Guide for Roundabouts (2023) / Chapter Skim
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11-1 Contents 11-2 11.1 Profile and Cross-Section Relationships 11-2 11.2 Profile Development 11-7 11.3 Roadways with Grades 11-10 11.4 Transverse and Cross-Slope Design 11-10 11.4.1 Outward Sloping Circulatory Roadway 11-10 11.4.2 Crowned Circulatory Roadway 11-12 11.4.3 Other Circulatory Roadway Grading Techniques 11-12 11.4.4 Fully Traversable Central Islands 11-13 11.5 Pedestrian Design Influences 11-14 11.6 Trucks 11-17 11.7 References This chapter discusses roundabout vertical design. Roundabout vertical design represents a combination of roadway profiles, cross-section elements, and resulting three-dimensional surfaces that must consider the interrelationships of all elements.
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11-2 Guide for Roundabouts the roadway cross section. This chapter discusses these elements as they relate to roundabout performance -- visibility, drainage, driver comfort and safety, and accessibility -- and the need for context-sensitive design.
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Vertical Alignment and Cross-Section Design 11-3 A general procedure to develop a profile is described below. The example is provided to emphasize design principles and intent, but a variety of techniques can be used to appropriately meet vertical design needs.
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11-4 Guide for Roundabouts • Assess existing ground profiles along the horizontal alignment. A sample of existing elevations along each roadway's profile grade line provides the existing topography and roadway approach as well as departure grades.
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Vertical Alignment and Cross-Section Design 11-5 crossings (see Section 11.5)
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11-6 Guide for Roundabouts Exhibit 11.4. Example profile along outer edge of central island.
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Vertical Alignment and Cross-Section Design 11-7 • Alternative approach: Proceed from the outside in. Alternatively, practitioners can develop the outside edges of pavement and the splitter islands first, establishing alignments and setting the circle height on the basis of existing grades at the outside edges of the roundabout.
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11-8 Guide for Roundabouts of negative superelevation. Research using simulation of truck dynamics over a range of conditions has found that speed management, cross section, and truck apron design are important design parameters for managing lateral forces on trucks (2)
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Vertical Alignment and Cross-Section Design 11-9 Exhibit 11.7. Example of profile with modified downgrade profile through the roundabout.
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11-10 Guide for Roundabouts 11.4 Transverse and Cross-Slope Design The cross section influences roundabout profiles and surfaces. Together, the profile, cross section, and resulting surface influence roundabout performance in terms of drainage and meeting user needs.
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11-12 Guide for Roundabouts may be easier to achieve using asphalt paving. Exhibit 11.11 shows an example of a crowned circulatory roadway.
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Vertical Alignment and Cross-Section Design 11-13 Mini-roundabouts and compact roundabouts should have raised or domed central islands to promote conspicuity (refer to Section 2.3.2)
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11-14 Guide for Roundabouts accessibility needs. This could increase the roundabout's footprint and require practitioners to reconfigure the vertical alignments of the roadway approach.
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Vertical Alignment and Cross-Section Design 11-15 Truck aprons are commonly sloped toward the outside of the roundabout (Exhibit 11.9, b and d)
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11-16 Guide for Roundabouts 200 mm) , though it could be less.
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Vertical Alignment and Cross-Section Design 11-17 can be reviewed by drawing a chord across the apron in the position where the trailer would sweep across. In some cases, the warping of the profile along the circulatory roadway can create high spots that could cause trailers to drag or scrape along the truck apron.
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