#37 Turbo Roundabouts

Turbo roundabouts originated in the Netherlands in the mid-1990s as a treatment for managing capacity and traffic conflicts on high-volume multi-lane arterial roads. Although the capacity of turbo roundabouts may be generally similar to conventional multi-lane roundabouts, their line-marking and delineation encourage driver lane discipline, leading to fewer conflicts. As described by Austroads, turbo roundabouts require drivers to enter “in specific lanes depending on which exit they want to take”, as indicated by arrows on the pavement and signage. As noted by the US FHWA, the term “turbo” refers to the spiral geometry of the design and is not a reference to speed, which is expected to be reduced by turbo and other roundabout treatments.

Effectiveness

In terms of capacity, while Austroads states that turbo roundabouts are generally similar to their conventional counterparts, some research shows substantial capacity increases (25-35%) depending on the balance of traffic flows from each of the approaches. On safety, case studies cited by Austroads have reported crash and casualty crash reductions of up to 70% and 82% respectively. Austroads also notes that detailed formal evaluations are limited. Referring to early (1994-2015) evaluations in Europe, fatality reductions of 50% or more at multiple trial sites are cited by the FHWA.

Considerations

Accommodation of vulnerable road users, including pedestrians, cyclists and motorcyclists, needs to be considered when planning for turbo and other roundabout types. Ideally, turbo roundabouts should be avoided where cyclist volumes are high. U-turns are not accommodated in turbo roundabouts, as the lane a driver enters in will feed to an exit and leave the roundabout before reaching the original entry point.

Appropriate sites for turbo roundabouts will have higher traffic volumes of up to 35,000 vehicles per day. Clear line-marking and delineation are essential for vehicles entering and circulating within the roundabout. Geometric design needs to consider the ability of longer vehicles to maintain lane position. Additional signage on approaches may be needed to encourage appropriate lane selection well before entry. Design guidance is provided in the Austroads’ (2021) Guide to Road Design Part 7: New and Emerging Treatments.