Insights

Roundabouts are continually evolving as transportation engineers develop configurations that address different operational, safety and site-related challenges. 

Turbo roundabouts are among the newer configurations attracting attention in North America. They use spiral lane geometry and lane dividers to guide drivers into the correct lane before entry and discourage lane changes within the roundabout. By keeping vehicles in defined paths, the design can reduce weaving and some conflicts found at conventional multilane roundabouts. 

However, turbo roundabouts are only one tool in a much larger design toolbox. The key is not choosing the newest configuration but identifying the solution that best fits the site-specific challenges at hand.

Roundabout Configurations and Design Strategies

Whether the goal is reducing specific crash issues, accommodating higher traffic volumes, producing less delay, reducing speeds or addressing site constraints, different roundabout types offer different solutions.

Single-lane Roundabouts 

Single-lane roundabouts are among the most widely used roundabout configurations in the United States. Featuring a single circulating lane, they are commonly used in rural, suburban and urban settings where improving safety and traffic flow are primary objectives.

Compact and Mini Roundabouts 

Compact and mini roundabouts are smaller modern roundabouts designed to conserve space and reduce costs. They are commonly used where right-of-way, cost, speed or community context makes a larger roundabout impractical while still providing many of the safety and operational benefits of a traditional roundabout.

Multilane Roundabouts 

Multilane roundabouts feature two or more circulating lanes and are designed to accommodate higher traffic volumes than a single-lane configuration. They are often used on major corridors, at larger intersections and in areas where traffic volumes are expected to increase.

Non-Circular Roundabouts 

Non-circular roundabouts adapt traditional roundabout principles to fit unique site constraints. Configurations such as elliptical, teardrop or peanut-shaped roundabouts can help address challenging roadway geometry, closely spaced intersections or other site-specific conditions.

Hybrid Roundabouts 

Hybrid roundabouts combine characteristics from multiple roundabout configurations to address a site's unique operational or safety needs. These designs, tailored to the specific conditions of an individual intersection, may incorporate spiral lane markings, enhanced lane guidance or other geometric features intended to improve driver understanding and reduce the number of conflict points.

Choosing the Right Roundabout Is More Complex Than Choosing a Design

While each roundabout type serves a distinct purpose, the more important lesson is that roundabouts are not a one-size-fits-all solution. The same design approach does not work everywhere because no two intersections, or even two approaches at the same roundabout, face the same challenges. Before any recommendation can be made, engineers must evaluate factors such as existing and future traffic volumes, turning movement patterns, crash history, driver behavior trends, truck and freight activity, right-of-way constraints, community context, roadway context and more. 

Understanding Challenges 

Transportation projects often involve balancing competing priorities, including safety, operations, cost, public understanding, future growth and constructability. 

Sometimes those priorities point toward a larger multilane roundabout, while other situations may be better suited to a compact or mini roundabout. In other cases, project teams may incorporate select principles associated with configurations such as turbo roundabouts without implementing a full turbo design.  

That decision is often driven by the same operational, geometric and community considerations that influence whether a full turbo roundabout is appropriate in the first place. Considerations such as traffic movement patterns, lane-selection needs, pedestrian and bicycle accommodations, freight activity, right-of-way availability, long-term maintenance requirements and project costs can influence the final recommendation. 

Evaluating Solutions 

Once traffic patterns, safety performance, vehicle turning movements and site constraints have been evaluated, the conversation shifts from identifying challenges to determining the most effective way to address them. At this stage, engineers begin weighing potential solutions. 

Key questions often include:

• What problem are we trying to solve?
• What is driving the current performance issues?
• What does success look like for this community?
• How much of the recommendation is based on today's needs versus future expectations?
• Are there multiple ways to achieve the desired outcome? 

The answers help determine which solution is the best fit. In some cases, project teams may apply principles commonly associated with turbo roundabouts to address specific operational or safety concerns. However, those decisions should be based on a comprehensive evaluation of traffic operations, geometry and user needs.

Case Study: Wendell Filer Roundabout

At the intersection of Filer Avenue, Martin Street and Wendell Street in Twin Falls, Idaho, three roadways meet in an unusual offset configuration. As traffic volumes increased, the intersection experienced congestion, unpredictable turning movements and safety concerns. A conventional roundabout would have required a larger footprint and greater right-of-way impacts. 

To address these challenges, the project team designed a peanut-shaped roundabout that connects the offset roadways through a single, low-speed intersection. While elongated overall, each end uses compact-roundabout inscribed circle diameters, allowing the design to fit the existing street pattern while minimizing additional right-of-way needs.

Because this is the City of Twin Falls’ first city-owned roundabout, simplicity was a key design consideration. The team prioritized clear vehicle paths and reduced decision points, creating an intuitive driving experience that helps motorists confidently enter, circulate and exit the roundabout. 

The design also incorporates rolled curb, which accommodates occasional off-tracking by trucks and vehicles pulling trailers without requiring a larger roundabout footprint. This approach preserves space while maintaining access for a wide range of vehicles. 

The Twin Falls project demonstrates how roundabout geometry can be adapted to site-specific conditions. By combining a peanut-shaped configuration, compact dimensions and intuitive vehicle paths, the design improves operations and safety while limiting impacts on surrounding properties.

We Can Help

Turbo roundabouts may be attracting attention, but they are not the right choice for every intersection. Identifying the best solution requires more than selecting the latest design approach. It starts with understanding the specific challenges at a location, including safety performance, traffic operations, freight and multimodal needs, available right of way, future growth and community context. 

While data and modeling play an important role, effective transportation solutions also rely on engineering judgment and real-world experience. Understanding why issues occur, how development patterns may change and what tradeoffs exist between alternatives helps communities make informed decisions that support long-term goals. 

B&N helps communities evaluate those tradeoffs and develop practical transportation solutions that balance safety, mobility and future needs. Our team supports projects from planning and alternative evaluation through design, public engagement and implementation. 

Contact Brian Walsh and Molly Loucks to learn more about practical transportation solutions tailored to your community's needs.