As roundabouts decrease traffic deaths for drivers, some designs of the controversial traffic treatment may actually increase non-deadly collisions for people on bikes. while scaring others off riding entirely, a series of studies suggests — and its raising thorny questions about who might get hurt along the road to Vision Zero.

As part of an ongoing study of the safety impacts of roundabouts, researchers Patrick Singleton and Nirajan Poudel surveyed 49 studies from roughly a dozen countries and concluded that, on the aggregate, roundabouts may actually increase crashes between bicyclists and drivers — and in some cases, they increase serious injuries and deaths of people on two wheels, too.