For a cul-de-sac, this could be described by `highway=turning_loop`. [turning loop][1] loop][1] However when the non-traversible island becomes large enough, it may be better modeled with the road centerline around the turning loop so that driveway entrances match the physical layout. [1]: https://wiki.openstreetmap.org/wiki/Tag:highway%3Dturning_loop

For a cul-de-sac, this could be described by `highway=turning_loop`. [turning loop][1] [1]: https://wiki.openstreetmap.org/wiki/Tag:highway%3Dturning_loop