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Roundabout Practices (2016)

Chapter: Chapter Four - Case Examples of Roundabout Practices

« Previous: Chapter Three - Survey Results
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Suggested Citation:"Chapter Four - Case Examples of Roundabout Practices ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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26 alternative, or a DOT employee at headquarters or a regional office champions the roundabout alternative. In Georgia, a roundabout can be dismissed from consideration without a preliminary scaled aerial sketch to analyze a roundabout’s potential impacts. The interviews also highlighted that in addition to dif- ferences among policies concerning the selection of round- abouts, there are also differences in the enforcement of policies. Maryland, for example, strongly enforces its policy requiring the evaluation of a roundabout as part of the devel- opment review process; because of this policy, developers and the transportation engineers working for them are now proposing roundabouts knowing that the SHA will require them to do so. In Georgia. more than half of the roundabouts in the cur- rent program were selected and funded as part of GDOT’s safety program. Five of the seven states profiled indicated that cities and towns now request roundabouts, while the other two have an ICE policy in place that mandates a round- about be considered as an alternative. Although many of Maryland’s original roundabouts were selected because of existing safety problems at intersections, roundabouts have become the first choice for many of Maryland’s district engi- neers, regardless of the safety record at that intersection. MODIFICATIONS TO EXISTING ROUNDABOUTS Several interviewees discussed modifications to existing round abouts resulting from identified safety or operational problems. Most of the modifications to date involved reducing the number of lanes because of excess capacity at the round- about. In many cases, this involved converting multilane facili- ties that were overbuilt for future traffic volumes to single-lane roundabouts. An example is the US-50/Connector Road round- about in Emporia, Kansas, which was opened as a multilane roundabout (Figure 29) with three lanes on the east and west approaches; and was later converted to a single-lane roundabout (Figure 30) through striping and an expanded truck apron. At two locations, WSDOT reduced the capacity of multi- lane roundabouts to minimize minor crash patterns. By reduc- ing the number of lanes on the approach to the roundabout, it was able to reduce the number of points where crashes can occur. Interviews were conducted with representatives of seven state DOT, who sere selected based on the following guide- lines and feedback from the Topic Panel: • Known leaders in the roundabout field • A mix of early adopter and more recent adoption states • A geographic mix of states • Preference given to panel member states • Preference given to states who have participated in similar interviews in the past to allow for a comparative review. The selected states, highlighted in Figure 28, are Califor- nia, Connecticut, Georgia, Kansas, Maryland, Minnesota, and Washington. The state DOT representatives were asked about their responses to the survey, and additional questions were asked about the following five topics: • Roundabout selection • Phased roundabout implementation • Modifications to existing roundabouts • Design vehicle accommodations • Accelerated low-cost roundabouts. This chapter summarizes the discussion of each of these items. ROUNDABOUT SELECTION As indicated in the survey, safety is still the number one factor in the selection of roundabouts. The interviews suggest that a policy, directive, or guide suggesting or mandating that a roundabout be considered as an option also drives the selection of roundabouts, as do political reasons or changes in policies. Before the adoption of the ICE policy in California (45), the performance of roundabouts was not being evaluated because of the increased degree of difficulty involved with assessing the feasibility of roundabouts compared with other inter- sections forms. After the implementation of the ICE policy, the number of roundabouts considered “increased overnight.” However, despite policies that indicate a roundabout should be considered, some of the state officials interviewed still struggle to get roundabouts included in the evaluation outside of certain situations. In Connecticut, a roundabout might only be considered if the municipality requests one, a consultant that has experience with roundabouts proposes one as an chapter four CASE EXAMPLES OF ROUNDABOUT PRACTICES

27 Minnesota DOT (MnDOT) has modified several round- abouts from their original design. The 20-year traffic fore- cast was used for the design, and a multilane roundabout was installed. One location had performance issues immediately, and it was found that traffic volumes were low enough that a single lane would be sufficient. The roundabout was con- verted to a single-lane roundabout using pavement markings and signage, and is operating acceptably. Because of this experience, MnDOT is now considering installing single- lane roundabouts if the operational analysis shows that the roundabout is at the margin of requiring multiple lanes. State DOTs have also updated other types of circular intersection to improve safety and operations. Connecticut DOT (ConnDOT) and Caltrans have modified rotaries using roundabout design techniques. FHWA and the Roadway Safety Foundation awarded ConnDOT a national roadway safety award for the rotary conversion project. PHASED ROUNDABOUT IMPLEMENTATION As mentioned previously, at least five of the seven states inter- viewed have modified roundabouts by reducing capacity. Because of this experience, all seven DOTs are now actively attempting to size their roundabouts for immediate and near- future vehicle demand. Some state DOTS are using phased roundabout implementation to address the issue of overbuild- ing them. Kansas, for instance, is now actively promoting the use of a phased roundabout implementation approach through its recently published second edition of the Kansas Round- about Guide. FIGURE 28 States interviewed. FIGURE 29 As-constructed US-50/Connector Road Roundabout in Emporia, Kansas. Source: Google Earth Aerial (46). FIGURE 30 Modified US-50/Connector Road roundabout in Emporia, Kansas. Source: Google Earth Aerial (47).

28 As mentioned in the questionnaire summary, most states use a 20-year design life for their roundabout analyses, and many states, including Georgia and Maryland, will open the roundabout as a single-lane roundabout if that configuration is expected to be sufficient for approximately 10 years. Another factor, as the ConnDOT respondent pointed out, is the uncer- tainty with current forecasting methods. The Maryland SHA pointed out that the Towson Circle roundabout in Towson, Maryland, was opened as a multilane roundabout with two entry, exiting, and circulatory lanes on all approaches, and was designed to be expanded to three lanes in the future (Fig- ure 31). However, the Towson Circle was modified in 2008 to reduce the number of lanes, and Maryland SHA indicated that it will likely never be expanded. Although multiple states that were interviewed said they have built a roundabout with a phased implementation strat- egy, only the Maryland SHA operates a roundabout that had been expanded. As shown in Figure 32, the I-495/Ritchie- Marlboro Road interchange ramp terminal intersections were expanded from two to three lanes in advance of development in the vicinity. DESIGN VEHICLE ACCOMMODATION The selection of a design vehicle is often based on roadway classification, with most states using WB-62, WB-65, or WB-67s as the default design vehicles for state routes at all intersections, not only roundabouts. The default design vehi- cle for a state route is typically the state route movements at a roundabout, such as through movements on the state route at a roundabout intersection of a state road and local road. The design vehicle for other movements at a roundabout is typically based on local agency guidelines and adjacent land uses. In an effort to limit the size of roundabouts and provide speed control, WSDOT determines the specific movements larger vehicles are expected to make at an intersection, and designs its roundabouts to accommodate large vehicles on those movements. FIGURE 31 As-constructed I-495/Ritchie-Marlboro Road ramp terminal roundabouts. Source: Google Earth Aerial (48). FIGURE 32 Modified I-495/Ritchie-Marlboro Road ramp terminal roundabouts. Source: Google Earth Aerial (49).

29 Oversize/overweight vehicles, including vehicles such as low-boys with low vertical clearance, are only used as design vehicles on certain state highways with special freight needs. Most of the states interviewed use four-inch truck aprons adjacent to the central island, but some of the state agencies are considering lower height truck aprons to accommodate vehicles with low vertical clearance. WSDOT recently modi- fied its truck apron height from three inches to two inches in certain applications. GDOT tracks the routes of oversize/overweight vehicles statewide, and is able to determine the types of vehicles likely to traverse the proposed roundabout, and the move- ments the large vehicles are expected to make at the inter- section. Alternatively, GDOT will consider parallel routes for infrequent movements of large vehicles. KDOT only tracks oversize/overweight vehicles, so it is unable to determine specific movements at intersections that need to accommo- date an oversize/overweight vehicle movement. Consequently, KDOT designs the proposed roundabout to accommodate all oversize/overweight vehicle movements. ACCELERATED, LOW-COST ROUNDABOUTS GDOT’s District 1 used state forces to install a mini-roundabout at the intersection of SR-11/SR-124/Galilee Church Road in Jackson County, Georgia, over several weekends in April and May of 2013. Figures 33–36 show the intersection before, during, and after construction. Flexible curbing was used around the circumference of the central island, which was backfilled with asphalt. Since its original construction, the district has installed a concrete curb. The project included widening on three of the four cor- ners of the intersection to accommodate the 90-foot inscribed circle diameter. Construction costs were $63,000, including full repaving and striping. Two additional projects were completed under the same quick response contract, which is a contracting mechanism with a $200,000 cap. In one of the locations, the GDOT Dis- trict 3 maintenance staff worked with a general contractor at the intersection of Cedar Rock Road and SR-81 in Hotch- kins, Georgia, to rebuild the intersection as a roundabout. The contractor constructed the central island and grading out of a rigid plastic modular curbing system and then District 3 main- tenance staff laid the asphalt overlay on all approaches. The total roundabout construction cost was $300,000. The second location was constructed of a rigid plastic modular system. FIGURE 33 Intersection of SR-11/SR-124/Galilee Church Road in Jackson County, Georgia, prior to construction. Source: Zehngraff (50). FIGURE 34 Intersection of SR-11/SR-124/Galilee Church Road in Jackson County, Georgia, during first weekend of construction of the accelerated low-cost roundabout. Source: Zehngraff (50). FIGURE 35 Intersection of SR-11/SR-124/Galilee Church Road in Jackson County, Georgia, during second weekend of construction of the accelerated low-cost roundabout. Source: Zehngraff (50).

30 Both of these low-cost implementations were delivered in less time that a typical roundabout project, which entails a rig- orous review process. These projects also yielded a high rate of return on investment. The SR-11/SR-124/Galilee Church Road roundabout in particular was found to have reduced crashes at the intersection and shortened a quarter-mile stopped queue to an eight-to-10-car rolling queue. Georgia is not alone in this practice. WSDOT has installed accelerated, low-cost roundabouts at four locations. These compact roundabouts were interim fixes, all constructed in a weekend. Although there is no policy in place for this type of installation, it has been embraced as a technique for rapid deployment of an intersection retrofit. Candidate locations for this type of installation are at the nodes of an interchange where there are recurring backups onto the mainline. These projects typically include pavement markings and signs, with a budget of $200,000 to $300,000. In 2007, Maryland SHA installed a 75-foot diameter mini- roundabout at an interchange off-ramp at the intersection of US 50 and Thompson Creek Road in Stevensville, Maryland (Figure 37). The roundabout was installed using only thermo- plastic pavement markings. No curbing or pavement was used for the installation. The only additional materials used were vertical flexible posts to delineate the center island. The cost of the installation was around $50,000, and the roundabout has been in place since 2007. EFFECTIVE PRACTICES This section provides a summary of effective practices and lessons learned identified from the DOT interviews. Because each state is at a different stage in their experience with round- abouts, the primary topics and key takeaways are unique. FIGURE 36 Accelerated low-cost roundabout installation at the intersection of SR-11/SR-124/Galilee Church Road in Jackson County, Georgia. Source: Zehngraff (50). FIGURE 37 Accelerated low-cost roundabout installation at the intersection of US-50/Thompson Creek in Stevensville, Maryland. Source: Jenior (51).

31 California Roundabouts have been being built in California since the early 1990s. Over the past decade roundabouts were primar- ily considered through the Caltrans safety program. With the advent of the ICE policy in 2013, roundabouts have emerged as a preferred intersection type, consistently having the opti- mum outcome in comparisons of different intersection types. Hence, the ICE process has had a dramatic effect on the con- sideration of roundabouts in California. Connecticut ConnDOT is both building roundabouts and modifying rota- ries, which has led to improved safety performance. The award-winning Killingworth Rotary Modification project reduced the frequency and severity of crashes by incorporat- ing elements of roundabout design. Although roundabouts are not the preferred type of intersection control, Connecticut is looking to incorporate stronger language in the upcoming revision to the roadway design manual. ConnDOT is look- ing at cost-effective ways to deliver all types of transporta- tion projects, including scaling back the design horizon to a 10-year design period. Georgia GDOT built its first single-lane roundabout in Georgia in 1999, but it was not until 2008 that multilane roundabouts were allowed to be built in the state. GDOT has become very judi- cious about the number of lanes they use in their roundabouts. When evaluating a multilane roundabout, GDOT attempts to use a phased implementation approach. If a single-lane round- about can operate effectively for seven to 10 years, GDOT will open it as a single-lane roundabout designed for future expan- sion. GDOT has developed a checklist for roundabout designs, and it has developed its own roundabout analysis tool based on the HCM 2010 model. For future-year analyses, GDOT uses calibration factors from California and Bend, Oregon. GDOT has built several low-cost roundabouts that were installed in a matter of days. GDOT sees the benefit of reduced crashes and improved operations through roundabout implementation, which may not always require a “textbook” approach. Kansas KDOT has been viewed as a leader in the implementation of roundabouts in the United States. The first edition of the Kansas Roundabout Guide was adopted by several other states, and even a Canadian province, British Columbia. In the sec- ond edition of the guide, released in 2014, KDOT developed guidance to support the oversize/overweight vehicles that are becoming more common in Kansas. Roundabouts in Kansas, especially in the more rural areas, are responses to identified safety concerns. Many have been constructed at the local level, especially in Lawrence and suburbs of Kansas City. In the past, KDOT has provided peer review support to local jurisdictions as well, which has helped keep the qual- ity of roundabout designs high, even when KDOT is not officially involved. Maryland Maryland (SHA) established the nation’s first state roundabout program in the early 1990s. In the early years, SHA concen- trated on identifying intersections with a crash history that could be mitigated by roundabouts, and required all SHA districts to identify intersections. Later, SHA developed a policy requiring the evaluation of roundabouts, and because of Maryland’s positive experience with roundabouts and a strict enforcement of the policy, roundabouts have become the default intersection choice in the state. Many of SHA’s early multilane roundabouts were later scaled back because of safety or operational issues, and because of this experience, SHA has made a strong push towards a phased implementation approach at roundabouts. Similar, because of its experience, SHA is spending much more on roundabout projects than in the early 1990s by making sure that all roundabouts have curbs, gutters, and illumination. Minnesota Since 2009, MnDOT has been using an ICE process for select- ing intersection types. However, it was not until a few years ago that MnDOT started formally evaluating roundabouts as part of the ICE process, which is the primary mechanism for roundabouts selection. MnDOT has found that excess capacity at multilane roundabouts has resulted in an increased number of property damage only crashes. Because of this experience, when the anticipated future traffic volumes dictate the need for a multilane roundabout, MnDOT evaluates whether the round- about can be opened as a single-lane roundabout with capac- ity added when required. Recently, two multi lane roundabouts were reduced to single-lane roundabouts, and the number of crashes was reduced accordingly. Washington WSDOT has constructed more than 100 roundabouts on the state highway system and has been building roundabouts for more than 15 years. WSDOT has developed state-specific roundabout design guidance. Working with the trucking indus- try, WSDOT developed design techniques aimed at accom- modating large vehicles while still maintaining compact roundabout designs, including specifying curbing details and allowing lane straddling. The combination of these design techniques helps both to accommodate larger vehicles and keep vehicle speeds low.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 488: Roundabout Practices summarizes roundabout policies, guidance, and practices within state departments of transportation (DOTs) as of 2015. The synthesis may be used as a reference for state agencies that are creating or updating their roundabout and intersection control policies.

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