Investigating the Conversion of a Signalized Intersection to a Turbo Roundabout

Turbo roundabouts are multilane roundabouts with helical pavement markings and raised structures. While they have seen widespread use in Europe, California is only the second state in the U.S. (after Florida) to have installed a turbo roundabout. Turbo roundabouts separate the ingress and circulating roadways. The concept was proposed and implemented in the Netherlands to mitigate congestion by improving traffic flow efficiency and addressing safety concerns on conventional multilane roundabouts. This research evaluates the first-ever turbo roundabout in California and compares its safety and operational performance with the previously existing 4-legged signalized intersection. The research team obtained safety and operational performance measures using well-calibrated simulation models and video analytics of the real-world conditions recorded before and after the installation of the turbo roundabout. The safety metrics include surrogate safety measures defining traffic conflicts between vehicles that enabled a quicker safety evaluation compared to multi-year collision data. The results show that the turbo roundabout effectively reduced crash potential and queuing delays at this site, meaning smoother,safer traffic flows. The real-world conflict data shows that more severe crossing and head-on conflicts (situations that could lead to collisions) of the previous 4-legged signalized intersection have all but been eliminated. The rear-end conflicts that do occur on the roundabout involve vehicles traveling at meaningfully reduced speeds compared to similar conflicts observed on the signalized intersection. The results show that turbo roundabouts may be an effective option at rural routes where the siting criteria for multilane roundabouts are satisfied. Given the timeline for this research project, the evaluation was conducted within months of the construction completion. It is therefore advised that Caltrans continue to monitor traffic crash data to ensure that the long-term crash data shows the expected improvement of safety. To support such evaluation, the counterfactual estimate of annual crash counts (i.e., the number of crashes that would have been expected had the intersection left as a signalized intersection) is provided in this study. The estimate provides a basis for comparison with future crash data to Caltrans. This study underscores the potential of turbo roundabouts to enhance roadway safety and operational performance,supporting their consideration in traffic management strategies across California and the nation.