Advancing Turbo Roundabouts in the United States: Synthesis Report

Porter, Richard J.; Gooch, Jeff; Chestnutt, Catherine; Moore, B.; Broeren, P.; Tigelaar, J. · 2019 · ROSA P / United States. Department of Transportation. Federal Highway Administration. Office of Safety

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Summary

This synthesis report, produced by the Federal Highway Administration (FHWA) in 2019, addresses the potential for turbo roundabouts to improve safety and operational performance at high-volume intersections in the United States. The research was motivated by the growing prevalence of multilane roundabouts in the U.S., which have experienced higher frequencies of low-severity crashes due to conflicts between exiting and circulating vehicles, as well as driver confusion regarding lane selection. While traditional roundabouts significantly reduce fatal and injury crashes compared to signalized intersections, negative public perception regarding multilane designs has hindered their adoption. The report aims to synthesize international and domestic resources to evaluate whether turbo roundabouts—a modified design originating in the Netherlands—can resolve these specific conflict issues while maintaining the safety benefits of roundabouts. The methodology involved a comprehensive review of existing published resources, including technical reports, academic papers, and design guidelines from countries with established turbo roundabout networks, such as the Netherlands, Poland, and Slovenia. The synthesis is organized into three primary sections: geometric design, capacity and operational performance, and safety performance. The report details specific geometric parameters, including inner radius values, cross-section elements, and the "turbo block" spiral alignment that forces drivers to select their lane before entering the circulatory roadway. It also examines capacity models, such as those from the Highway Capacity Manual (HCM) 2010, and evaluates safety data through crash modification factors (CMFs) and surrogate safety assessments. Key findings indicate that turbo roundabouts utilize raised lane dividers and spiral geometries to eliminate lane-changing within the circulatory roadway, thereby removing the weaving conflicts common in standard multilane roundabouts. Geometrically, the design is governed by the swept path of design vehicles and fastest-path analysis for passenger cars, with standard inner radii ranging from 10.5 meters for "mini" designs to 20 meters for "large" designs. Capacity estimates suggest that turbo roundabouts perform similarly to modern multilane roundabouts, with HCM 2010 methods providing reasonable estimates based on Polish data. Safety evaluations reveal that the reduced vehicle speeds and elimination of lane changes are expected to result in fewer and less severe crashes than multilane roundabouts. Specifically, international data suggests a 76% reduction in injury crash frequency when converting traditional intersections to turbo roundabouts. The significance of this report lies in its provision of a technical foundation for adopting turbo roundabouts in the U.S. context. By demonstrating that turbo roundabouts can mitigate the specific operational and safety drawbacks of multilane designs, the FHWA aims to encourage agency and stakeholder buy-in. The report concludes that turbo roundabouts offer an effective solution for higher-volume intersections, providing a safer alternative to both traditional multilane roundabouts and signalized intersections. It further emphasizes the need for appropriate education and public outreach to address driver unfamiliarity with the unique geometric requirements, such as mandatory lane selection prior to entry.

Key finding

Conversion of an intersection to a turbo roundabout is associated with a 76 percent reduction in injury crash frequency compared to yield-controlled, signalized, or old-style rotary intersections.

Methodology

review

Provenance

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Topics

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• intersection design