Objective: The objectives of this project were to (1) develop methods of estimating the safety and operational impacts of U.S. roundabouts and (2) refine the design criteria used for them.

Status: The final report has been published as NCHRP Report 572:  Roundabouts in the United States (this page includes the presentation materials developed in Task 10) and NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572. 

Background: Although traffic circles have been used in the United States since 1905, their use has been limited since the 1950s because they were found to work neither efficiently nor safely. The modern roundabout was developed in the United Kingdom in the 1960s to address these problems. Two key characteristics of the modern roundabout are (1) entering traffic that yields to circulating traffic and (2) geometric constraints that slow entering vehicles. Many studies have shown that modern roundabouts (hereafter referred to as roundabouts) can be safe and effective, and they are now widely used internationally.

Because modern roundabout design is relatively new to the United States, there has been some reluctance to apply it. Perceived differences in driver behavior raise questions about how appropriate some international research and practices are for the United States. Therefore, additional information on the safety and operation of roundabouts in the United States will be very helpful to planners and designers in determining where roundabouts would reduce intersection crashes and congestion and in refining the design criteria currently being used.

Although available information suggests that roundabouts are relatively safe, there is concern about the effects of different design configurations on the safety of bicycles and pedestrians, particularly pedestrians with disabilities. For example, pedestrians with blindness and low vision use cues from traffic sounds to determine when to cross the roadway. The free-flowing traffic at a roundabout can sometimes make this task extremely difficult.

Tasks:

Task 1.  Describe, analyze, and critique pertinent domestic and international operational and safety analysis models and design criteria, on the basis of applicability to roundabouts in the United States. Submit a working paper summarizing the results of this effort and identifying those geometric, traffic, and other characteristics that are expected to affect the safety and operations of all roundabout users, including bicycles, pedestrians, and pedestrians with disabilities. The working paper should also identify known relationships between the characteristics and performance measures for safety and operations for all users.

Task 2.  Propose traffic analysis model formulations that predict capacity, delay, and queuing at roundabouts. Propose crash-frequency and severity prediction model formulations for roundabouts. Use these formulations to identify the data that will be needed to develop the models in subsequent tasks.

Task 3.  Prepare a detailed data collection plan for Task 5. This plan should describe the number and characteristics of data-collection sites (including a list of candidate sites); the data that will be collected; and the method(s) that will be used to collect the data. Comparisons of intersection operation and safety before and after installation of roundabouts are encouraged. The plan should describe how information on the effectiveness of the design criteria and approaches used by various agencies will be gathered.

Task 4.  Prepare an interim report summarizing the efforts and findings of Tasks 1 through 3. The report must include an updated work plan and budget for the remaining tasks. The interim report must present a realistic discussion on the potential for achieving the project's objectives within the time and financial constraints. A critical part of this discussion will be the explanation of the proposed allocation of resources among Tasks 6, 7, 8, and 9. This explanation should illustrate the expected results of each task, describe the value of these results to practitioners, assess the likelihood of success in accomplishing the results, and consider the availability of alternative methods to provide the needed information to practitioners.

Task 5.  Execute the approved data-collection plan.

Task 6.  Update and expand the inventory of U.S. roundabouts compiled during the FHWA research and make it available to transportation professionals. The inventory should be structured so that practitioners can readily find situations similar to those where they are considering installation of a roundabout. The inventory should include information on the operation and safety of the intersection before and after the installation of the roundabout. The inventory should also include basic geometric and traffic characteristics of the before and after conditions (including accommodations for pedestrians and bicycles) as well as site characteristics. The inventory should include contact information. The inventory should be based largely on existing documentation though, where practical, key pieces of missing information should be developed.

Task 7.  Evaluate existing operational analysis models using U.S. data. Refine or develop computational procedures to estimate capacity, delay, and queue lengths for approaches to single and multi-lane roundabouts using U.S. data. The definition of delay shall be compatible with the Highway Capacity Manual (HCM) to permit aggregation of delay on arterials. The traffic data requirements should not exceed those of the current HCM procedures for other unsignalized intersections, although additional geometric data are expected. Propose criteria and thresholds for defining the level of service for vehicular traffic. Document the procedures in a form that could be submitted to the TRB Committee on Highway Capacity and Quality of Service for adoption. Documentation must include the proposed text changes for HCM Chapters 10 and 17, three fully solved sample calculations that illustrate the procedures, and hypothetical benchmark data sets that may be used to compare the proposed procedures with other existing procedures.

Task 8.  Develop crash prediction models or methods that relate crashes to traffic and geometric characteristics for all roundabout sizes and types. The models or methods should estimate total crashes and fatal and injury crashes separately. A summary analysis is also required to assess collision types and severity of multi-vehicles, single vehicles, pedestrians, and bicyclists. Model requirements should conform to those of the Highway Safety Manual under development (see NCHRP Project 17-18(4)).

Task 9.  Refine geometric and traffic control design criteria used for roundabouts, including inscribed circle diameters; entry and exit widths, angles, radii, and flares; circulating roadway widths; intersection sight distances; treatments for bicycles and pedestrians (including pedestrians with disabilities and including the impact of accessible pedestrian signals on pedestrian access and vehicle operations); markings; and signs. This information should build on the current literature based on knowledge gained during the course of the project and information gleaned from use of the models developed in Tasks 7 and 8. Of particular interest are criteria for multi-lane roundabouts and those with more than four legs. The criteria should consider the traffic volume, composition (including emergency vehicles), and speed and whether the intersection is in a rural or urban area.

Task 10.  Develop materials that transportation agencies can use to demonstrate the benefits of roundabouts in a public forum.

Task 11.  Submit a final report that briefly documents the entire research effort, fully presents the design criteria from Task 9, summarizes the results of Tasks 7 and 8 (including comparisons of the performance of roundabouts and signalized intersections based on the data collected and use of the models or methods), and demonstrates how this information can be used in planning and designing a roundabout. The full specification of the models developed in Tasks 7 and 8 should be included as appendices. Deliver the Task 6 inventory in a suitable format for practitioners to access it and for it to be maintained.