Roundabouts are a useful form of intersection design that have applicability in a variety of situations. Roundabouts can be used at low-volume and high-volume intersections as an alternative to traditional intersection control of stop signs and traffic signals, where such control forms are not warranted or efficient. Because roundabouts require a low speed on entry and circulation, they necessitate more adaptation to a high-speed setting, in order to encourage drivers to slow to the lower speeds suitable for entering and circulating the roundabout. Appropriate deceleration on approaches to roundabouts is primarily accomplished through the use of applicable geometric design principles; however, traffic control devices (specifically signing and markings on the approach) also serve a vital role in communicating to the approaching driver what speed profile should be anticipated. Information on suitable treatments to use at these locations is valuable to practitioners and can aid in the decision about which treatment(s) to install and/or study at a given location. This report provides a resource for engineers to identify and select appropriate speed reduction treatments for high-speed approaches to roundabouts. The research examines best practices and research literature on speed reduction techniques for high-speed approaches for all intersection types, as well as treatments for work zones and horizontal curves. Included in the review is a summary of current design and traffic control device guidance in Minnesota, from the MnDOT Road Design Manual and Minnesota Manual on Uniform Traffic Control Devices, and a compilation of responses from practitioners with experience in designing and operating roundabouts with high-speed approaches. Guidance from within Minnesota includes discussion of how to determine a roundabout’s fastest path and providing appropriate radii for entry, circulation, and exit manoeuvres; selecting appropriate design vehicles and approach offsets; landscaping and maintenance concerns; and recommended signs, plaques, and pavement markings. Feedback from practitioners was in the form of responses to 10 questions about their experiences with roundabouts with high-speed approaches, including geometric treatments, traffic control device treatments, changes in maintenance practices, crash history, and countermeasures considered or applied. Researchers contacted three groups of practitioners, based on input and guidance from the TAP: the Transportation Research Board (TRB) Roundabout Committee and listserv used heavily by more than 400 practitioners, the technical consultants on MnDOT list of pre-qualified contractors for roundabout work types, and the city and county staff who are on MnDOT’s state-aid contact list. In total, during the month in which the practitioner request was conducted, researchers received nine responses from the TRB listserv and one updated contact person from the MnDOT prequalified list. Of these responses, four completed questionnaires were received, representing respondents from four states (Minnesota, Arizona, Kansas, and Wisconsin) and were employed by two state DOTs, a city, and a consultant. Common treatments described by the practitioners included splitter islands, approach curves, guide signs and diagrammatic signs (with spacing adjusted for speed-appropriate distances), and illumination. Respondents indicated very little change in routine maintenance, and they indicated that the number and severity of crashes declined after installation of roundabouts. Researchers also reviewed previous research and existing guidance on speed reduction techniques from national documents (FHWA’s Roundabouts: An Informational Guide, first and second editions) and guidance documents from four other states (Iowa, Kansas, Wisconsin, and Washington). These documents provide a representative sample of the material available to practitioners seeking guidance on design of high-speed roundabout approaches. Collectively these documents discuss appropriate ranges of values for inscribed circle diameter (90-180 ft) and splitter island length (50-200 ft), as well as considerations for illumination, advance signing and marking, and potential countermeasures to encourage speed reduction. Based on the findings from the aforementioned efforts, researchers synthesized a selection of treatments, including traditional signs with and without beacons, pavement markings, illumination, speed-activated signs, and transition zones. Information on the effectiveness of these treatments, as well as potential costs of installation and maintenance, are provided for the practitioner to determine which treatment(s) best suit the site under consideration. Guidance is also provided for the methodology of conducting a speed study to determine the speed characteristics of a site, as well as links to resources for additional information. Finally, the project identifies a number of research needs specific to particular treatments as well as the general need for field research of the recommended countermeasures specifically on approaches to high-speed rural roundabouts. Three general research needs apply broadly to the treatments discussed in this research: * Establish that these countermeasures would achieve speed reductions on roundabout approaches that are similar to other locations (e.g., horizontal curves) where they are more commonly used. * Determine the effects of a combination of multiple countermeasures, compared to single treatments used individually. * Document the effectiveness of a single treatment relative to another single treatment, particularly for treatments of similar cost (e.g., compare two different warning signs, or two pavement marking treatments). More specific research needs statements are also provided for infrastructure treatments (gateway treatments, illumination), pavement markings (transverse markings, lane narrowing), and signing (transition zones, long-term effectiveness). (Author/publisher)
Abstract