Investigating the conversion of a signalized intersection to a Turbo Roundabout

Abstract: Turbo roundabouts are multilane roundabouts with helical pavement markings and raised structures. While they have seen widespread use in Europe, California is the second state in the US (after Florida) to have installed a turbo roundabout. Turbo roundabouts separate the ingress and circulating roadways. The concept was first proposed and implemented in the Netherlands to address safety concerns and improve traffic flow efficiency 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 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, enabling a more prompt 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. The real-world conflict data show that more severe crossing and head-on conflicts at the former 4-legged signalized intersection have all but been eliminated.
The rear-end conflicts that do occur on the roundabout involve vehicles traveling at significantly reduced speeds compared to similar conflicts observed at the signalized intersection. The results show that turbo roundabouts may be an effective option on rural routes where the sitting criteria for multilane roundabouts are satisfied. Given the timeline for this research project, the evaluation was conducted within months of the roundabout’s completion. It is therefore advised that Caltrans continues to monitor traffic crash data to ensure that the long-term crash data shows the expected safety improvement. 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 remained a signalized intersection) is provided in this study. The estimate provides a basis for comparison with future crash data for Caltrans.