Federal Highway Administration
Mini-roundabouts are a type of roundabout characterized by a small diameter and traversable islands (central island and splitter islands). Mini-roundabouts offer most of the benefits of regular roundabouts with the added benefit of a smaller footprint. As with roundabouts, mini-roundabouts are a type of intersection rather than merely a traffic calming measure, although they may produce some traffic calming effects. They are best suited to environments where speeds are already low and environmental constraints would preclude the use of a larger roundabout with a raised central island. Mini-roundabouts are common in the United Kingdom (U.K.) and France and are emerging in the United States (including states such as Maryland and Michigan), Germany, and other countries.
This technical summary focuses on single-lane mini-roundabouts. Because experience with mini-roundabouts is limited in the United States, the information presented here draws primarily upon guidance and experience from other countries with reference to American guidance as appropriate. This technical summary explores the unique characteristics of mini-roundabouts while reinforcing the need to apply the principles-based approach common to all roundabout design. It provides an overview of the key considerations for planning, analysis, and design of mini-roundabouts.
The information presented herein is a summary of principles outlined in the FHWA document Roundabouts: An Informational Guide and the forthcoming 2nd Edition (hereafter referred to as the Roundabout Guide), which is in progress at the time of this writing and due to be published in 2010. Specific considerations for single-lane and multilane roundabouts are summarized in a companion Technical Summary on Roundabouts also produced by FHWA. Figures are from the forthcoming 2nd Edition unless otherwise noted.
A mini-roundabout is a type of intersection that can be used at physically-constrained locations in place of stop controlled or signalized intersections to help improve safety problems and excessive delays at minor approaches.
Mini-Roundabout Design Features
This figure illustrates the design features of a mini-roundabout; these are described further later in this summary. Mini-roundabouts generally have an inscribed circle that is small enough to stay within the existing right-of-way (or within the existing curb lines if adequate space is available). They operate in the same manner as larger roundabouts, with yield control on all entries and counterclockwise circulation around a mountable (traversable) central island.
Mini-roundabouts are distinguished from neighborhood traffic circles primarily by their traversable islands and yield control on all approaches, which allows them to function as other roundabouts do. Neighborhood traffic circles are typically built at the intersections of local streets for reasons of traffic calming and/or aesthetics. They typically are operated as two-way or all-way stop-controlled intersections and frequently do not include raised channelization to guide approaching traffic into the circulatory roadway. At some neighborhood traffic circles, left-turning vehicles must turn in front of the central island, potentially conflicting with other circulating traffic.
To help promote safe operations, the design generally aligns passenger cars on the approach in such a way as to naturally follow the circulatory roadway and minimize running over the central island to the extent possible. Due to the small footprint, large vehicles are typically required to over-run the fully traversable central island.
Mini-roundabouts may be an optimal solution for a safety or operational issue at an existing stop-controlled or signalized intersection where there is insufficient right-of-way for a standard roundabout installation. Of course, mini-roundabouts are not always feasible or optimal solutions for every problem. The benefits of mini-roundabouts, and some constraining factors (derived largely from international experience, particularly in the U.K., where mini-roundabouts were invented), are described in this section.
Benefits of Mini-Roundabouts
Benefits of Mini-Roundabouts (Continued)
The various user types of a mini-roundabout have unique characteristics that should be considered in the planning and design process. Some of the characteristics of four primary road user groups–motorists, pedestrians, bicyclists, and large vehicles such as trucks and emergency vehicles–are discussed here; a more complete discussion can be found in the Roundabout Guide.
Motorist and Emergency Vehicle Considerations
As with other types of roundabouts, mini-roundabouts can enhance the safety of drivers, including older drivers, by:
Attention should be paid to the placement of signs and pavement markings to make them clear, visible, and unambiguous to all users, including older drivers. Trucks and other large vehicles can be accommodated at a mini-roundabout by using mountable islands. Further details on design vehicles are provided later in this technical summary.
Because of the traversable design of the central island and splitter islands, emergency vehicles are unlikely to have significant difficulty negotiating a mini-roundabout.
Pedestrian and Bicyclist Considerations
Pedestrians are accommodated at pedestrian crosswalks around the perimeter of the mini-roundabout. The splitter islands at mini-roundabouts typically do not provide the same degree of refuge as those at other roundabouts, thus typically requiring pedestrians to cross the street in one stage (as with many conventional intersections).
The Americans with Disabilities Act requires that all new and modified intersections, including roundabouts, be accessible to and usable by people with disabilities. The accessibility of mini-roundabouts to pedestrians with vision disabilities has not been specifically researched but is not expected to require treatments beyond those provided for similar single-lane roundabouts. Further discussion can be found in the Roundabout Guide.
Mini-roundabouts are generally located in environments where bicyclists are comfortable negotiating the roundabout as a motor vehicle. In the event a bicyclist desires to navigate the intersection as a pedestrian, sidewalks and crosswalks are provided.
As noted previously, mini-roundabouts are an intersection form that may have some traffic calming properties. Because of their design characteristics, mini-roundabouts are most effective in lower-speed environments in which all approaching roadways have posted speed of 30 mph or less and an 85th-percentile speed of less than 35 mph (55 km/h) near the proposed yield and/or entrance line. For any location with an 85th-percentile speed above 35 mph (55 km/h), the mini-roundabout can be included as part of a broader system of traffic calming measures to achieve an appropriate speed environment.
There are a number of locations where mini-roundabouts are commonly found to be advantageous and a number of situations that may adversely affect their feasibility. As with any decision regarding intersection treatments, care should be taken to understand the particular benefits and trade-offs for each project site.
Common Site Applications
Mini-roundabouts can be used at existing intersections to replace two-way stop control, all-way stop control, or a traffic signal. Mini-roundabouts can improve the operation of an intersection by reducing the dominance of the traffic flow from one direction over others, facilitating access and reducing delay to minor street movements, and improving overall intersection capacity.
Mini-roundabouts generally have a narrower range of applications than other types of roundabouts. The following applications represent some of the situations at which mini-roundabouts may be advantageous (further discussion can be found in the Roundabout Guide):
Potential Site Constraints
Due to their smaller proportions, mini-roundabouts are not suitable for all locations. Certain site-related factors may significantly influence the design, requiring that a more detailed investigation of some aspects of the site be carried out. A number of these factors (many of which are valid for any intersection type) are listed below:
Methods to Address Site Constraints
The existence of one or more of these conditions does not necessarily preclude the installation of a mini-roundabout. Experience in the United States is limited to date, but there may be comparable conditions in other countries where mini-roundabouts have successfully overcome one or more of the conditions listed above. To address these conditions, additional analysis, design work, and coordination with affected parties may be needed to resolve conflicts and help in the decision-making process. In some cases, the conditions identified on the previous slide cannot be overcome, and another intersection type may be more suitable.
Mini-roundabouts are generally recommended for intersections in which the total entering daily traffic volume is no more than approximately 15,000 vehicles. While a mini-roundabout may perform acceptably at higher volume locations, there has been limited experience for such sites in the United States. Multilane mini-roundabouts have been used in the U.K. but are rare elsewhere.
Operational performance models for mini-roundabouts have not been developed for U.S. conditions. The calibration to U.S. drivers of international models, such as those from the U.K., has not been determined.
The geometric design of a mini-roundabout, as with other types of roundabouts, requires the balancing of competing design objectives. Roundabouts operate most safely when their geometry forces traffic to enter and circulate at slow speeds. Poor roundabout geometry has been found to negatively impact roundabout operations by affecting driver lane choice and behavior through the roundabout. Many of the geometric parameters are governed by the maneuvering requirements of the design vehicle and the accommodation of nonmotorized users. Thus, designing a roundabout is a process of determining the optimal balance between safety provisions, operational performance, and accommodation of design users. For these reasons, roundabout design techniques are difficult to standardize, and there is rarely only one "right" way to design a roundabout.
Key Objectives of Mini-Roundabout Design
Mini-roundabout design applies many of the same principles used for other types of roundabouts, including:
The Roundabout Guide and/or the Technical Summary on Roundabouts provide more detailed design guidelines. The remainder of this document focuses on the design aspects and considerations that are unique to mini-roundabouts.
Mini-roundabout design applies many of the same principles and details of the design of larger roundabouts but with different emphasis areas. Given that the central island of a mini-roundabout is fully traversable, the overall design should provide channelization that naturally guides drivers to the intended path. Sub-optimal designs may result in drivers turning left in front of the central island (or driving over the top of it), improperly yielding, or traveling at excess speeds through the intersection.
The following key horizontal design areas for considerations will be highlighted: entrance line placement, splitter islands, size, design vehicle, design speed, and central island.
Horizontal Design Consideration: Placement of Entrance Line
The entrance line is integral to the geometric design of a mini-roundabout, and incorrect placement can introduce undesirable driver behavior. This figure illustrates one particular situation where the design allows passenger cars to turn left in front of the central island, introducing the risk of drivers taking this undesirable action. In this case, the combination of the intersection skew angle, small size of the central island, small size of the splitter islands, and large width of the circulatory roadway makes it comfortable for a driver to turn left in front of the central island instead of navigating around it.
Horizontal Design Consideration: Placement of Entrance Line (Continued)
This slide illustrates two possible design improvements: (a) advancing the entrance line forward, or (b) simultaneously enlarging the central island and reducing the circulatory roadway width, with the entrance line coincident with the inscribed circle of the roundabout. For the option of advancing the entrance line forward, the outer swept path of passenger cars and the largest vehicle likely to use the intersection are identified for all turning movements, and the advanced entrance line is placed at least 2 ft (0.6 m) outside of the vehicle paths. Skewed approaches are one particular situation where advancing the yield line may be beneficial to discourage vehicles from making a left-turn in front of the central island. However, this may result in a reduction of capacity, as advancing the yield line may affect yielding behavior at the entry.
Other Horizontal Design Considerations
Other Horizontal Design Considerations (Continued)
Pedestrian and Bicycle Design Treatments
At conventional intersections, pedestrian ramps and pedestrian crossings are typically located near the curb returns at the corners of the intersection. When converting to a mini-roundabout, these corner pedestrian crossing locations will likely require relocation. The pedestrian crossing is recommended to be located 20 to 25 ft (6.1 to 7.6 m) upstream of the entrance line to accommodate one vehicle queue ahead of the crossing.
Where a mountable or raised splitter island is used, the walkway through the splitter island should be "cut-through" instead of ramped. This is less cumbersome for wheelchair users and allows the cut-through walkway to be aligned with the crosswalks, providing guidance for all pedestrians, but particularly for those who are visually-impaired. The cut-through walkway should be approximately the same width as the crosswalk, ideally a minimum width of 10 ft (3 m). Where a minimum splitter island width of 6 ft (1.8 m) is available on the approach, a pedestrian refuge can be provided within the splitter island. In some cases, the available roadway width may not be sufficient to provide an adequate refuge area, in which case pedestrians will need to cross in one stage.
Since typical on-road bicycle travel speeds are approximately 12 to 20 mph (20 to 30 km/h), the speeds of vehicles approaching and traveling through mini-roundabouts are similar to those of bicyclists. Bicyclists are encouraged to navigate through a mini-roundabout like other vehicles. Where bicycle lanes are provided on the approaches to a mini-roundabout, they should be terminated to alert drivers and bicyclists of the need for bicyclists to merge into traffic. One suggested practice is to terminate the bike lane at least 100 ft (30 m) upstream of the entrance line, provide a 50-ft (15-m) taper ending prior to the crosswalk at the roundabout entry, and use a dotted bike lane stripe for the last 50 to 200 ft (15 to 60 m) prior to the beginning of the taper.
Sight Distance and Visibility and Vertical Design
The principles of sight distance and visibility at mini-roundabouts are consistent with other roundabouts and other intersections. Detailed guidelines for evaluating sight distance and visibility are provided in the Roundabout Guide and the Technical Summary on Roundabouts.
Mini roundabouts should generally be designed to be outward draining to place the central island at the highest point of the intersection for maximum visibility. This technique of sloping outward is recommended primarily because it:
This is consistent with most standard intersection grading, where the high-point is located near the center of the intersection and slopes towards the outer curbs. Therefore, in most retrofit situations, installation of a mini-roundabout would not necessarily require significant grade modifications to the intersection.
Pavement Markings and Signs
At mini-roundabouts, pavement markings and signs work together to create a comprehensive system to guide and regulate road users. Pavement markings and signs are simpler at mini-roundabouts than at other types of roundabouts. The Federal Highway Administration has published the 2009 Manual on Uniform Traffic Control Devices (MUTCD), which includes major revisions and additions related to signage and markings at roundabouts. A number of examples and general design principles are highlighted below.
Pavement markings for mini-roundabouts are largely similar to those for other roundabouts. However, because the islands may be either flush or mountable, additional pavement markings can be used to improve the visibility of key features, including the direction of circulation and splitter islands. This figure depicts a sample pavement marking plan for a mini-roundabout. A wide white dotted line is used to designate the entrance location, similar to other roundabouts. Some optional features include the following (not necessarily shown in the figure):
Pavement Markings and Signs (Continued)
The principal difference in signing at mini-roundabouts compared to other roundabouts is that no signs can be located within the fully mountable central island. As a result, the Circular Intersection (W2-6) warning sign is typically used on each approach in advance of the YIELD sign. YIELD signs are typically placed as close as practical to the entrance line and can be supplemented with a Roundabout Circulation plaque (R6-5P). Advance directional guide signs and exit guide signs are typically unnecessary given the size of the mini-roundabout and the nature of the approach roadways (generally low-speed local streets). However, standard street name signs should be used and are typically mounted on the same posts as the yield signs (similar to conventional intersections). This figure gives a sample signing plan for a mini-roundabout.
For splitter islands that are either painted or are fully mountable, KEEP RIGHT signs cannot be used. KEEP RIGHT signs may be provided for raised non-mountable islands, particularly where a pedestrian refuge is provided; however, care should be taken to ensure the sign does not obscure the view of the central island approaching the mini-roundabout. Some agencies are experimenting with illuminated bollards to mark splitter islands.
Lighting and Landscaping
It is important that mini-roundabouts, including their pedestrian crossing areas, be visible to approaching drivers. Consideration needs to be given to ensuring the intersection is conspicuous at night, which may mean providing additional street lighting. The Design Guide for Roundabout Lighting, published by the Illuminating Engineering Society, is the primary resource that should be consulted in completing a lighting plan for all roundabout types including mini-roundabouts. The Roundabout Guide also provides a summary of lighting principles, and the same principles for lighting traditional intersections apply to mini-roundabouts.
Landscaping of mini-roundabouts is minimal due to the traversable nature of the central island and (often) splitter islands. However, it is possible to provide landscaping around the perimeter of the intersection. Any landscaping that is provided should be designed to minimize roadside hazards and to maintain adequate stopping and intersection sight distance throughout the roundabout.
Other Design Details and Applications
Construction costs for mini-roundabouts vary widely depending upon the extent of sidewalk modifications or other geometric improvements and the types of materials used. In most cases, mini-roundabouts have been installed with little or no pavement widening and with only minor changes to curbs and sidewalks. Construction costs have ranged from about $50,000 for an installation consisting entirely of pavement markings and signage to $250,000 or more for mini-roundabouts that include raised islands and pedestrian improvements.
A benefit-cost analysis may be useful for programming purposes, as it is recognizes that not all of the benefits and costs can be quantified by pure construction costs. The safety, operational, and environmental benefits of mini-roundabouts can be quantified and compared to the initial construction and ongoing maintenance cost over the life cycle of the roundabout. Although research is needed on the service lives of mini-roundabouts in the United States, they are likely to be comparable to the intersections they replace, depending on construction materials, weather conditions, traffic conditions, and other factors. When compared to signalized intersections, mini-roundabouts are likely to have longer service lives due to less maintenance. More detail can be found in the Roundabout Guide.
For More Information
How to drive a roundabout (WSDOT)