A GDF2 is a notional line indicating the centerline of a Way A GDF2 is a notional line indicating the centerline of a Way excepting that in the case of a dual carriageway or motorway, it indicates the central median between opposing carriageways and in the case of a roundabout, it deviates from following the centreline around the roundabout and, instead, proceeds to a meeting point at the centre of the roundabout where it intersects with the centerlines of other Ways feeding into the roundabout.