Guide for Roundabouts (2023)

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13-1   Contents 13-1 13.1 Roadway Pavement Type 13-3 13.2 Truck Apron Material 13-5 13.3 Pavement Jointing 13-7 13.4 Curb Type 13-9 13.5 Splitter Islands with Sloped Noses 13-10 13.6 References This chapter provides guidance on design, surface, and material details that bridge a roundabout’s design from the horizontal and vertical layouts described in Chapter 10: Horizontal Alignment and Design and Chapter 11: Vertical Alignment and Cross-Section Design to the construction process described in Chapter 15: Construction and Maintenance. This chapter discusses • Pavement and surface treatments, • Curb types, and • Median and splitter island features that facilitate maintenance. This chapter is not an exhaustive discussion of all relevant design details. Many geometric design considerations related to these topics are fundamental to all intersection types, including those presented in Chapters 10 and 11. 13.1 Roadway Pavement Type Pavement type and surface material treatment preference vary by agency and are based on climate and established local practice. Two material types are common for the top surface of approach, departure, and circulatory roadways at roundabouts: asphalt concrete (AC) and portland cement concrete (PCC). Whether to use AC or PCC depends on local preferences and the pave- ment type on the approach roadways. Some trade-offs for each are described in Exhibit 13.1. Project context often dictates the choice between AC and PCC. For example, a complex staging and traffic management scenario may be more readily executed using PCC. Conversely, AC may be beneficial for a multilane roundabout with complex lane configurations that need a spiral marking pattern. C H A P T E R 1 3 Curb and Pavement Details

13-2 Guide for Roundabouts Asphalt Concrete Portland Cement Concrete Maintenance • Mill and overlay of the existing roundabout are easier with traffic present. • Easier to surface repair. • Joint repair is not possible with traffic present, so more detailed traffic control plans are necessary for maintenance. Pavement marking • Best contrast with any type of marking material. • Requires using contrast tape, which is more costly to maintain. Staging • May be able to take advantage of temporary pavement as a base below the leveling course. • No joint lines to address. • Temporary pavement and other surfaces may be covered if the top asphalt layer is placed all at once in a later stage. • If joint lines are not perpendicular, longitudinal cracking will occur over time. • Adding curb in later stages requires a sawcut and backfilling with specified compaction. • Joint lines provide a clean break for stages. • Temporary AC adjacent to PCC can be easily removed without a sawcut. • Staging requires stage cuts at joint lines, which may be more difficult with traffic present. • Overall construction time may be longer because of curing time. Durability • Shorter design life. • More prone to rutting, which affects drainage characteristics. • Requires substantial base material and depth and surface mix design for optimal durability and skid resistance. • Rehabilitation is feasible, cost-effective, and practical. • Longer design life; more likely to retain initial drainage characteristics. • Near the end of the design life, complete reconstruction is required—pavement rehabilitation is not possible. Color contrast • If splitter islands are AC, there will be minimal contrast as color lightens over time. • Provides good contrast with pavement markings and with PCC truck aprons. • Colored AC is possible but less common than colored PCC. • Mixes can be colored to provide contrast and textured for aesthetics. • More difficult to establish contrast with pavement markings and truck apron. • Requires using contrast tape (more costly to maintain). Other • AC placement will be more rounded through cross-slope transitions, which may help oversized vehicles and lowboy trailers traverse through the roundabout. • Easier to construct a smooth crown line. • Skid resistance declines more rapidly. • Joint lines may resemble lane lines in the multilane design. Expandability or staged design expansion • Adjustability of grades is easier, and drainage is less impacted. • The PCC joint pattern may not be compatible with an expanded layout. • Dowels may be required for expansion areas. SOURCE: Adapted from American Concrete Pavement Association (1). Exhibit 13.1. Considerations for pavement type at roundabouts.

Curb and Pavement Details 13-3   13.2 Truck Apron Material Agencies use a range of aesthetic treatments, including simple broomed PCC, stamped/ patterned PCC (broomed finish), and various types of paver materials—there is no preferred texture for the truck apron. However, it is beneficial for the truck apron to be a different color and texture from the circulatory roadway and non-traversable surfaces (e.g., sidewalks) to improve its visibility to drivers and distinguish it from other elements. There are multiple ways to accomplish this, three of which are discussed below. • PCC and AC. Some agencies use PCC for truck aprons (for durability) and AC for the circu- latory roadway. This way, the truck apron is less prone to rutting. The visual distinction also helps communicate the difference in the intended uses. • PCC only. For roundabouts constructed entirely of PCC, mixtures can be colored to provide visual distinction. The apron may also be textured or patterned. With this approach, an agency preferably will not select a color that may be mistaken for one that the MUTCD designates for a specific use (2). For example, red and green coloring denotes travel lanes for transit and bicycle use. Therefore, some agencies have adopted designated colors for associated round- about design elements. • AC and pavers. Some agencies use pavers (or other stone material) in the truck apron (see Exhibit 13.2 and Exhibit 13.3), with the uneven driving surface dissuading drivers from LOCATION: US 5/Route 9, Brattleboro, Vermont. SOURCE: Lee Rodegerdts. Exhibit 13.2. Example of raised truck apron using pavers. LOCATION: Broad Street/Coburn Avenue/Chuck Druding Drive, Nashua, New Hampshire. SOURCE: Lee Rodegerdts. Exhibit 13.3. Example of flush truck apron using pavers.

13-4 Guide for Roundabouts traversing the area. Pavers can be an effective design element that provides an aesthetic quality, but they require more time, money, and expertise—paver installation is labor intensive and can result in water ponding or erosion if pavers are improperly installed. Exhibit 13.4 and Exhibit 13.5 demonstrate the means and methods for constructing a PCC truck apron. In this example, the truck apron curb was located and constructed before the remainder of the truck apron. Steel reinforcing in the truck apron adds durability for anticipated design vehicle loads. The shape of the truck apron curb more effectively discourages passenger cars from using the apron than the material it is made from. The curb helps reinforce the use of the circulatory roadway while allowing the intended vehicles to track upon the truck apron; this practice permits the use of smooth truck aprons that are easier to construct and maintain. Section 13.4 further discusses curb types. Pedestrians should not access the roundabout’s central island. Truck apron material and color alone are not appropriate ways to reinforce this because these differences are not adequate for people who are blind or have low vision. Instead, landscaping and buffer strips or LOCATION: Route 9–Route 126/I-95 Northbound Ramps/Service Plaza Drive, West Gardiner, Maine. SOURCE: Jonathan French. Exhibit 13.4. Example of placing truck apron curb. LOCATION: Route 9–Route 126/I-95 Northbound Ramps/Service Plaza Drive, West Gardiner, Maine. SOURCE: Jonathan French. Exhibit 13.5. Example of steel reinforcing being placed for truck apron.

Curb and Pavement Details 13-5   other detectable edges between the sidewalk and circulatory roadway, discussed in Chapter 10: Horizontal Alignment and Design, more effectively discourage pedestrians of all abilities from accessing the central island. 13.3 Pavement Jointing If PCC is used for the circulatory roadway of a multilane roundabout, expansion joint design and location are key considerations. Practitioners must carefully develop a jointing plan to avoid joint lines being mistaken for lane lines. In general, the best joint patterns are those that are concentric and radial to the circulatory roadway within the roundabout. • On single-lane roundabouts, jointing must not split the circulatory roadway into equal parts, as this can give the illusion of a two-lane roundabout. This can be particularly problematic at night and in wet conditions when vehicles may drive along the joints. This introduces the potential for side-by-side movements. Alternatively, the jointing may split the roadway into larger and smaller segments that are unlikely to be confused for distinct travel lanes (see Exhibit 13.6). • On multilane roundabouts, circumferential joints within the circulatory roadway need to follow pavement markings to the extent practical (see Exhibit 13.7). Cracking in PCC has been a problem at some roundabouts, particularly around the outside of the circulatory roadway near outside curbs or splitter islands. Practitioners can solve this issue by isolating the circulatory roadway portion with an expansion joint and constructing special monolithic sections in key areas on the approaches and around splitter islands. When the joints are laid out independently of each other, the joint spacing adjacent to the truck apron and the outside of the circulatory roadway can be more uniform, rather than closer together near the truck apron and farther apart on the outside of the circulatory roadway. Practitioners need to prepare a jointing plan and associated detail sheets as part of the final design plan set and submit them to the review authority. Exhibit 13.6 provides an example concrete jointing plan and the resulting roundabout that illustrates the radial pattern. Exhibit 13.7 shows an example of jointing alignment with pavement markings. LOCATION: US 75/K-31/K-268, Osage County, Kansas. SOURCE: Kansas Department of Transportation. Exhibit 13.6. Example jointing plan for a single-lane roundabout.

13-6 Guide for Roundabouts A jointing plan is needed so the joint layout will be constructed properly; the plan is the key by which the joints will be correctly located. The American Concrete Pavement Association (ACPA) identifies a six-step process for developing a jointing plan (1): 1. Draw all pavement edge and back-of-curb lines in plan view. Draw locations of all manholes, drainage inlets, and valve covers so that joints can intersect them. 2. Draw all lane lines on approach legs and in the circulatory roadway. Confirm that joint spacing does not exceed the maximum recommended width of 15 ft (4.5 m). 3. In the circulatory roadway, add transverse joints radiating out from the center of the circle. Extend these joints through the back of the curb and gutter. 4. On the approaches, add transverse joints at all locations where a width change occurs in the pavement (at bullnose of splitter islands; beginning and ending of curves, tapers, tangents, curb returns; etc.). Extend these joints through the back of the curb and gutter. 5. Add transverse joints beyond and between those added in Step 4. Space joints evenly between other joints, making sure to not violate maximum joint spacing. 6. Adjust for in-pavement objects and fixtures and to eliminate L-shapes, small triangular slabs, and so on. The ACPA recommends considering the following when preparing a jointing plan for a round- about (1): • Match existing joints and cracks wherever possible. • Place joints to meet in-pavement structures. • Set maximum joint spacing as follows: – 24 times concrete thickness (on unstabilized base) – 21 times concrete thickness (on stabilized base) – Maximum of 15 ft (4.5 m) for streets and highways • Understand that practical adjustments can be made to joint locations. Similarly, the ACPA recommends avoiding the following (1): • Slabs less than 1 ft (300 mm) wide • Slabs greater than 15 ft (4.5 m) wide • Angles less than 60 degrees created by dog-legging joints through curve radius points (approx- imately 90 degrees is best) LOCATION: Rice Road/I-70 Eastbound Ramps/Cyprus Drive, Topeka, Kansas. SOURCE: Lee Rodegerdts. Exhibit 13.7. Example of pavement markings aligned with concrete jointing at a single-lane roundabout.

Curb and Pavement Details 13-7   • Creating interior corners (L-shaped slabs) • Creating odd shapes (keep slabs square or pie-shaped) As noted in Section 10.8, a roundabout may be built as a single-lane roundabout with plans for expansion in the future based on projected traffic volumes. However, because concrete jointing can be mistaken for striping, the jointing plans established for the opening-year roundabout should be reasonably compatible with plans for the ultimate roundabout configuration. Exhibit 13.8 illustrates an example of a jointing plan for a multilane roundabout with a spiral design. Exhibit 13.9 shows how the painted lane lines are aligned with the longitudinal jointing. 13.4 Curb Type Two general curb (or curb and gutter) types are used at roundabouts. One group of curb types is traversable and accommodates vehicles driving onto and over them if necessary. Other curb types are non-traversable, typically with more vertical rise, and are expressly designed to discourage any driver from mounting or driving over them. Exhibit 13.10 shows a wide array of concrete curb types, demonstrating that curbs and local design standards dictate the details but that various designs can support the traversable or non-traversable intent. Variations using granite curbs are possible. • Traversable curbs. These are sometimes called rolled or mountable curbs. These are most common for the leading edges of truck aprons but are sometimes used throughout a roundabout LOCATION: Radio Road/Paulson Road, Saint Croix County, Wisconsin. SOURCE: Wisconsin Department of Transportation. Exhibit 13.8. Example of multilane roundabout jointing plan.

13-8 Guide for Roundabouts in higher-speed environments. In the higher-speed context, mountable curbs may be used at splitter islands or medians with a taller height and different profile from those at truck aprons. Traversable curbs generally have a low vertical component of 1 in. to 2 in. (25 mm to 50 mm), a low overall rise of 3 in. to 4 in. (75 mm to 100 mm), and a broad slope in a vertical-to-horizontal ratio of 1:4. Curb types can vary, but general practice favors keeping the vertical component and the overall rise lower to minimize the adverse effects on truck dynamics or damage to truck tires while still discouraging passenger car use. There is considerable variation from agency to agency. Exhibit 13.10 provides a few examples of curb type variations. LOCATION: Mason Street / I-41 Southbound Ramps, Green Bay, Wisconsin. SOURCE: Lee Rodegerdts. Exhibit 13.9. Example of multilane roundabout jointing along pavement markings. NOTE: Variations using granite curbs are possible but not shown. SOURCE: Adapted from Florida Department of Transportation and Washington State Department of Transportation details (4, 5). Exhibit 13.10. Concrete curb type variations.

Curb and Pavement Details 13-9   • Non-traversable curbs. These are sometimes called vertical or slope-faced curbs. They are most common in the vicinity of pedestrian crossings but are sometimes used throughout a round- about in lower-speed environments (except for the front edge of truck aprons). Non-traversable portions of central islands can also be separated from truck aprons with vertical curbs. Agency preferences vary widely, but these are often 6-in. (150 mm) vertical curbs that provide pro- tection and detectable edging for pedestrians, typical of what is provided in most urban areas. These curb types may also have a sloped face but remain unmountable (see Exhibit 13.10, d). Curb strikes are one of the leading causes of motorcyclist fatalities, which are overrepresented at roundabouts compared with other intersection forms (3). Mountable curb types around the perimeter and along the inside of the truck apron may improve recovery for these vehicle types. Curb types need to be consistent with the intersection plan to accommodate oversize or overweight vehicles. 13.5 Splitter Islands with Sloped Noses As discussed in Chapter 10: Horizontal Alignment and Design, channelization of splitter islands includes the fundamental principles of offsetting and matching the radius to the intended driver- funneling effect (6). For roundabouts in locations with snowfall that needs to be cleared, plowable end treatments (depicted in Exhibit 13.11 and Exhibit 13.12) can allow snowplows to push LOCATION: CR 17/52nd Avenue S, West Fargo, North Dakota. SOURCE: Lee Rodegerdts. Exhibit 13.11. Example of plowable end treatment at splitter island. LOCATION: US 50/California State Route 89, El Dorado County, California. SOURCE: Michael Alston. Exhibit 13.12. Example of plowable end treatment at bypass island.

13-10 Guide for Roundabouts snow up to and on the splitter island. Some agencies also use sloped edges at pedestrian ramps and pedestrian routes through splitter islands to reduce damage from plowing operations. This is a viable treatment where winter weather is a serious consideration. 13.6 References 1. Research & Technology Update No. 6.03: Concrete Roundabouts. American Concrete Pavement Association, Washington, DC, 2005. 2. Manual on Uniform Traffic Control Devices for Streets and Highways, 2009 ed., Including Revision 1, Dated May 2012; Revision 2, Dated May 2012; and Revision 3, Dated August 2022. FHWA, US Department of Transportation, 2022. http://mutcd.fhwa.dot.gov/. 3. Steyn, H. J., A. Griffin, and L. Rodegerdts. A Review of Fatal and Severe Injury Crashes at Roundabouts. Vol. IV of VII, Accelerating Roundabout Implementation in the United States. Publication FHWA-SA-15-072. FHWA, US Department of Transportation, 2015. 4. FY 2022–23 Standard Plans: Curb and Gutter. Index 520-001. Florida Department of Transportation, Tallahassee, 2021. https://www.fdot.gov/design/standardplans/current/default.shtm. (Accessed June 9, 2022.) 5. Roundabout Cement Concrete Curbs: Standard Plan F-10.18-02. Washington State Department of Transporta- tion, Olympia, 2020. https://wsdot.wa.gov/engineering-standards/all-manuals-and-standards/standard-plans. (Accessed June 9, 2022.) 6. A Policy on Geometric Design of Highways and Streets, 7th ed. AASHTO, Washington, DC, 2018.