A capacity model for multilane roundabouts has existed in the United States for several years. However, the multilane roundabout capacity model in the Highway Capacity Manual (HCM) was calibrated using data collected roughly 10 years ago, and driver performance at multilane roundabouts likely changed during that time as the number of such installations has increased across the United States. Also, the current model in the HCM is limited in several important ways, such as no consideration of the diameter of the circle, width of the splitter island, or other key geometric elements, and new data could provide insight on some of those aspects.

Furthermore, recent advances in roundabout design bring about a need for an updated model. Turbo roundabout, first developed in the Netherlands and now widely adopted across Europe, represents one such advancement. Despite their popularity internationally, only four turbo roundabouts are currently open in the United States (Jacksonville, FL; Hollister, CA; Delaware County, OH; and Jefferson County, AL). Nevertheless, many state, local, and tribal transportation agencies are actively considering turbo roundabouts for intersection improvements because of potential safety benefits.

A key knowledge gap for implementing turbo roundabouts in the United States involves their traffic capacity. At present, neither the HCM nor commonly used traffic analysis software includes models specific to turbo roundabouts. Turbo roundabouts have unique geometric features, such as tighter lane separations and distinctive 90-degree entries, which affect vehicle gap-acceptance behavior, traffic distribution patterns, and the maneuverability of larger vehicles, such as buses and trucks. Understanding how these characteristics influence capacity is essential for accurately assessing their feasibility and performance in the U.S. context.

The objectives of this research are to develop a robust capacity model tailored for turbo roundabouts that builds on U.S. driver behavior characteristics and to update or refine the existing multilane roundabout capacity model with additional data. The new model is expected to help agencies accurately estimate delays, travel times, queue lengths, levels of service, and other measures for turbo roundabouts and/or other multilane roundabout designs, ultimately assisting agencies in selecting the most suitable intersection configuration.