chapter 6/Geometric Design Page 6-27 Roundabouts: An Informational Guide desired entry path radius. Provided sufficient clearance is given for the design vehicle, approaching vehicles will adjust their path accordingly and negotiate through the entry geometry into the circulatory roadway.
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Roundabouts: An Informational Guide Page 6-28 Chapter 6/Geometric Design The exit curb is commonly designed to be curvilinearly tangential to the outside edge of the circulatory roadway. Likewise, the projection of the inside (left)
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Chapter 6/Geometric Design Page 6-29 Roundabouts: An Informational Guide At single-lane roundabouts in urban environments, exits should be designed to enforce slow exit path speeds to maximize safety for pedestrians crossing the exiting traffic stream. Pedestrian activity should be considered at all exits except where separate pedestrian facilities (grade separated paths, etc.)
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Roundabouts: An Informational Guide Page 6-30 Chapter 6/Geometric Design Larger-diameter roundabouts may be required to accommodate large vehicles while maintaining low speeds for passenger vehicles. However, in some cases, land constraints may limit the ability to accommodate large semi-trailer combinations while achieving adequate deflection for small vehicles.
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Chapter 6/Geometric Design Page 6-31 Roundabouts: An Informational Guide For locations with a high volume of truck traffic, special consideration may be given to the size of the roundabout to require use of the truck apron by only the largest of vehicles. For the example illustrated in Exhibit 6-20, the high volume of truck traffic traversing through the intersection dictated the use of a larger inscribed circle diameter.
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Roundabouts: An Informational Guide Page 6-32 Chapter 6/Geometric Design in turn limit the visibility of the central island on the approach. Additionally, wider entries and larger entry radii are typically required for a small diameter roundabout to accommodate the design vehicle.
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Chapter 6/Geometric Design Page 6-33 Roundabouts: An Informational Guide the material used for the sidewalks so that pedestrians are not encouraged to cross the circulatory roadway. In addition, the truck apron features should be designed to encourage heavy vehicles to use this portion of the central island when necessary.
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From page 177... ...
Roundabouts: An Informational Guide Page 6-34 Chapter 6/Geometric Design components becomes more difficult with multilane roundabout design. Due to this balancing of design elements that is required to meet the design principles, the use or creation of boilerplate or standard designs is discouraged.
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From page 178... ...
Chapter 6/Geometric Design Page 6-35 Roundabouts: An Informational Guide On multilane roundabouts, it is also desirable to achieve balanced lane utilization in order to be able to achieve predicted capacity. There are a number of design variables that can produce lane imbalance, such as poorly designed entry or exit alignments or turning movement patterns.
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Roundabouts: An Informational Guide Page 6-36 Chapter 6/Geometric Design designs. Entry widths should therefore be minimized and flare lengths maximized to achieve the desired capacity with minimal effect on crashes.
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Chapter 6/Geometric Design Page 6-37 Roundabouts: An Informational Guide minor street has been reduced to a single lane to provide consistency in the lane configurations. For the portions of a multilane roundabout where the circulatory roadway is reduced to a single lane, the guidance for circulatory roadway width contained in Section 6.4.3 should be used.
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Roundabouts: An Informational Guide Page 6-38 Chapter 6/Geometric Design where a portion of the circulatory roadway is required to have three lanes despite the fact that all of the entries have only two lanes. 6.5.4 ENTRY GEOMETRY AND APPROACH ALIGNMENT At multilane roundabouts, the design of the entry curvature should balance the competing objectives of speed control, adequate alignment of the natural paths, and the need for appropriate visibility lines.
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Chapter 6/Geometric Design Page 6-39 Roundabouts: An Informational Guide The engineer should balance the need to control entry speed with the need to provide good path alignment at multilane entries. The desired result of the entry design is for vehicles to naturally be aligned into their correct lane within the circulatory roadway, as illustrated in Exhibit 6-29.
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From page 183... ...
Roundabouts: An Informational Guide Page 6-40 Chapter 6/Geometric Design of the entry curve from the entrance line. If it is located too close to the circulatory roadway, the tangent (or large radius portion of the compound curve)
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Chapter 6/Geometric Design Page 6-41 Roundabouts: An Informational Guide As discussed previously for single-lane roundabouts, a useful surrogate for capturing the effects of entry speed, path alignment, and visibility to the left is entry angle (phi)
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From page 185... ...
Roundabouts: An Informational Guide Page 6-42 Chapter 6/Geometric Design regarding the minimum dimensions and design details for splitter islands are provided under the discussion of single-lane roundabouts in Section 6.4.1. Additional discussion of pedestrian crosswalk design is provided in Section 6.8.1 and considerations for signalized pedestrian crossing are discussed in Chapter 7.
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From page 186... ...
Chapter 6/Geometric Design Page 6-43 Roundabouts: An Informational Guide Exhibit 6-34 illustrates a possible low-cost fix that involves modifications to the lane arrangements using a combination of striping and physical modifications. This may be acceptable if the traffic volumes are compatible.
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Roundabouts: An Informational Guide Page 6-44 Chapter 6/Geometric Design 6.5.7 DESIGN VEHICLE CONSIDERATIONS Design vehicle considerations should be made for both tracking on the entry/ exit and within the circulatory roadway (as previously discussed in Section 6.5.3)
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Chapter 6/Geometric Design Page 6-45 Roundabouts: An Informational Guide 6.5.8 OTHER DESIGN PRACTICES Throughout the world there continues to be advancement in the design practices for multilane roundabouts. One practice initiated in the Netherlands and being tested elsewhere is the turbo-roundabout (13)
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Roundabouts: An Informational Guide Page 6-46 Chapter 6/Geometric Design to naturally follow the circulatory roadway and minimize running over of the central island to the extent possible. 6.6.1 GENERAL DESIGN CRITERIA FOR MINI-ROUNDABOUTS Many of the same principles are used in the design of mini-roundabouts as in full-sized roundabouts.
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From page 190... ...
Chapter 6/Geometric Design Page 6-47 Roundabouts: An Informational Guide location should be at the center of the of the left-turning inner swept paths, which will be near, but not necessarily on, the center of the inscribed circle (14)
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Roundabouts: An Informational Guide Page 6-48 Chapter 6/Geometric Design beneficial to discourage vehicles from making a left turn in front of the central island. However, this may result in a reduction of capacity since advancing the yield line may affect yielding behavior at the entry.
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From page 192... ...
Chapter 6/Geometric Design Page 6-49 Roundabouts: An Informational Guide For intersections with excessive skew or offset approach alignments, the use of dual mini-roundabouts is another option for providing adequate vehicle channelization through the intersection (14–15, 17–18)
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From page 193... ...
Roundabouts: An Informational Guide Page 6-50 Chapter 6/Geometric Design 6.6.1.2 Pedestrian Treatments at Mini-Roundabouts At conventional intersections, pedestrian ramps and crosswalks are typically located near the curb returns at the corners of the intersection. When converting to a mini-roundabout, these corner pedestrian-crossing locations may require relocation.
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Chapter 6/Geometric Design Page 6-51 Roundabouts: An Informational Guide 6.6.1.3 Bicycles at Mini-Roundabouts Since typical on-road bicycle travel speeds are between 12 and 20 mph (20 to 30 km/h) , the speeds of vehicles approaching and traveling through mini-roundabouts are similar to those of bicyclists.
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Roundabouts: An Informational Guide Page 6-52 Chapter 6/Geometric Design A third option achieves deflection for all movements by shifting the inscribed circle along the minor street axis, as illustrated in Exhibit 6-44. This option will likely require modification of all intersection curb lines and may require additional realignment of the approach legs upstream of the intersection.
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Chapter 6/Geometric Design Page 6-53 Roundabouts: An Informational Guide 6.7 PERFORMANCE CHECKS Performance checks are a vital part of roundabout design. These checks help an engineer determine whether the design meets its performance objectives.
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Roundabouts: An Informational Guide Page 6-54 Chapter 6/Geometric Design Recommended maximum theoretical entry design speeds for roundabouts at various intersection site categories are provided in Exhibit 6-47. Site Category Recommended Maximum Theoretical Entry Design Speed Mini-Roundabout 20 mph (30 km/h)
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Chapter 6/Geometric Design Page 6-55 Roundabouts: An Informational Guide more critical than the through movement. The fastest path should be drawn and checked for all approaches of the roundabout.
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Roundabouts: An Informational Guide Page 6-56 Chapter 6/Geometric Design Exhibit 6-50 Example of Critical Right-Turn Movement Exhibit 6-51 Guidance on Drawing and Measuring the Entry Path Radius then measure the minimum radii using suitable curve templates or by replicating the path in CAD and using it to determine the radii. The Wisconsin Department of Transportation Roundabout Guide (7)
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From page 200... ...
Chapter 6/Geometric Design Page 6-57 Roundabouts: An Informational Guide 6.7.1.2 Vehicle Speed Estimation The relationship between travel speed and horizontal curvature is documented in the AASHTO "Green Book" (4)
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Roundabouts: An Informational Guide Page 6-58 Chapter 6/Geometric Design speed. Analysts should use caution in using deceleration as a limiting factor to establish entry speeds for design.
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Chapter 6/Geometric Design Page 6-59 Roundabouts: An Informational Guide With all predicted speeds, the engineer is cautioned to look at the entire trajectory of the subject movement to determine what speeds are reasonable for each part of the trajectory. The above discussion highlights observed limitations on entry and exit speed based on circulating speed.
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Roundabouts: An Informational Guide Page 6-60 Chapter 6/Geometric Design and the vehicle cuts across adjacent travel lanes, ignoring all lane markings. In addition to evaluating the fastest path, at multilane roundabouts the engineer should also consider the natural vehicle paths.
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Chapter 6/Geometric Design Page 6-61 Roundabouts: An Informational Guide 6.7.3.1 Stopping Sight Distance Stopping sight distance is the distance along a roadway required for a driver to perceive and react to an object in the roadway and to brake to a complete stop before reaching that object. Stopping sight distance should be provided at every point within a roundabout and on each entering and exiting approach.
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From page 205... ...
Roundabouts: An Informational Guide Page 6-62 Chapter 6/Geometric Design Exhibit 6-55 Stopping Sight Distance on the Approach Exhibit 6-56 Stopping Sight Distance on Circulatory Roadway Exhibit 6-57 Sight Distance to Crosswalk on Exit
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Chapter 6/Geometric Design Page 6-63 Roundabouts: An Informational Guide 6.7.3.2 Intersection Sight Distance Intersection sight distance is the distance required for a driver without the rightof-way to perceive and react to the presence of conflicting vehicles. Intersection sight distance is achieved through the establishment of sight triangles that allow a driver to see and safely react to potentially conflicting vehicles.
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Roundabouts: An Informational Guide Page 6-64 Chapter 6/Geometric Design entry. If the approach leg of the sight triangle is greater than 50 ft (15 m)
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Chapter 6/Geometric Design Page 6-65 Roundabouts: An Informational Guide 6.7.3.5 Combined Sight Distance Diagram During design and review, roundabouts should be checked to ensure that adequate stopping and intersection sight distance is being provided. Checks for each approach should be overlaid onto a single drawing, as shown in Exhibit 6-60, to illustrate the clear vision areas for the intersection.
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Roundabouts: An Informational Guide Page 6-66 Chapter 6/Geometric Design At roundabouts, the intersection angle may be measured as the angle between a vehicle's alignment at the entrance line and the sight line required according to intersection sight-distance guidelines. Exhibit 6-61 shows an example design with a severe angle of visibility to the left, and Exhibit 6-62 shows a possible correction.
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Chapter 6/Geometric Design Page 6-67 Roundabouts: An Informational Guide 6.8 DESIGN DETAILS This section provides a discussion of a variety of design details that are common to all types of roundabouts. 6.8.1 PEDESTRIAN DESIGN CONSIDERATIONS 6.8.1.1 Sidewalks Wherever possible, sidewalks at roundabouts should be set back from the edge of the circulatory roadway with a landscape strip.
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Roundabouts: An Informational Guide Page 6-68 Chapter 6/Geometric Design The recommended sidewalk width at roundabouts is 6 ft (1.8 m) , and the minimum width is 5 ft (1.5 m)
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Chapter 6/Geometric Design Page 6-69 Roundabouts: An Informational Guide the crossing is from the roundabout, the more likely pedestrians will choose a shorter route that may put them in greater danger. On the other hand, placing crosswalks at distances away from the entrance line that are approximately in increments of vehicle lengths reduces the chance that queued vehicles will be stopped on the crosswalk, blocking convenient crossing movements by pedestrians.
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Roundabouts: An Informational Guide Page 6-70 Chapter 6/Geometric Design – Place the entire crosswalk perpendicular to the centerline of the approach roadway. This results in angled crossings of the entry and exit lanes.
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Chapter 6/Geometric Design Page 6-71 Roundabouts: An Informational Guide with a planting strip as recommended above, ramps do not need to have flares and should simply have curbed edges aligned with the crosswalk. This provides alignment cues for pedestrians, especially those who are blind or who have low vision.
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Roundabouts: An Informational Guide Page 6-72 Chapter 6/Geometric Design traffic predictions suggest that a multilane roundabout may be desirable. In addition, the use of a roundabout with two-lane entries for the major roadway and one-lane entries for the minor roadway can be a good solution to minimize complexity for bicyclists where a roundabout is proposed at an intersection of a major multilane street and a minor street.
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Chapter 6/Geometric Design Page 6-73 Roundabouts: An Informational Guide abouts. As described in Section 6.8.2, multilane roundabouts are more challenging for cyclists, and bike ramps can be used to provide the option to travel through the roundabout like a pedestrian.
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Roundabouts: An Informational Guide Page 6-74 Chapter 6/Geometric Design Exhibit 6-67 Possible Treatments for Bicycles Exhibit 6-68 Bicycle Ramp Design Options Since bike ramps can be confusing for pedestrians with vision impairments, detectable warnings should be included on the ramp. Where the ramp is placed in a planter strip, the detectable warning field should be placed at the top of the ramp since the ramp itself is part of the vehicular area for which the detectable warning is used.
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Chapter 6/Geometric Design Page 6-75 Roundabouts: An Informational Guide help keep pedestrians from misconstruing the bike ramp as a pedestrian crossing location. These aspects include the angle of the ramp, the possible steeper slope of the ramp, and location of the ramp relatively far from the roundabout and crosswalk.
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Roundabouts: An Informational Guide Page 6-76 Chapter 6/Geometric Design attempting to merge into traffic. It may also be possible at multilane roundabouts in slow-speed urban environments to include a bus stop without a bus pullout immediately after the crosswalk, as exiting traffic has an opportunity to pass the waiting bus.
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From page 220... ...
Chapter 6/Geometric Design Page 6-77 Roundabouts: An Informational Guide provided in advance of the splitter island. It may be desirable to extend the curbing from the approach for at least the length of the required deceleration distance to the roundabout.
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Roundabouts: An Informational Guide Page 6-78 Chapter 6/Geometric Design and exiting–circulating vehicle crash rates (see Chapter 5)
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Chapter 6/Geometric Design Page 6-79 Roundabouts: An Informational Guide the lower expectation of drivers to stop may increase the risk of collisions with pedestrians. They also introduce additional complexity for pedestrians with visual impairments who are attempting to navigate the intersection.
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Roundabouts: An Informational Guide Page 6-80 Chapter 6/Geometric Design There are two design options for right-turn bypass lanes. The first option, shown in Exhibit 6-72 (full bypass)
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Chapter 6/Geometric Design Page 6-81 Roundabouts: An Informational Guide operational performance than the second. However, the second option generally requires less construction and right-of-way than the first.
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Roundabouts: An Informational Guide Page 6-82 Chapter 6/Geometric Design 6.8.7 VERTICAL CONSIDERATIONS Components of vertical alignment design for roundabouts include profiles, superelevation, approach grades, and drainage. Vertical design should account for the likelihood of large truck overturning or load shifting, which can sometimes be induced by excessive cross slopes.
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Chapter 6/Geometric Design Page 6-83 Roundabouts: An Informational Guide may also be beneficial to the engineer, reviewers, and contractor. The combination of the central island, inscribed circle, and curb profiles allows for quick verification of cross slopes and drainage and provides additional information to contractors for staking out the roundabout.
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Roundabouts: An Informational Guide Page 6-84 Chapter 6/Geometric Design The outward cross-slope design means vehicles making through and left-turn movements must negotiate the roundabout at negative superelevation. Excessive negative superelevation can result in an increase in single-vehicle crashes and lossof-load incidents for trucks, particularly if speeds are high.
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From page 228... ...
Chapter 6/Geometric Design Page 6-85 Roundabouts: An Informational Guide be no more than 2%; greater slopes may increase the likelihood of loss-of-load incidents. Within the United States, truck aprons are commonly sloped toward the outside of the roundabout.
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Roundabouts: An Informational Guide Page 6-86 Chapter 6/Geometric Design 6.8.7.5 Locating Roundabouts on Grades It is generally not desirable to place roundabouts in locations where grades through the intersection are greater than 4%, although roundabouts have been installed on grades of 10% or more. Installing roundabouts on roadways with grades lower than 3% is generally not problematic (25)
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Chapter 6/Geometric Design Page 6-87 Roundabouts: An Informational Guide roundabout should not necessarily be eliminated from consideration at such a location. Rather, the intersection should be relocated or the vertical profile modified, if possible.
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Roundabouts: An Informational Guide Page 6-88 Chapter 6/Geometric Design under truck traffic. However, few agencies have reported problems with rutting on well-constructed asphalt concrete pavement.
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Chapter 6/Geometric Design Page 6-89 Roundabouts: An Informational Guide 6.8.8.3 Truck Apron Material For the truck apron, concrete pavement or concrete with a brick paver surface is commonly used. Other options include using large [4 in (100 mm)
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From page 233... ...
Roundabouts: An Informational Guide Page 6-90 Chapter 6/Geometric Design 6.9 CLOSELY SPACED ROUNDABOUTS It is sometimes desirable to consider the operation of two or more roundabouts in close proximity to each other. In these cases, the expected queue length at each roundabout becomes important.
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Chapter 6/Geometric Design Page 6-91 Roundabouts: An Informational Guide intersections without providing a significant operational detriment to the other intersection, provided that adequate capacity is available at both intersections. Exhibit 6-81(b)
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Roundabouts: An Informational Guide Page 6-92 Chapter 6/Geometric Design This interchange form has been used successfully in some cases to defer the need to widen bridges. Unlike signalized ramps that may require exclusive leftturn lanes across the bridge and extra queue storage, this type of roundabout interchange exhibits very little queuing between the intersections since these movements are almost unopposed.
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From page 236... ...
Chapter 6/Geometric Design Page 6-93 Roundabouts: An Informational Guide Carmel, Indiana Avon, Colorado Exhibit 6-85 Example of a Compact Interchange with Raindrop-Shaped Central Islands Exhibit 6-86 Example of Interchange with Raindrop-Shaped Central Islands Raindrop central islands make wrong-way movements more difficult, but require navigating two roundabouts to make a U-turn. The second configuration uses raindrop-shaped central islands that preclude some turns at the roundabout; examples are shown in Exhibit 6-85 and Exhibit 6-86.
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From page 237... ...
Roundabouts: An Informational Guide Page 6-94 Chapter 6/Geometric Design 6.10.2 SINGLE-POINT DIAMOND INTERCHANGE Another type of diamond interchange is a single-point diamond interchange. This incorporates a single large-diameter roundabout centered over or under a freeway.
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From page 238... ...
Chapter 6/Geometric Design Page 6-95 Roundabouts: An Informational Guide 6.11 ACCESS MANAGEMENT Access points near an intersection or along an arterial create additional conflicts within the roadway system that affect operations and safety. Managing access points can improve the overall effectiveness of the system by streamlining the roadway operations and reducing the number of conflicts.
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Roundabouts: An Informational Guide Page 6-96 Chapter 6/Geometric Design 6.11.2 ACCESS NEAR THE ROUNDABOUT Public and private access points near a roundabout often have restricted operations due to the channelization of the roundabout. Driveways between the crosswalk and entrance line complicate the pedestrian ramp treatments and introduce conflicts in an area critical to operations of the roundabout.
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Chapter 6/Geometric Design Page 6-97 Roundabouts: An Informational Guide (a) Driveway between crosswalk and roundabout (Bend, Oregon)
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Roundabouts: An Informational Guide Page 6-98 Chapter 6/Geometric Design downstream of a traffic signal. Queuing from nearby intersections (the roundabout or others nearby)
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Chapter 6/Geometric Design Page 6-99 Roundabouts: An Informational Guide additional lanes. Where a single-lane roundabout will be sufficient for much of its design life, engineers should evaluate whether it is best to first construct a singlelane roundabout until traffic volumes dictate the need for expansion to a multilane roundabout.
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Roundabouts: An Informational Guide Page 6-100 Chapter 6/Geometric Design is used, grinding off of the pavement markings may leave a permanent mark on the roadway surface that may be confusing to drivers. Therefore, particular care should be taken in locating the markings in the interim configuration where concrete paving is used to minimize the need for relocation of the markings in the ultimate configuration.
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Chapter 6/Geometric Design Page 6-101 Roundabouts: An Informational Guide (a) Staged Multilane Roundabout: Interim Configuration (b)
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Roundabouts: An Informational Guide Page 6-102 Chapter 6/Geometric Design 6.13 REFERENCES 1.
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Chapter 6/Geometric Design Page 6-103 Roundabouts: An Informational Guide 16. Brilon, W
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